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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 virtual void Add(
	Object key,
	Object value


Type: System.Object
The key of the element to add.
Type: System.Object
The value of the element to add. The value can be null.


IDictionary.Add(Object, Object)


key is null.


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 null, 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.

using System;
using System.Collections;
public class SamplesHashtable  {

   public static void Main()  {

      // Creates and initializes a new Hashtable.
      Hashtable myHT = 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:" );
      PrintKeysAndValues( myHT );

   public static void PrintKeysAndValues( Hashtable myHT )  {
      Console.WriteLine( "\t-KEY-\t-VALUE-" );
      foreach ( DictionaryEntry de in myHT )
         Console.WriteLine( "\t{0}:\t{1}", de.Key, de.Value );
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.