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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)

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 nullptr.


IDictionary::Add(Object, Object)


key is nullptr.


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 nullptr, 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 namespace System;
using namespace System::Collections;
void PrintKeysAndValues( Hashtable^ myHT );
int main()

   // Creates and initializes a new Hashtable.
   Hashtable^ myHT = gcnew 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 );

void PrintKeysAndValues( Hashtable^ myHT )
   Console::WriteLine( "\t-KEY-\t-VALUE-" );
   IEnumerator^ myEnum = myHT->GetEnumerator();
   while ( myEnum->MoveNext() )
      DictionaryEntry de = *safe_cast<DictionaryEntry ^>(myEnum->Current);
      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

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

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

.NET Framework

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

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0