Namespaces

Namespaces are a categorization mechanism for grouping all types related to a particular area of functionality. Namespaces are hierarchical and can have arbitrarily many levels in the hierarchy, though namespaces with more than half a dozen levels are rare. Typically the hierarchy begins with a company name, and then a product name, and then the functional area. For example, in Microsoft.Win32.Networking, the outermost namespace is Microsoft, which contains an inner namespace Win32, which in turn contains an even more deeply nested Networking namespace.

Namespaces are primarily used to organize types by area of functionality so that they can be more easily found and understood. However, they can also be used to avoid type name collisions. For example, the compiler can distinguish between two types with the name Button as long as each type has a different namespace. Thus you can disambiguate types System.Web.UI.WebControls.Button and System.Windows.Controls.Button.

In Listing 5.2, the Console type is found within the System namespace. The System namespace contains the types that enable the programmer to perform many fundamental programming activities. Almost all C# programs use types within the System namespace. Table 5.1 provides a listing of other common namespaces.

It is not always necessary to provide the namespace when calling a method. For example, if the call expression appears in a type in the same namespace as the called method, the compiler can infer the namespace to be the namespace that contains the type. Later in this chapter, you will see how the using directive eliminates the need for a namespace qualifier as well.

Type Name

Calls to static methods require the type name qualifier as long as the target method is not within the same type.¹ (As discussed later in the chapter, a using static directive allows you to omit the type name.) For example, a call expression of Console.WriteLine() found in the method HelloWorld.Main() requires the type, Console, to be specified. However, just as with the namespace, C# allows the omission of the type name from a method call whenever the method is a member of the type containing the call expression. (Examples of method calls such as this appear in Listing 5.4.) The type name is unnecessary in such cases because the compiler infers the type from the location of the call. If the compiler can make no such inference, the name must be provided as part of the method call.

1. Or base class.

At their core, types are a means of grouping together methods and their associated data. For example, Console is the type that contains the Write(), WriteLine(), and ReadLine() methods (among others). All of these methods are in the same group because they belong to the Console type.

Scope

In Chapter 4, you learned that the scope of a program element is the region of text in which it can be referred to by its unqualified name. A call that appears inside a type declaration to a method declared in that type does not require the type qualifier because the method is in scope throughout its containing type. Similarly, a type is in scope throughout the namespace that declares it; therefore, a method call that appears in a type in a particular namespace need not specify that namespace in the method call name.

Method Name

Every method call contains a method name, which might or might not be qualified with a namespace and type name, as we have discussed. After the method name comes the argument list, which is a parenthesized, comma-separated list of the values that correspond to the parameters of the method.

Parameters and Arguments

A method can take any number of parameters, and each parameter is of a specific data type. The values that the caller supplies for parameters are called the arguments; every argument must correspond to a particular parameter. For example, the following method call has three arguments:

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System.IO.File.Copy(
    oldFileName, newFileName, false)

The method is found on the class File, which is located in the namespace System.IO. It is declared to have three parameters, with the first and second being of type string and the third being of type bool. In this example, we use variables (oldFileName and newFileName) of type string for the old and new filenames, and then specify false to indicate that the copy should fail if the new filename already exists.

Method Return Values

In contrast to System.Console.WriteLine(), the method call System.Console.ReadLine() in Listing 5.2 does not have any arguments because the method is declared to take no parameters. However, this method happens to have a method return value. The method return value is a means of transferring results from a called method back to the caller. Because System.Console.ReadLine() has a return value, it is possible to assign the return value to the variable firstName. In addition, it is possible to pass this method return value itself as an argument to another method call, as shown in Listing 5.3.

Listing 5.3: Passing a Method Return Value as an Argument to Another Method Call

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class Program
{
  static void Main()
  {
      System.Console.Write("Enter your first name: ");
      System.Console.WriteLine("Hello {0}!",
          System.Console.ReadLine());
  }
}

Instead of assigning the returned value to a variable and then using that variable as an argument to the call to System.Console.WriteLine(), Listing 5.3 calls the System.Console.ReadLine() method within the call to System.Console.WriteLine(). At execution time, the System.Console.ReadLine() method executes first, and its return value is passed directly into the System.Console.WriteLine() method, rather than into a variable.

Not all methods return data. Both versions of System.Console.Write() and System.Console.WriteLine() are examples of such methods. As you will see shortly, these methods specify a return type of void, just as the HelloWorld declaration of Main returned void.

Statement versus Method Call

Listing 5.3 provides a demonstration of the difference between a statement and a method call. Although System.Console.WriteLine("Hello {0}!", System.Console.ReadLine()); is a single statement, it contains two method calls. A statement often contains one or more expressions, and in this example, two of those expressions are method calls. Therefore, method calls form parts of statements.

