C H A P T E R  24

Reflection and Attributes

Metadata and Reflection

The Type Class

Getting a Type Object

What Is an Attribute?

Applying an Attribute

Predefined, Reserved Attributes

More About Applying Attributes

Custom Attributes

Accessing an Attribute

Metadata and Reflection

Most programs are written to work on data. They read, write, manipulate, and display data. (Graphics are a form of data.) The types that you as the programmer create and use are designed for these purposes, and it is you, at design time, who must understand the characteristics of the types you use.

For some types of programs, however, the data they manipulate is not numbers, text, or graphics but information about programs and program types.

• Data about programs and their types is called metadata and is stored in the programs' assemblies.
• A program can look at the metadata of other assemblies or of itself, while it's running. When a running program looks at its own metadata, or that of other programs, it's called reflection.

An object browser is an example of a program that displays metadata. It can read assemblies and display the types they contain, along with all the characteristics and members.

This chapter will look at how your programs can reflect on data using the Type class and how you can add metadata to your types using attributes.

The Type Class

Throughout this text I've described how to declare and use the types available in C#. These include the predefined types (int, long, string, and so on), types from the BCL (Console, IEnumerable, and so on), and user-defined types (MyClass, MyDel, and so on). Every type has its own members and characteristics.

The BCL declares an abstract class called Type, which is designed to contain the characteristics of a type. Using objects of this class allows you to get information about the types your program is using.

Since Type is an abstract class, it cannot have actual instances. Instead, at run time, the CLR creates instances of a class derived from Type (RuntimeType) that contains the type information. When you access one of these instances, the CLR returns a reference, not of the derived type but of the base class Type. For simplicity's sake, though, throughout the rest of the chapter, I'll call the object pointed at by the reference an object of type Type, although technically it's an object of a derived type that is internal to the BCL.

Important things to know about Type are the following:

• For every type used in a program, the CLR creates a Type object that contains the information about the type.
• Every type used in a program is associated with a separate Type object.
• Regardless of the number of instances of a type that are created, there is only a single Type object associated with all the instances.

Figure 24-1 shows a running program with two MyClass objects and an OtherClass object. Notice that although there are two instances of MyClass, there is only a single Type object representing it.

Figure 24-1. The CLR instantiates objects of type Type for every type used in a program.

You can get almost anything you need to know about a type from its Type object. Table 24-1 lists some of the more useful members of the class.

Table 24-1. Selected Members of Class System.Type

Member	Member Type	Description
Name	Property	Returns the name of the type.
Namespace	Property	Returns the namespace containing the type declaration.
Assembly	Property	Returns the assembly in which the type is declared. If the type is generic, it returns the assembly in which the type is defined.
GetFields	Method	Returns a list of the type's fields.
GetProperties	Method	Returns a list of the type's properties.
GetMethods	Method	Returns a list of the type's methods.

Getting a Type Object

There are several ways to get a Type object. We'll look at using the GetType method and using the typeof operator.

Type object contains a method called GetType, which returns a reference to an instance's Type object. Since every type is ultimately derived from object, you can call the GetType method on an object of any type to get its Type object, as shown here:

The following code shows the declarations of a base class and a class derived from it. Method Main creates an instance of each class and places the references in an array called bca for easy processing. Inside the outer foreach loop, the code gets the Type object and prints out the name of the class. It then gets the fields of the class and prints them out. Figure 24-2 illustrates the objects in memory.

This code produces the following output:

Figure 24-2. The base class and derived class objects along with their Type objects

You can also use the typeof operator to get a Type object. Just supply the name of the type as the operand, and it returns a reference to the Type object, as shown here:

The following code shows a simple example of using the typeof operator:

This code produces the following output:

What Is an Attribute?

An attribute is a language construct that allows you to add metadata to a program's assembly. It's a special type of class for storing information about program constructs.

• The program construct to which you apply an attribute is called its target.
• Programs designed to retrieve and use metadata, such as object browsers, are said to be consumers of the attributes.
• There are attributes that are predefined in .NET, and you can also declare custom attributes.

Figure 24-3 is an overview of the components involved in using attributes and illustrates the following points about them:

• You apply attributes to program constructs in the source code.
• The compiler takes the source code and produces metadata from the attributes and places that metadata in the assembly.
• Consumer programs can access the metadata of the attributes along with the metadata for the rest of the components of the program. Notice that the compiler both produces and consumes attributes.

