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Dictionary<TKey, TValue> Constructor (Int32)

Initializes a new instance of the Dictionary<TKey, TValue> class that is empty, has the specified initial capacity, and uses the default equality comparer for the key type.

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

public Dictionary(
	int capacity


Type: System.Int32
The initial number of elements that the Dictionary<TKey, TValue> can contain.


capacity is less than 0.

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

The capacity of a Dictionary<TKey, TValue> is the number of elements that can be added to the Dictionary<TKey, TValue> before resizing is necessary. As elements are added to a Dictionary<TKey, TValue>, 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<TKey, TValue>.

Dictionary<TKey, TValue> requires an equality implementation to determine whether keys are equal. 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. Alternatively, you can specify an implementation of the IEqualityComparer<T> generic interface by using a constructor that accepts a comparer parameter.

This constructor is an O(1) operation.

The following code example creates a dictionary with an initial capacity of 4 and populates it with 4 entries.

using System;
using System.Collections.Generic;

public class Example
    public static void Main()
        // Create a new dictionary of strings, with string keys and
        // an initial capacity of 4.
        Dictionary<string, string> openWith = 
                               new Dictionary<string, string>(4);

        // 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");

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

/* This code example produces the following output:

Key = txt, Value = notepad.exe
Key = bmp, Value = paint.exe
Key = dib, Value = paint.exe
Key = rtf, Value = wordpad.exe

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.