Dictionary<TKey, TValue> Constructor

July 28, 2014

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

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

public Dictionary()

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

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.

NoteNote:

If you can estimate the size of the collection, using a constructor that specifies the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Dictionary<TKey, TValue>.

This constructor is an O(1) operation.

The following code example creates an empty Dictionary<TKey, TValue> of strings with string keys and uses the Add method to add some elements. The example demonstrates that the Add method throws an ArgumentException when attempting to add a duplicate key.

This code example is part of a larger example provided for the Dictionary<TKey, TValue> class.


// Create a new dictionary of strings, with string keys.
//
Dictionary<string, string> openWith =
    new Dictionary<string, string>();

// Add some elements to the dictionary. There are no 
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is 
// already in the dictionary.
try
{
   openWith.Add("txt", "winword.exe");
}
catch (ArgumentException)
{
   outputBlock.Text += "An element with Key = \"txt\" already exists." + "\n";
}


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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