Although coding multiple method calls in a single statement often reduces the amount of code, it does not necessarily increase the readability and seldom offers a significant performance advantage. Developers should favor readability over brevity.

Formal Parameter Declaration

Consider the declarations of the DisplayGreeting(), GetFullName(), and the GetInitials() methods. The text that appears between the parentheses of a method declaration is the formal parameter list. (As we will see when we discuss generics, methods may also have a type parameter list. When it is clear from the context which kind of parameters we are discussing, we simply refer to them as parameters in a parameter list.) Each parameter in the parameter list includes the type of the parameter along with the parameter name. A comma separates each parameter in the list.

Behaviorally, most parameters are virtually identical to local variables, and the naming convention of parameters follows accordingly. Therefore, parameter names use camelCase. Also, it is not possible to declare a local variable (a variable declared inside a method) with the same name as a parameter of the containing method, because this would create two local variables of the same name.

Method Return Type Declaration

In addition to GetUserInput(), GetFullName(), and the GetInitials() methods requiring parameters to be specified, each of these methods includes a method return type. You can tell that a method returns a value because a data type appears immediately before the method name in the method declaration. Each of these method examples specifies a string return type. Unlike with parameters, of which there can be any number, only one method return type is allowable.

As with GetUserInput() and GetInitials(), methods with a return type almost always contain one or more return statements that return control to the caller. A return statement consists of the return keyword followed by an expression that computes the value the method is returning. For example, the GetInitials() method’s return statement is return $"{ firstName[0] }. { lastName[0] }.";. The expression (an interpolated string in this case) following the return keyword must be compatible with the stated return type of the method.

If a method has a return type, the block of statements that makes up the body of the method must not have an unreachable end point. That is, there must be no way for control to “fall off the end” of a method without returning a value. Often the easiest way to ensure that this condition is met is to make the last statement of the method a return statement. However, return statements can appear in locations other than at the end of a method implementation. For example, an if or switch statement in a method implementation could include a return statement within it; see Listing 5.5 for an example.

Listing 5.5: A return Statement before the End of a Method

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class Program
{
  static bool MyMethod()
  {
      string command = ObtainCommand();
      switch(command)
      {
          case "quit":
              return false;
          // ... omitted, other cases
          default:
              return true;
      }
  }
}

(Note that a return statement transfers control out of the switch, so no break statement is required to prevent illegal fall-through in a switch section that ends with a return statement.)

In Listing 5.5, the last statement in the method is not a return statement; it is a switch statement. However, the compiler can deduce that every possible code path through the method results in a return, so that the end point of the method is not reachable. Thus this method is legal even though it does not end with a return statement.

If particular code paths include unreachable statements following the return, the compiler will issue a warning that indicates the additional statements will never execute.

Though C# allows a method to have multiple return statements, code is generally more readable and easier to maintain if there is a single exit location rather than having multiple returns sprinkled through various code paths of the method.

Specifying void as a return type indicates that there is no return value from the method. As a result, a call to the method may not be assigned to a variable or used as a parameter type at the call site. A void method call may be used only as a statement. Furthermore, within the body of the method the return statement becomes optional, and when it is specified, there must be no value following the return keyword. For example, the return of Main() in Listing 5.4 is void, and there is no return statement within the method. However, DisplayGreeting() includes an (optional) return statement that is not followed by any returned result.

Although, technically, a method can have only one return type, the return type could be a tuple. As a result, starting with C# 7.0, it is possible to return multiple values packaged as a tuple using C# tuple syntax. For example, you could declare a GetName() method, as shown in Listing 5.6.

Listing 5.6: Returning Multiple Values Using a Tuple

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class Program
{
  static string GetUserInput(string prompt)
  {
      System.Console.Write(prompt);
      return System.Console.ReadLine();
  }
  static (string First, string Last) GetName()                                  
  {
      string firstName, lastName;
      firstName = GetUserInput("Enter your first name: ");
      lastName = GetUserInput("Enter your last name: ");
      return (firstName, lastName);
  }
  static public void Main()
  {
      (string First, string Last) name = GetName();                             
      System.Console.WriteLine($"Hello { name.First } { name.Last }!");
  }
}

Technically, we are still returning only one data type, a ValueTuple<string, string>. However, effectively, you can return any (preferably reasonable) number you like.

Expression Bodied Methods

To support the simplest of method declarations without the formality of a method body, C# 6.0 introduced expression bodied methods, which are declared using an expression rather than a full method body. Listing 5.4’s GetFullName() method provides an example of the expression bodied method:

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static string GetFullName( string firstName, string lastName) =>

In place of the curly brackets typical of a method body, an expression bodied method uses the “goes to” operator (fully introduced in Chapter 13), for which the resulting data type must match the return type of the method. In other words, even though there is no explicit return statement in the expression bodied method implementation, it is still necessary that the return type from the expression match the method declaration’s return type.