Figure 24-3. The components involved with creating and using attributes

By convention, attribute names use Pascal casing and end with the suffix Attribute. When applying an attribute to a target, you can leave off the suffix. For example, for attributes SerializableAttribute and MyAttributeAttribute, you can use the short names Serializable and MyAttribute when applying them to a construct.

Applying an Attribute

The purpose of an attribute is to tell the compiler to emit a certain set of metadata about a program construct to the assembly. You do this by applying the attribute to the construct.

• You apply an attribute by placing an attribute section immediately before the construct.
• An attribute section consists of square brackets enclosing an attribute name and sometimes a parameter list.

For example, the following code shows the headings of two classes. The first few lines of code show an attribute named Serializable applied to class MyClass. Notice that Serializable has no parameter list. The second class declaration has an attribute called MyAttribute, which has a parameter list with two string parameters.

Some important things to know about attributes are the following:

• Most attributes apply only to the construct immediately following the attribute section or sections.
• A construct with an attribute applied to it is said to be decorated, or adorned, with the attribute. Both terms are common.

Predefined, Reserved Attributes

Before looking at how you can define your own attributes, this section describes two attributes predefined and reserved by .NET: the Obsolete and Conditional attributes.

The Obsolete Attribute

The Obsolete attribute allows you to mark a program construct as obsolete and to display a helpful warning message when the code is compiled. The following code shows an example of its use:

Notice that method Main calls PrintOut even though it's marked as obsolete. In spite of this, the code compiles and runs fine and produces the following output:

During compilation, though, the compiler produces the following CS0618 warning message to inform you that you're using an obsolete construct:

Another overload of the Obsolete attribute takes a second parameter of type bool. This parameter specifies whether use of the target should be flagged as an error instead of just a warning. The following code specifies that it should be flagged as an error:

The Conditional Attribute

The Conditional attribute allows you to either include or exclude all the invocations of a particular method. To use the Conditional attribute, apply it to the method declaration, along with a compilation symbol as a parameter.

• If the compilation symbol is defined, the compiler includes the code for all the invocations of the method, the way it would for any normal method.
• If the compilation symbol is not defined, the compiler omits all the method invocations throughout the code.

The CIL code defining the method itself is always included in the assembly. It's just the invocations that are either inserted or omitted.

For example, in the following code, the Conditional attribute is applied to the declaration of a method called TraceMessage. The attribute has a single parameter, which in this case is the string DoTrace.

• When the compiler is compiling the code, it checks whether there is a compilation symbol named DoTrace defined.
• If DoTrace is defined, the compiler includes all the calls to method TraceMessage, as usual.
• If there is no DoTrace compilation symbol defined, it doesn't output code for any of the calls to TraceMessage.

Example of the Conditional Attribute

The following code shows a full example of using the Conditional attribute.

• Method Main contains two calls to method TraceMessage.
• The declaration for method TraceMessage is decorated with the Conditional attribute, which has the compilation symbol DoTrace as its parameter. So if DoTrace is defined, the compiler will include the code for all the calls to TraceMessage.
• Since the first line of code defines a compilation symbol named DoTrace, the compiler will include the code for both calls to TraceMessage.

This code produces the following output:

If you comment out the first line so that DoTrace is not defined, the compiler will not insert the code for the two calls to TraceMessage. This time, when you run the program, it produces the following output:

Predefined Attributes

The .NET Framework predefines a number of attributes that are understood and interpreted by the compiler and the CLR. Table 24-2 lists some of these. The table uses the short names, without the “Attribute” suffix. For example, the full name of CLSCompliant is CLSCompliantAttribute.

Table 24-2. Important Attributes Defined in .NET

Attribute	Meaning
CLSCompliant	Declares that the publicly exposed members should be checked by the compiler for compliance with the CLS. Compliant assemblies can be used by any .NET-compliant language.
Serializable	Declares that the construct can be serialized.
NonSerialized	Declares that the construct cannot be serialized.
Obsolete	Declares that the construct should not be used. The compiler also produces a compile-time warning or error message, if the construct is used.
DLLImport	Declares that the implementation is unmanaged code.
WebMethod	Declares that the method should be exposed as part of an XML web service.
AttributeUsage	Declares what types of program constructs the attribute can be applied to. This attribute is applied to attribute declarations.

More About Applying Attributes

The simple attributes shown so far have used a single attribute applied to a method. This section describes other types of attribute usage.

