Dictionary(TKey, TValue) Constructor (IDictionary(TKey, TValue), IEqualityComparer(TKey))

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


Initializes a new instance of the Dictionary<TKey, TValue> class that contains elements copied from the specified IDictionary<TKey, TValue> and uses the specified IEqualityComparer<T>.

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

public Dictionary(
	IDictionary<TKey, TValue> dictionary,
	IEqualityComparer<TKey> comparer


Type: System.Collections.Generic.IDictionary<TKey, TValue>

The IDictionary<TKey, TValue> whose elements are copied to the new Dictionary<TKey, TValue>.

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.

Exception Condition

dictionary is null.


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<TKey, TValue> 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<TKey, TValue> is large enough to contain all the elements in dictionary.

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(n) operation, where n is the number of elements in dictionary.

The following code example shows how to use the Dictionary<TKey, TValue>(IDictionary<TKey, TValue>, IEqualityComparer<TKey>) constructor to initialize a Dictionary<TKey, TValue> with case-insensitive sorted content from another dictionary. The code example creates a SortedDictionary<TKey, TValue> with a case-insensitive comparer and populates it with data in random order, then passes the SortedDictionary<TKey, TValue> to the Dictionary<TKey, TValue>(IDictionary<TKey, TValue>, IEqualityComparer<TKey>) constructor, along with a case-insensitive equality comparer, creating a Dictionary<TKey, TValue> 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<TKey, TValue> to a Dictionary<TKey, TValue> 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

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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