Expression bodied methods are syntactic shortcuts to the fuller method body declaration. As such, their use should be limited to the simplest of method implementations—generally expressible on a single line.

using static Directive

The using directive allows you to abbreviate a type name by omitting the namespace portion of the name—such that just the type name can be specified for any type within the stated namespace. In contrast, the using static directive allows you to omit both the namespace and the type name from any member of the stated type. A using static System.Console directive, for example, allows you to specify WriteLine() rather than the fully qualified method name of System.Console.WriteLine(). Continuing with this example, we can update Listing 5.2 to leverage the using static System.Console directive to create Listing 5.9.

Listing 5.9: using static Directive

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using static System.Console;

class HeyYou
{
  static void Main()
  {
      string firstName;
      string lastName;

      WriteLine("Hey you!");

      Write("Enter your first name: ");

      firstName = ReadLine();
      Write("Enter your last name: ");
      lastName = ReadLine();
      WriteLine(
          $"Your full name is { firstName } { lastName }.");
  }
}

In this case, there is no loss of readability of the code: WriteLine(), Write(), and ReadLine() all clearly relate to a console directive. In fact, one could argue that the resulting code is simpler and therefore clearer than before.

However, sometimes this is not the case. For example, if your code uses classes that have overlapping behavior names, such as an Exists() method on a file and an Exists() method on a directory, then perhaps a using static directive would reduce clarity when you invoke Exists(). Similarly, if the class you were writing had its own members with overlapping behavior names—for example, Display() and Write()—then perhaps clarity would be lost to the reader.

This ambiguity would not be allowed by the compiler. If two members with the same signature were available (through either using static directives or separately declared members), any invocation of them that was ambiguous would result in a compile error.

Aliasing

The using directive also allows aliasing a namespace or type. An alias is an alternative name that you can use within the text to which the using directive applies. The two most common reasons for aliasing are to disambiguate two types that have the same name and to abbreviate a long name. In Listing 5.10, for example, the CountDownTimer alias is declared as a means of referring to the type System.Timers.Timer. Simply adding a using System.Timers directive will not sufficiently enable the code to avoid fully qualifying the Timer type. The reason is that System.Threading also includes a type called Timer; therefore, using just Timer within the code will be ambiguous.

Listing 5.10: Declaring a Type Alias

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using System;
using System.Threading;                                  
using CountDownTimer = System.Timers.Timer;              

class HelloWorld
{
  static void Main()
  {
      CountDownTimer timer;                              

      // ...
  }
}

Listing 5.10 uses an entirely new name, CountDownTimer, as the alias. It is possible, however, to specify the alias as Timer, as shown in Listing 5.11.

Listing 5.11: Declaring a Type Alias with the Same Name

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using System;
using System.Threading;

// Declare alias Timer to refer to System.Timers.Timer to               
// avoid code ambiguity with System.Threading.Timer                     
using Timer = System.Timers.Timer;                                      

class HelloWorld
{
  static void Main()
  {
      Timer timer;                                                      
      // ...
  }
}

Because of the alias directive, “Timer” is not an ambiguous reference. Furthermore, to refer to the System.Threading.Timer type, you will have to either qualify the type or define a different alias.

Value Parameters

Arguments to method calls are usually passed by value, which means the value of the argument expression is copied into the target parameter. For example, in Listing 5.13, the value of each variable that Main() uses when calling Combine() will be copied into the parameters of the Combine() method. Output 5.5 shows the results of this listing.

Listing 5.13: Passing Variables by Value

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class Program
{
  static void Main()
  {
      // ...
      string fullName;
      string driveLetter = "C:";
      string folderPath  = "Data";
      string fileName    = "index.html";

      fullName = Combine(driveLetter, folderPath, fileName);

      Console.WriteLine(fullName);
      // ...
  }
  static string Combine(
      string driveLetter, string folderPath, string fileName)
  {
      string path;
      path = string.Format("{1}{0}{2}{0}{3}",
          System.IO.Path.DirectorySeparatorChar,
          driveLetter, folderPath, fileName);
      return path;
  }
}

Output 5.5

C:Dataindex.html

Even if the Combine() method assigns null to driveLetter, folderPath, and fileName before returning, the corresponding variables within Main() will maintain their original values because the variables are copied when calling a method. When the call stack unwinds at the end of a call, the copied data is thrown away.

