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Hashtable.Add Method

Adds an element with the specified key and value into the Hashtable.

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

Public Overridable Sub Add ( _
	key As Object, _
	value As Object _


Type: System.Object
The key of the element to add.
Type: System.Object
The value of the element to add. The value can be Nothing.


IDictionary.Add(Object, Object)


key is Nothing.


An element with the same key already exists in the Hashtable.


The Hashtable is read-only.


The Hashtable has a fixed size.

A key cannot be Nothing, but a value can be.

An object that has no correlation between its state and its hash code value should typically not be used as the key. For example, String objects are better than StringBuilder objects for use as keys.

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Hashtable; for example, myCollection["myNonexistentKey"] = myValue. However, if the specified key already exists in the Hashtable, setting the Item property overwrites the old value. In contrast, the Add method does not modify existing elements.

If Count is less than the capacity of the Hashtable, this method is an O(1) operation. If the capacity needs to be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.

The following example shows how to add elements to the Hashtable.

Imports System
Imports System.Collections

Public Class SamplesHashtable

    Public Shared Sub Main()

        ' Creates and initializes a new Hashtable.
        Dim myHT As New Hashtable()
        myHT.Add("one", "The")
        myHT.Add("two", "quick")
        myHT.Add("three", "brown")
        myHT.Add("four", "fox")

        ' Displays the Hashtable.
        Console.WriteLine("The Hashtable contains the following:")

    End Sub 'Main

    Public Shared Sub PrintKeysAndValues(myHT As Hashtable)
        Console.WriteLine(vbTab + "-KEY-" + vbTab + "-VALUE-")
        Dim de As DictionaryEntry
        For Each de In  myHT
            Console.WriteLine(vbTab + "{0}:" + vbTab + "{1}", de.Key, de.Value)
        Next de
    End Sub 'PrintKeysAndValues

End Class 'SamplesHashtable

' This code produces the following output.
' The Hashtable contains the following:
'         -KEY-   -VALUE-
'         two:    quick
'         three:  brown
'         four:   fox
'         one:    The

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.