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Dictionary.Dictionary(Int32, Generic IEqualityComparer) Constructor

Initializes a new instance of the Dictionary class that is empty, has the specified initial capacity, and uses the specified IEqualityComparer.

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

public Dictionary (
	int capacity,
	IEqualityComparer<TKey> comparer
public Dictionary (
	int capacity, 
	IEqualityComparer<TKey> comparer
public function Dictionary (
	capacity : int, 
	comparer : IEqualityComparer<TKey>
Not applicable.



The initial number of elements that the Dictionary can contain.


The IEqualityComparer implementation to use when comparing keys, or a null reference (Nothing in Visual Basic) to use the default EqualityComparer for the type of the key.

Exception typeCondition


capacity is less than 0.

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 must be unique according to the specified comparer.

The capacity of a Dictionary is the number of elements that can be added to the Dictionary before resizing is necessary. As elements are added to a Dictionary, the capacity is automatically increased as required by reallocating the internal array.

If the size of the collection can be estimated, specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Dictionary.

Dictionary requires an equality implementation to determine whether keys are equal. If comparer is a null reference (Nothing in Visual Basic), this constructor uses the default generic equality comparer, EqualityComparer.Default. If type TKey implements the System.IEquatable generic interface, the default equality comparer uses that implementation.

This constructor is an O(1) operation.

The following code example creates a Dictionary with an initial capacity of 5 and 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, an
        // initial capacity of 5, and a case-insensitive equality 
        // comparer.
        Dictionary<string, string> openWith = 
                      new Dictionary<string, string>(5, 
        // Add 4 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.
            openWith.Add("BMP", "paint.exe");
        catch (ArgumentException)
            Console.WriteLine("\nBMP is already in the dictionary.");
        // List the contents of the dictionary.
        foreach( KeyValuePair<string, string> kvp in openWith )
            Console.WriteLine("Key = {0}, Value = {1}", kvp.Key, 

/* 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

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0