The value of a reference type variable is, as the name implies, a reference to the location where the data associated with the object is stored. How the runtime chooses to represent the value of a reference type variable is an implementation detail of the runtime; typically it is represented as the address of the memory location in which the object’s data is stored, but it need not be.

If a reference type variable is passed by value, the reference itself is copied from the caller to the method parameter. As a result, the target method cannot update the caller variable’s value, but it may update the data referred to by the reference.

Alternatively, if the method parameter is a value type, the value itself is copied into the parameter, and changing the parameter in the called method will not affect the original caller’s variable.

Reference Parameters (ref)

Consider Listing 5.14, which calls a function to swap two values, and Output 5.6, which shows the results.

Listing 5.14: Passing Variables by Reference

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class Program
{
  static void Main()
  {
      // ...
      string first = "hello";
      string second = "goodbye";
      Swap(ref first, ref second);                                       

      Console.WriteLine(
          $@"first = ""{ first }"", second = ""{ second }""" );
      // ...
  }
  static void Swap(ref string x, ref string y)                           
  {
      string temp = x;
      x = y;
      y = temp;
  }
}

Output 5.6

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first = "goodbye", second = "hello"

The values assigned to first and second are successfully switched. To do this, the variables are passed by reference. The obvious difference between the call to Swap() and Listing 5.13’s call to Combine() is the inclusion of the keyword ref in front of the parameter’s data type. This keyword changes the call such that the variables used as arguments are passed by reference, so the called method can update the original caller’s variables with new values.

When the called method specifies a parameter as ref, the caller is required to supply a variable, not a value, as an argument and to place ref in front of the variables passed. In so doing, the caller explicitly recognizes that the target method could reassign the values of the variables associated with any ref parameters it receives. Furthermore, it is necessary to initialize any local variables passed as ref because target methods could read data from ref parameters without first assigning them. In Listing 5.14, for example, temp is assigned the value of first, assuming that the variable passed in first was initialized by the caller. Effectively, a ref parameter is an alias for the variable passed. In other words, it is essentially giving a parameter name to an existing variable, rather than creating a new variable and copying the value of the argument into it.

Output Parameters (out)

As mentioned earlier, a variable used as a ref parameter must be assigned before it is passed to the called method, because the called method might read from the variable. The “swap” example given previously must read and write from both variables passed to it. However, it is often the case that a method that takes a reference to a variable intends to write to the variable but not to read from it. In such cases, clearly it could be safe to pass an uninitialized local variable by reference.

To achieve this, code needs to decorate parameter types with the keyword out. This is demonstrated in the TryGetPhoneButton() method in Listing 5.15, which returns the phone button corresponding to a character.

Listing 5.15: Passing Variables Out Only

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class ConvertToPhoneNumber
{
  static int Main(string[] args)
  {
      if(args.Length == 0)
      {
          Console.WriteLine(
              "ConvertToPhoneNumber.exe <phrase>");
          Console.WriteLine(
              "'_' indicates no standard phone button");
          return 1;
      }
      foreach(string word in args)
      {
          foreach(char character in word)
          {
              if(TryGetPhoneButton(character, out char button))                          
              {
                  Console.Write(button);
              }
              else
              {
                  Console.Write('_');
              }
          }
      }
      Console.WriteLine();
      return 0;
  }
  static bool TryGetPhoneButton(char character, out char button)                          
  {
      bool success = true;
      switch( char.ToLower(character) )
      {
          case '1':
              button = '1';
              break;
          case '2': case 'a': case 'b': case 'c':
              button = '2';
              break;
          // ...
          case '-':
              button = '-';
              break;
          default:
                // Set the button to indicate an invalid value
                button = '_';
              success = false;
              break;
      }
      return success;
  }
}

Output 5.7 shows the results of Listing 5.15.

Output 5.7

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>ConvertToPhoneNumber.exe CSharpIsGood
274277474663

In this example, the TryGetPhoneButton() method returns true if it can successfully determine the character’s corresponding phone button. The function also returns the corresponding button by using the button parameter, which is decorated with out.

An out parameter is functionally identical to a ref parameter; the only difference is which requirements the language enforces regarding how the aliased variable is read from and written to. Whenever a parameter is marked with out, the compiler checks that the parameter is set for all code paths within the method that return normally (i.e., the code paths that do not throw an exception). If, for example, the code does not assign button a value in some code path, the compiler will issue an error indicating that the code didn’t initialize button. Listing 5.15 assigns button to the underscore character because even though it cannot determine the correct phone button, it is still necessary to assign a value.

A common coding error when working with out parameters is to forget to declare the out variable before you use it. Starting with C# 7.0, it is possible to declare the out variable inline when invoking the function. Listing 5.15 uses this feature with the statement TryGetPhoneButton(character, out char button) without ever declaring the button variable beforehand. Prior to C# 7.0, it would be necessary to first declare the button variable and then invoke the function with TryGetPhoneButton(character, out button).

