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

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

Dictionary<(Of <(TKey, TValue>)>) requires an equality implementation to determine whether keys are equal. This constructor uses the default generic equality comparer, EqualityComparer<(Of <(T>)>)..::.Default. If type TKey implements the System..::.IEquatable<(Of <(T>)>) generic interface, the default equality comparer uses that implementation. Alternatively, you can specify an implementation of the IEqualityComparer<(Of <(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.

Imports System
Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main() 

        ' Create a new dictionary of strings, with string keys and
        ' an initial capacity of 4.
        Dim openWith As New Dictionary(Of 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.
        Console.WriteLine()
        For Each kvp As KeyValuePair(Of String, String) In openWith
            Console.WriteLine("Key = {0}, Value = {1}", _
                kvp.Key, kvp.Value)
        Next kvp

    End Sub

End Class

' 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

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

.NET Framework

.NET Compact Framework

XNA Framework

Reference