Chapter 9. Collections

Collections are groups of items; in .NET, collections contain objects (including boxed value types). Each object contained in a collection is called an element. Some collections contain a straightforward list of elements, while others (dictionaries) contain a list of key and value pairs. The following collection types consist of a straightforward list of elements:

ArrayList
BitArray
Queue
Stack

The following collection types are dictionaries:

Hashtable
SortedList

These collection classes are organized under the System.Collections namespace. In addition to this namespace, there is also another namespace called System.Collections.Specialized, which contains a few more useful collection classes. These classes might not be as well known as the previous classes, so here is a short explanation of the list:

ListDictionary

This class operates very similar to the Hashtable. However, this class beats out the Hashtable on performance when it contains 10 or fewer elements.

HybridDictionary

This class consists of two internal collections, the ListDictionary and the Hashtable. Only one of these classes is used at any one time. The ListDictionary is used while the collection contains 10 or fewer elements, and then a switch is made to use a Hashtable when the collection grows beyond 10 elements. This switch is made transparently to the developer. Once the Hashtable is used, this collection cannot revert to using the ListDictionary even if the elements number 10 or fewer. Also note that when using strings as the key, this class supports both case-sensitive (with respect to the invariant culture) and case-insensitive string searches through setting a Boolean value in the constructor.

CollectionsUtil

This class contains two static methods: one to create a case-insensitive Hashtable and another to create a case-insensitive SortedList. By directly creating a Hashtable and SortedList object, you will always create a case-sensitive Hashtable or SortedList, unless you use one of the constructors that take an IComparer and pass CaseInsensitiveComparer.Default to it.

NameValueCollection

This collection consists of key and value pairs, which are both of type String. The interesting thing about this collection is that it can store multiple string values with a single key. The multiple string values are comma-delimited. The String.Split method is useful when breaking up multiple strings in a value.

StringCollection

This collection is a simple list containing string elements. This list accepts null elements as well as duplicate strings. This list is case-sensitive.

StringDictionary

This is a Hashtable that stores both the key and value as strings. Keys are converted to all lowercase letters before being added to the Hashtable, allowing for case-insensitive comparisons. Keys cannot be null, but values may be set to null.

The C# compiler also supports a fixed-size array. Arrays of any type may be created using the following syntax:

int[] foo = new int[2];

where foo is an integer array containing exactly 2 elements.

Arrays come in several styles as well: multidimensional, jagged, and even multidimensional jagged. Multidimensional arrays are defined as shown here:

int[,] foo = new int[2,3];      // A 2-dimensional array containing up to 6 elements
int[,,] bar = new int[2,3,4];   // A 3-dimensional array containing up to 24 elements

A two-dimensional array is usually described as a table with rows and columns. The foo array would be described as a table of two rows each containing three columns of elements. A three-dimensional array can be described as a cube with layers of tables. The bar array could be described as four layers of two rows each containing three columns of elements.

Jagged arrays are arrays of arrays. Therefore, if you picture a jagged array as a type of two-dimensional array, it could have a different number of elements on each row. A jagged array is defined as follows:

int[][] baz = new int[2][] {new int[2], new int[3]};

The baz array consists of a one-dimensional array containing two elements. Each of these elements consists of another array, the first array having two elements and the second array having three.

The rest of this chapter contains recipes dealing with arrays and the various collection types.

9.1. Swapping Two Elements in an Array

Problem

You want an efficient method to swap two elements that exist within a single array.

Solution

Use a temporary object to hold one of the items being swapped: 

public static void SwapElementsInArray(object[] theArray, int index1, int index2)
{
    object tempHolder = theArray[index1];
    theArray[index1] = theArray[index2];
    theArray[index2] = tempHolder;
}

You can make this method strongly typed by setting theArray parameter type to a specific type. The following overload of the SwapElementsInArray method has been modified to accept an array of integers. This fix will prevent any costly boxing operations in the code that actually swaps the two elements:

public static void SwapElementsInArray(int[] theArray, int index1, int index2)
{
    int tempHolder = theArray[index1];
    theArray[index1] = theArray[index2];
    theArray[index2] = tempHolder;
}

Discussion

There is no specific method in the .NET Framework that allows only two specific elements to be swapped within an array. The SwapElementsInArray method presented in this recipe allows for only two specified elements of an array (specified in the index1 and index2 arguments to this method).

The following code uses the SwapElementsInArray method to swap the zeroth and fourth elements in an array of integers:

public static void TestSwapArrayElements( )
{
    int[] someArray = new int[5] {1,2,3,4,5};

    for (int counter = 0; counter < someArray.Length; counter++)
    {
        Console.WriteLine("Element " + counter + " = " + someArray[counter]);
    }

    SwapElementsInArray(someArray, 0, 4);

    for (int counter = 0; counter < someArray.Length; counter++)
    {
        Console.WriteLine("Element " + counter + " = " + someArray[counter]);
    }
}

This code produces the following output:

Element 0 = 1    ← The original array
Element 1 = 2
Element 2 = 3
Element 3 = 4
Element 4 = 5

Element 0 = 5    ← The array with reversed elements
Element 1 = 2
Element 2 = 3
Element 3 = 4
Element 4 = 1
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