Another C# 7.0 feature is the ability to discard an out parameter entirely. If, for example, you simply wanted to know whether a character was a valid phone button but not actually return the numeric value, you could discard the button parameter using an underscore: TryGetPhoneButton(character, out _).

Prior to C# 7.0’s tuple syntax, a developer of a method might declare one or more out parameters to get around the restriction that a method may have only one return type; a method that needs to return two values can do so by returning one value normally, as the return value of the method, and a second value by writing it into an aliased variable passed as an out parameter. Although this pattern is both common and legal, there are usually better ways to achieve that aim. For example, if you are considering returning two or more values from a method and C# 7.0 is available, it is likely preferable to use C# 7.0 tuple syntax. Prior to that, consider writing two methods, one for each value, or still using the System.ValueTuple type but without C# 7.0 syntax.

Read-Only Pass by Reference (in)

In C# 7.2, support was added for passing a value type by reference that was read only. Rather than passing the value type to a function so that it could be changed, read-only pass by reference was added: It allows the value type to be passed by reference so that not only copy of the value type occurs but, in addition, the invoked method cannot change the value type. In other words, the purpose of the feature is to reduce the memory copied when passing a value while still identifying it as read only, thus improving the performance. This syntax is to add an in modifier to the parameter. For example:

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int Method(in int number) { ... }

With the in modifier, any attempts to reassign number (number++, for example) will result in a compile error indicating that number is read only.

Return by Reference

Another C# 7.0 addition is support for returning a reference to a variable. Consider, for example, a function that returns the first pixel in an image that is associated with red-eye, as shown in Listing 5.16.

Listing 5.16: ref Return and ref Local Declaration

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// Returning a reference
public static ref byte FindFirstRedEyePixel(byte[] image)                       
{
  // Do fancy image detection perhaps with machine learning
  for (int counter = 0; counter < image.Length; counter++)
  {
    if(image[counter] == (byte)ConsoleColor.Red)
    {
      return ref image[counter];
    }
  }
  throw new InvalidOperationException("No pixels are red.");
}
public static void Main()
{
  byte[] image = new byte[254];
  // Load image
  int index = new Random().Next(0, image.Length - 1);
  image[index] =
      (byte)ConsoleColor.Red;
  System.Console.WriteLine(
      $"image[{index}]={(ConsoleColor)image[index]}");
  // ...

  // Obtain a reference to the first red pixel                              
  ref byte redPixel = ref FindFirstRedEyePixel(image);                      
  // Update it to be Black                                                  
  redPixel = (byte)ConsoleColor.Black;                                      
  System.Console.WriteLine(
      $"image[{index}]={(ConsoleColor)image[redPixel]}");
}

By returning a reference to the variable, the caller is then able to update the pixel to a different color, as shown in the highlighted lines of Listing 5.16. Checking for the update via the array shows that the value is now black.

There are two important restrictions on return by reference, both due to object lifetime: (1) Object references shouldn’t be garbage collected while they’re still referenced, and (2) they shouldn’t consume memory when they no longer have any references. To enforce these restrictions, you can only return the following from a reference-returning function:

• References to fields or array elements

• Other reference-returning properties or functions

• References that were passed in as parameters to the by-reference-returning function

For example, FindFirstRedEyePixel() returns a reference to an item in the image array, which was a parameter to the function. Similarly, if the image was stored as a field within the class, you could return the field by reference:

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byte[] _Image;
public ref byte[] Image { get {  return ref _Image; } }

In addition, ref locals are initialized to refer to a particular variable and can’t be modified to refer to a different variable.

There are several return-by-reference characteristics of which to be cognizant:

• If you’re returning a reference, you obviously must return it. This means, therefore, that in the example in Listing 5.16, even if no red-eye pixel exists, you still need to return a reference byte. The only workaround would be to throw an exception. In contrast, the by-reference parameter approach allows you to leave the parameter unchanged and return a bool indicating success. In many cases, this might be preferable.