Multiple Attributes

You can apply multiple attributes to a single construct.

• Multiple attributes can be listed in either of the following formats:
  • Separate attribute sections stacked on top of each other
  • A single attribute section, with the attributes separated by commas
• You can list the attributes in any order.

For example, the following two sections of code show the two ways of applying multiple attributes. The sections of code are equivalent.

Other Types of Targets

Besides classes, you can also apply attributes to other program constructs such as fields and properties. The following declaration shows an attribute on a field, and multiple attributes on a method:

You can also explicitly label attributes to apply to a particular target construct. To use an explicit target, place the target type, followed by a colon, at the beginning of the attribute section. For example, the following code decorates the method with an attribute and also applies an attribute to the return value.

The C# language defines ten standard attribute targets, which are listed in Table 24-3. Most of the target names are self-explanatory, but type covers classes, structs, delegates, enums, and interfaces. The typevar target name specifies type parameters to constructs that use generics.

Table 24-3. Attribute Targets

event	field
method	param
property	return
type	typevar
assembly	module

Global Attributes

You can also use an explicit target to set attributes at the assembly and module level, by using the assembly and module target names. (Assemblies and modules were explained in Chapter 10.) Some important points about assembly-level attributes are the following:

• Assembly-level attributes must be placed outside any namespace scope and are usually placed in the AssemblyInfo.cs file.
• The AssembyInfo.cs file usually contains metadata about the company, product, and copyright information.

The following are lines from an AssemblyInfo.cs file:

Custom Attributes

You've probably noticed that the syntax for applying an attribute is very different from anything you've seen so far. From that, you might get the impression that attributes are an entirely different type of construct. They're not—they're just a special kind of class.

Some important points about attribute classes are the following:

• User-defined attribute classes are called custom attributes.
• All attribute classes are derived from class System.Attribute.

Declaring a Custom Attribute

Declaring an attribute class is, for the most part, the same as declaring any other class. There are, however, several things to be aware of:

• To declare a custom attribute, do the following:
  • Declare a class derived from System.Attribute.
  • Give it a name ending with the suffix Attribute.
• For security, it's generally suggested that you declare your attribute classes as sealed.

For example, the following code shows the beginning of the declaration of attribute MyAttributeAttribute:

Since an attribute holds information about the target, the public members of an attribute class generally consist only of the following:

• Fields
• Properties
• Constructors

Using Attribute Constructors

Attributes, like other classes, have constructors. Every attribute must have at least one public constructor.

• As with other classes, if you don't declare a constructor, the compiler will produce an implicit, public, parameterless constructor for you.
• Attribute constructors, like other constructors, can be overloaded.
• When declaring the constructor, you must use the full class name, including the suffix. You can use the shortened name only when applying an attribute.

For example, with the following constructor, the compiler would produce an error message if the name did not include the suffix:

Specifying the Constructor

When you apply an attribute to a target, you are specifying which constructor should be used to create the instance of the attribute. The parameters listed in the attribute application are the actual parameters for the constructor.

For example, in the following code, MyAttribute is applied to a field and to a method. For the field, the declaration specifies a constructor with a single string parameter. For the method, it specifies a constructor with two string parameters.

Other important points about attribute constructors are the following:

• When applying an attribute, the actual parameters for the constructor must be constant expressions whose values can be determined at compile time.
• If you apply an attribute constructor with no parameters, you can leave off the parentheses. For example, both classes in the following code use the parameterless constructor for the attribute MyAttr. The meanings of the two forms are the same.

Using the Constructor

You cannot call the constructor explicitly. An instance of an attribute is created, and a constructor called, only when an attribute consumer accesses the attribute. This is very different from other class instances, which are created at the position where you use an object-creation expression. Applying an attribute is a declarative statement that does not determine when an object of the attribute class should be constructed.

Figure 24-4 compares the use of a constructor for a regular class and the use of a constructor with attributes.

• The imperative statement says, in effect, “Create a new class object here.”
• The declarative statement says, “This attribute is associated with this target, and in case the attribute needs to be constructed, use this constructor.”

Figure 24-4. Comparing the use of constructors

Positional and Named Parameters in Constructors

Like the methods and constructors of regular classes, the attribute constructors can also use positional and named parameters.

The following code shows the application of an attribute using a positional parameter and two named parameters:

The following code shows the declaration of the attribute class, as well as its application on class MyClass. Notice that the constructor declaration lists only a single formal parameter. And yet, by using named parameters, you can give the constructor three actual parameters. The two named parameters set the values of fields Ver and Reviewer.

