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Dictionary<TKey, TValue> Constructor (Int32, IEqualityComparer<TKey>)

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

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

public Dictionary(
	int capacity,
	IEqualityComparer<TKey> comparer
)

Parameters

capacity
Type: System.Int32

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

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.

ExceptionCondition
ArgumentOutOfRangeException

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<TKey, TValue> must be unique according to the specified 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. 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.

This constructor is an O(1) operation.

The following code example creates a Dictionary<TKey, TValue> 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, 
                          StringComparer.CurrentCultureIgnoreCase);

        // 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. 
        try
        {
            openWith.Add("BMP", "paint.exe");
        }
        catch (ArgumentException)
        {
            Console.WriteLine("\nBMP is already in the dictionary.");
        }

        // List the contents of the 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
 */

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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