• When declaring a reference local variable, initialization is required. This involves assigning it a ref return from a function or a reference to a variable:

  ref string text;  // Error

• Although it’s possible in C# 7.0 to declare a reference local variable, declaring a field of type ref isn’t allowed:

  class Thing { ref string _Text;  /* Error */ }

• You can’t declare a by-reference type for an auto-implemented property:

  class Thing { ref string Text { get;set; }  /* Error */ }

• Properties that return a reference are allowed:

  class Thing { string _Text = "Inigo Montoya";
  ref string Text { get { return ref _Text; } } }

• A reference local variable can’t be initialized with a value (such as null or a constant). It must be assigned from a by-reference-returning member or a local variable, field, or array element:

  ref int number = 42;  // ERROR

Parameter Arrays (params)

In the examples so far, the number of arguments that must be passed has been fixed by the number of parameters declared in the target method declaration. However, sometimes it is convenient if the number of arguments may vary. Consider the Combine() method from Listing 5.13. In that method, you passed the drive letter, folder path, and filename. What if the path had more than one folder, and the caller wanted the method to join additional folders to form the full path? Perhaps the best option would be to pass an array of strings for the folders. However, this would make the calling code a little more complex, because it would be necessary to construct an array to pass as an argument.

To make it easier on the callers of such a method, C# provides a keyword that enables the number of arguments to vary in the calling code instead of being set by the target method. Before we discuss the method declaration, observe the calling code declared within Main(), as shown in Listing 5.17.

Listing 5.17: Passing a Variable Parameter List

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using System;
using System.IO;
class PathEx
{
  static void Main()
  {
      string fullName;

      // ...

      // Call Combine() with four arguments                               
      fullName = Combine(                                                 
          Directory.GetCurrentDirectory(),                                
          "bin", "config", "index.html");                                 
      Console.WriteLine(fullName);

      // ...

      // Call Combine() with only three arguments                          
      fullName = Combine(                                                  
          Environment.SystemDirectory,                                     
          "Temp", "index.html");                                           
      Console.WriteLine(fullName);

      // ...

      // Call Combine() with an array                                      
      fullName = Combine(                                                  
          new string[] {                                                   
              "C:\", "Data",                                              
              "HomeDir", "index.html"} );                                  
      Console.WriteLine(fullName);
      // ...
  }

  static string Combine(params string[] paths)                             
  {
      string result = string.Empty;
      foreach (string path in paths)
      {
          result = Path.Combine(result, path);
      }
      return result;
  }
}

Output 5.8 shows the results of Listing 5.17.

Output 5.8

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C:Datamarkinconfigindex.html
C:WINDOWSsystem32Tempindex.html
C:DataHomeDirindex.html

In the first call to Combine(), four arguments are specified. The second call contains only three arguments. In the final call, a single argument is passed using an array. In other words, the Combine() method takes a variable number of arguments—presented either as any number of string arguments separated by commas or as a single array of strings. The former syntax is called the expanded form of the method call, and the latter form is called the normal form.

To allow invocation using either form, the Combine() method does the following:

1. Places params immediately before the last parameter in the method declaration

2. Declares the last parameter as an array

With a parameter array declaration, it is possible to access each corresponding argument as a member of the params array. In the Combine() method implementation, you iterate over the elements of the paths array and call System.IO.Path.Combine(). This method automatically combines the parts of the path, appropriately using the platform-specific directory-separator character. Note that PathEx.Combine() is identical to Path.Combine(), except that PathEx.Combine() handles a variable number of parameters rather than simply two.

There are a few notable characteristics of the parameter array:

• The parameter array is not necessarily the only parameter on a method.

• The parameter array must be the last parameter in the method declaration. Since only the last parameter may be a parameter array, a method cannot have more than one parameter array.

• The caller can specify zero arguments that correspond to the parameter array parameter, which will result in an array of zero items being passed as the parameter array.

• Parameter arrays are type-safe: The arguments given must be compatible with the element type of the parameter array.

• The caller can use an explicit array rather than a comma-separated list of arguments. The resulting CIL code is identical.

• If the target method implementation requires a minimum number of parameters, those parameters should appear explicitly within the method declaration, forcing a compile error instead of relying on runtime error handling if required parameters are missing. For example, if you have a method that requires one or more integer arguments, declare the method as int Max(int first, params int[] operands) rather than as int Max(params int[] operands) so that at least one value is passed to Max().

Using a parameter array, you can pass a variable number of arguments of the same type into a method. The section “Method Overloading,” which appears later in this chapter, discusses a means of supporting a variable number of arguments that are not necessarily of the same type.

By the way, a path Combine() function is a contrived example since, in fact, System.IO.Path.Combine() is an existing function that is overloaded to support parameter arrays.

Trapping Errors

To indicate to the calling method that the parameter is invalid, int.Parse() will throw an exception. Throwing an exception halts further execution in the current control flow and jumps into the first code block within the call stack that handles the exception.

Since you have not yet provided any such handling, the program reports the exception to the user as an unhandled exception. Assuming there is no registered debugger on the system, the error will appear on the console with a message such as that shown in Output 5.12.