Restricting the Usage of an Attribute

You've seen that you can apply attributes to classes. But attributes themselves are classes, and there is one important predefined attribute that you can apply to your custom attributes. It's the AttributeUsage attribute. You can use it to restrict the usage of an attribute to a specific set of target types.

For example, if you want your custom attribute MyAttribute to be applied only to methods, you could use the following form of AttributeUsage:

AttributeUsage has three important public properties, which are listed in Table 24-4. The table shows the names of the properties and their meanings. For the second two properties, it also shows their default values.

Table 24-4. Public Properties of AttributeUsage

Name	Meaning	Default
ValidOn	Stores a list of the types of targets to which the attribute can be applied. The first parameter of the constructor must be an enum value of type AttributeTarget.
Inherited	A Boolean value that specifies whether the attribute can be inherited by derived classes of the decorated type.	true
AllowMultiple	A Boolean value that specifies whether the target can have multiple instances of the attribute applied to it.	false

The Constructor for AttributeUsage

The constructor for AttributeUsage takes a single, positional parameter that specifies which target types are allowed for the attribute. It uses this parameter to set the ValidOn property. The acceptable target types are members of the AttributeTarget enumeration. Table 24-5 shows the complete set of the members of the AttributeTarget enumeration.

You can combine the usage types by using the bitwise OR operator. For example, the attribute declared in the following code can be applied only to methods and constructors.

Table 24-5. Members of Enum AttributeTarget

All	Assembly	Class	Constructor
Delegate	Enum	Event	Field
GenericParameter	Interface	Method	Module
Parameter	Property	ReturnValue	Struct

When you apply AttributeUsage to an attribute declaration, the constructor will have at least the one required parameter, which contains the target types to be stored in ValidOn. You can also optionally set the Inherited and AllowMultiple properties by using named parameters. If you don't set them, they'll have their default values, as shown in Table 24-4.

As an example, the next code block specifies the following about MyAttribute:

• MyAttribute must be applied only to classes.
• MyAttribute is not inherited by classes derived from classes to which it is applied.
• There cannot be multiple instances of MyAttribute applied to the same target.

Suggested Practices for Custom Attributes

The following practices are strongly suggested when writing custom attributes:

• The attribute class should represent some state of the target construct.
• If the attribute requires certain fields, include a constructor with positional parameters to collect that data, and let optional fields be initialized with named parameters, as needed.
• Don't implement public methods or other function members other than properties.
• For additional security, declare the attribute class as sealed.
• Use the AttributeUsage attribute on your attribute declaration to explicitly specify the set of attribute targets.

The following code illustrates these guidelines:

Accessing an Attribute

At the beginning of the chapter, you saw that you can access information about a type using its Type object. You can access custom attributes in the same way. There are two methods of Type that are particularly useful in this: IsDefined and GetCustomAttributes.

Using the IsDefined Method

You can use the IsDefined method of the Type object to determine whether a particular attribute is applied to a particular class.

For example, the following code declares an attributed class called MyClass and also acts as its own attribute consumer by accessing an attribute declared and applied in the program itself. At the top of the code are declarations of the attribute ReviewComment and the class MyClass, to which it is applied. The code does the following:

• First, Main creates an object of the class. It then retrieves a reference to the Type object by using the GetType method, which it inherited from its base class, object.
• With the reference to the Type object, it can call the IsDefined method to find out whether attribute ReviewComment is applied to this class.
  • The first parameter takes a Type object of the attribute you are checking for.
  • The second parameter is of type bool and specifies whether to search the inheritance tree of MyClass to find the attribute.

This code produces the following output:

Using the GetCustomAttributes Method

The GetCustomAttributes method returns an array of the attributes applied to a construct.

• The actual object returned is an array of objects, which you must then cast to the correct attribute type.
• The Boolean parameter specifies whether to search the inheritance tree to find the attribute.
  
  
  object[] AttArr = t.GetCustomAttributes(false);
  
  
• When the GetCustomAttributes method is called, an instance of each attribute associated with the target is created.

The following code uses the same attribute and class declarations as the previous example. But in this case, it doesn't just determine whether an attribute is applied to the class. Instead, it retrieves an array of the attributes applied to the class and cycles through them, printing out their member values.

This code produces the following output: