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Dictionary<TKey, TValue> Constructor (IEqualityComparer<TKey>)

 

Initializes a new instance of the Dictionary<TKey, TValue> class that is empty, has the default initial capacity, and uses the specified IEqualityComparer<T>.

Namespace:   System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

public Dictionary(
	IEqualityComparer<TKey> comparer
)

Parameters

comparer
Type: System.Collections.Generic.IEqualityComparer<TKey>

The IEqualityComparer<T> implementation to use when comparing keys, or null to use the default EqualityComparer<T> for the type of the key.

Use this constructor with the case-insensitive string comparers provided by the StringComparer class to create dictionaries with case-insensitive string keys.

Every key in a Dictionary<TKey, TValue> must be unique according to the specified comparer.

Dictionary<TKey, TValue> requires an equality implementation to determine whether keys are equal. If comparer is null, this constructor uses the default generic equality comparer, EqualityComparer<T>.Default. If type TKey implements the System.IEquatable<T> generic interface, the default equality comparer uses that implementation.

System_CAPS_noteNote

If you can estimate the size of the collection, using a constructor that specifies the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Dictionary<TKey, TValue>.

This constructor is an O(1) operation.

The following code example creates a Dictionary<TKey, TValue> with a case-insensitive equality comparer for the current culture. The example adds four elements, some with lower-case keys and some with upper-case keys. The example then attempts to add an element with a key that differs from an existing key only by case, catches the resulting exception, and displays an error message. Finally, the example displays the elements in the dictionary.

using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        // Create a new Dictionary of strings, with string keys 
        // and a case-insensitive comparer for the current culture.
        Dictionary<string, string> openWith = 
                      new Dictionary<string, string>( 
                          StringComparer.CurrentCultureIgnoreCase);

        // Add some elements to the dictionary.
        openWith.Add("txt", "notepad.exe");
        openWith.Add("bmp", "paint.exe");
        openWith.Add("DIB", "paint.exe");
        openWith.Add("rtf", "wordpad.exe");

        // Try to add a fifth element with a key that is the same 
        // except for case; this would be allowed with the default
        // comparer.
        try
        {
            openWith.Add("BMP", "paint.exe");
        }
        catch (ArgumentException)
        {
            Console.WriteLine("\nBMP is already in the dictionary.");
        }

        // List the contents of the sorted dictionary.
        Console.WriteLine();
        foreach( KeyValuePair<string, string> kvp in openWith )
        {
            Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, 
                kvp.Value);
        }
    }
}

/* This code example produces the following output:

BMP is already in the dictionary.

Key = txt, Value = notepad.exe
Key = bmp, Value = paint.exe
Key = DIB, Value = paint.exe
Key = rtf, Value = wordpad.exe
 */

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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