Output 5.12

Click here to view code image

Hey you!
Enter your first name: Inigo
Enter your age: forty-two
Unhandled Exception: System.FormatException: Input string was
        not in a correct format.
    at System.Number.ParseInt32(String s, NumberStyles style,
        NumberFormatInfo info)
    at ExceptionHandling.Main()

Obviously, such an error is not particularly helpful. To fix this, it is necessary to provide a mechanism that handles the error, perhaps reporting a more meaningful error message back to the user.

This process is known as catching an exception. The syntax is demonstrated in Listing 5.23, and the output appears in Output 5.13.

Listing 5.23: Catching an Exception

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using System;

class ExceptionHandling
{
  static int Main()
  {
      string firstName;
      string ageText;
      int age;
      int result = 0;

      Console.Write("Enter your first name: ");
      firstName = Console.ReadLine();

      Console.Write("Enter your age: ");
      ageText = Console.ReadLine();

      try
      {
          age = int.Parse(ageText);
          Console.WriteLine(
              $"Hi { firstName }! You are { age*12 } months old.");
      }
      catch (FormatException )
      {
          Console.WriteLine(
              $"The age entered, { ageText }, is not valid.");
          result = 1;
      }
      catch(Exception exception)
      {
          Console.WriteLine(
              $"Unexpected error:  { exception.Message }");
          result = 1;
      }
      finally
      {
          Console.WriteLine($"Goodbye { firstName }");
      }
      return result;
  }
}

Output 5.13

Click here to view code image

Enter your first name: Inigo
Enter your age: forty-two
The age entered, forty-two, is not valid.
Goodbye Inigo

To begin, surround the code that could potentially throw an exception (age = int.Parse()) with a try block. This block begins with the try keyword. It indicates to the compiler that the developer is aware of the possibility that the code within the block might throw an exception, and if it does, one of the catch blocks will attempt to handle the exception.

One or more catch blocks (or the finally block) must appear immediately following a try block. The catch block header (see Advanced Topic: General Catch later in this chapter) optionally allows you to specify the data type of the exception. As long as the data type matches the exception type, the catch block will execute. If, however, there is no appropriate catch block, the exception will fall through and go unhandled as though there were no exception handling. The resultant control flow appears in Figure 5.1.

For example, assume the user enters “forty-two” for the age in the previous example. In this case, int.Parse() will throw an exception of type System.FormatException, and control will jump to the set of catch blocks. (System.FormatException indicates that the string was not of the correct format to be parsed appropriately.) Since the first catch block matches the type of exception that int.Parse() threw, the code inside this block will execute. If a statement within the try block threw a different exception, the second catch block would execute because all exceptions are of type System.Exception.

If there were no System.FormatException catch block, the System.Exception catch block would execute even though int.Parse throws a System.FormatException. This is because a System.FormatException is also of type System.Exception. (System.FormatException is a more specific implementation of the generic exception, System.Exception.)

The order in which you handle exceptions is significant. Catch blocks must appear from most specific to least specific. The System.Exception data type is least specific, so it appears last. System.FormatException appears first because it is the most specific exception that Listing 5.23 handles.

Regardless of whether control leaves the try block normally or because the code in the try block throws an exception, the finally block of code will execute after control leaves the try-protected region. The purpose of the finally block is to provide a location to place code that will execute regardless of how the try/catch blocks exit—with or without an exception. Finally blocks are useful for cleaning up resources, regardless of whether an exception is thrown. In fact, it is possible to have a try block with a finally block and no catch block. The finally block executes regardless of whether the try block throws an exception or whether a catch block is even written to handle the exception. Listing 5.24 demonstrates the try/finally block, and Output 5.14 shows the results.

Listing 5.24: Finally Block without a Catch Block

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using System;

class ExceptionHandling
{
  static int Main()
  {
      string firstName;
      string ageText;
      int age;
      int result = 0;

      Console.Write("Enter your first name: ");
      firstName = Console.ReadLine();

      Console.Write("Enter your age: ");
      ageText = Console.ReadLine();

      try
      {
          age = int.Parse(ageText);
          Console.WriteLine(
              $"Hi { firstName }! You are { age*12 } months old.");
      }
      finally
      {
          Console.WriteLine($"Goodbye { firstName }");
      }
      return result;

  }
}

Output 5.14

Click here to view code image

Enter your first name: Inigo
Enter your age: forty-two
Unhandled Exception: System.FormatException: Input string was not in a
correct format.
   at System.Number.StringToNumber(String str, NumberStyles options,
NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style,
NumberFormatInfo info)
   at ExceptionHandling.Main()
Goodbye Inigo

The attentive reader will have noticed something interesting here: The runtime first reported the unhandled exception and then ran the finally block. What explains this unusual behavior?

