Dictionary.Dictionary(Generic IDictionary, Generic IEqualityComparer) Constructor

Initializes a new instance of the Dictionary class that contains elements copied from the specified IDictionary and uses the specified IEqualityComparer.

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

public Dictionary (
	IDictionary<TKey,TValue> dictionary,
	IEqualityComparer<TKey> comparer
public Dictionary (
	IDictionary<TKey,TValue> dictionary, 
	IEqualityComparer<TKey> comparer
public function Dictionary (
	dictionary : IDictionary<TKey,TValue>, 
	comparer : IEqualityComparer<TKey>
Not applicable.



The IDictionary whose elements are copied to the new Dictionary.


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


dictionary is a null reference (Nothing in Visual Basic).


dictionary contains one or more duplicate keys.

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; likewise, every key in the source dictionary must also be unique according to the specified comparer.


For example, duplicate keys can occur if comparer is one of the case-insensitive string comparers provided by the StringComparer class and dictionary does not use a case-insensitive comparer key.

The initial capacity of the new Dictionary is large enough to contain all the elements in 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(n) operation, where n is the number of elements in dictionary.

The following code example shows how to use the Dictionary(Generic IDictionary,Generic IEqualityComparer) constructor to initialize a Dictionary with case-insensitive sorted content from another dictionary. The code example creates a SortedDictionary with a case-insensitive comparer and populates it with data in random order, then passes the SortedDictionary to the Dictionary(Generic IDictionary,Generic IEqualityComparer) constructor, along with a case-insensitive equality comparer, creating a Dictionary that is sorted. This is useful if you need to build a sorted dictionary that at some point becomes static; copying the data from a SortedDictionary to a Dictionary improves retrieval speed.


When you create a new dictionary with a case-insensitive comparer and populate it with entries from a dictionary that uses a case-sensitive comparer, as in this example, an exception occurs if the input dictionary has keys that differ only by case.

using System;
using System.Collections.Generic;

public class Example
    public static void Main()
        // Create a new sorted dictionary of strings, with string 
        // keys and a case-insensitive comparer.
        SortedDictionary<string, string> openWith = 
                new SortedDictionary<string, string>(
        // 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");
        // Create a Dictionary of strings with string keys and a
        // case-insensitive equality comparer, and initialize it
        // with the contents of the sorted dictionary.
        Dictionary<string, string> copy = 
                new Dictionary<string, string>(openWith, 

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

/* This code example produces the following output:

Key = Bmp, Value = paint.exe
Key = DIB, Value = paint.exe
Key = rtf, Value = wordpad.exe
Key = txt, Value = notepad.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

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