First, the behavior is legal because when an exception is unhandled, the behavior of the runtime is implementation defined—any behavior is legal! The runtime chooses this particular behavior because it knows before it chooses to run the finally block that the exception will be unhandled; the runtime has already examined all of the activation frames on the call stack and determined that none of them is associated with a catch block that matches the thrown exception.

As soon as the runtime determines that the exception will be unhandled, it checks whether a debugger is installed on the machine, because you might be the software developer who is analyzing this failure. If a debugger is present, it offers the user the chance to attach the debugger to the process before the finally block runs. If there is no debugger installed or if the user declines to debug the problem, the default behavior is to print the unhandled exception to the console and then see if there are any finally blocks that could run. Due to the “implementation-defined” nature of the situation, the runtime is not required to run finally blocks in this situation; an implementation may choose to do so or not.

A method could throw many exception types. Table 5.2 lists some of the more common ones within the framework.

Listing 5.25: General Catch Blocks

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// A previous catch clause already catches all exceptions
#pragma warning disable CS1058
...
try
{
    age = int.Parse(ageText);
    System.Console.WriteLine(
        $"Hi { firstName }!  You are { age*12 } months old.");
}
catch (System.FormatException exception)
{
    System.Console.WriteLine(
        $"The age entered ,{ ageText }, is not valid.");
    result = 1;
}
catch(System.Exception exception)
{
    System.Console.WriteLine(
        $"Unexpected error:  { exception.Message }");
    result = 1;
}
catch
{
    System.Console.WriteLine("Unexpected error!");
    result = 1;
}
finally
{
    System.Console.WriteLine($"Goodbye { firstName }");
}
...

A catch block with no data type, called a general catch block, is equivalent to specifying a catch block that takes an object data type—for instance, catch(object exception){...}. For this reason, a warning is triggered stating that the catch block already exists; hence the #pragma warning disable directive.

Because all classes ultimately derive from object, a catch block with no data type must appear last.

General catch blocks are rarely used because there is no way to capture any information about the exception. In addition, C# doesn’t support the ability to throw an exception of type object. (Only libraries written in languages such as C++ allow exceptions of any type.)

Reporting Errors Using a throw Statement

C# allows developers to throw exceptions from their code, as demonstrated in Listing 5.26 and Output 5.15.

Listing 5.26: Throwing an Exception

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// A previous catch clause already catches all exceptions
using System;
public class ThrowingExceptions
{
  public static void Main()
  {
      try
      {
          Console.WriteLine("Begin executing");
          Console.WriteLine("Throw exception");
              throw new Exception("Arbitrary exception");                          
          Console.WriteLine("End executing");
      }
      catch(FormatException exception)
      {
         Console.WriteLine(
             "A FormateException was thrown");
      }
      catch(Exception exception)
      {
        Console.WriteLine(
            $"Unexpected error: { exception.Message }");
      }
      catch
      {
           Console.WriteLine("Unexpected error!");
      }

      Console.WriteLine(
          "Shutting down...");
  }
}

Output 5.15

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Begin executing
Throw exception...
Unexpected error:  Arbitrary exception
Shutting down...

As the arrows in Listing 5.26 depict, throwing an exception causes execution to jump from where the exception is thrown into the first catch block within the stack that is compatible with the thrown exception type.⁹ In this case, the second catch block handles the exception and writes out an error message. In Listing 5.26, there is no finally block, so execution falls through to the System.Console.WriteLine() statement following the try/catch block.

9. Technically it could be caught by a compatible catch filter as well.

To throw an exception, it is necessary to have an instance of an exception. Listing 5.26 creates an instance using the keyword new followed by the type of the exception. Most exception types allow a message to be generated as part of throwing the exception, so that when the exception occurs, the message can be retrieved.

Sometimes a catch block will trap an exception but be unable to handle it appropriately or fully. In these circumstances, a catch block can rethrow the exception using the throw statement without specifying any exception, as shown in Listing 5.27.

Listing 5.27: Rethrowing an Exception

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...
      catch(Exception exception)
      {
          Console.WriteLine(
              $@"Rethrowing unexpected error:  {
                  exception.Message }");
          throw;
      }
...

In Listing 5.27, the throw statement is “empty” rather than specifying that the exception referred to by the exception variable is to be thrown. This illustrates a subtle difference: throw; preserves the call stack information in the exception, whereas throw exception; replaces that information with the current call stack information. For debugging purposes, it is usually better to know the original call stack.

if (int.TryParse(ageText, out int age))
{
    Console.WriteLine(
        $"Hi { firstName }!  "
        + $"You are { age*12 } months old.");
}
else
{
    Console.WriteLine(
        $"The age entered, { ageText }, is not valid.");
}