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Dictionary<TKey, TValue>.IDictionary.Add Method

Adds the specified key and value to the dictionary.

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

void IDictionary.Add(
	Object key,
	Object value


Type: System.Object

The object to use as the key.

Type: System.Object

The object to use as the value.


IDictionary.Add(Object, Object)


key is null.


key is of a type that is not assignable to the key type TKey of the Dictionary<TKey, TValue>.


value is of a type that is not assignable to TValue, the type of values in the Dictionary<TKey, TValue>.


A value with the same key already exists in the Dictionary<TKey, TValue>.

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

If Count is less than the capacity, this method approaches 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 code example shows how to access the Dictionary<TKey, TValue> class through the System.Collections.IDictionary interface. The code example creates an empty Dictionary<TKey, TValue> of strings with string keys and uses the IDictionary.Add method to add some elements. The example demonstrates that the IDictionary.Add method throws an ArgumentException when attempting to add a duplicate key, or when a key or value of the wrong data type is supplied.

The code example demonstrates the use of several other members of the System.Collections.IDictionary interface.

using System;
using System.Collections;
using System.Collections.Generic;

public class Example
    public static void Main()
        // Create a new dictionary of strings, with string keys, 
        // and access it using the IDictionary interface. 
        IDictionary openWith = new Dictionary<string, string>();

        // Add some elements to the dictionary. There are no  
        // duplicate keys, but some of the values are duplicates. 
        // IDictionary.Add throws an exception if incorrect types 
        // are supplied for key or value.
        openWith.Add("txt", "notepad.exe");
        openWith.Add("bmp", "paint.exe");
        openWith.Add("dib", "paint.exe");
        openWith.Add("rtf", "wordpad.exe");
            openWith.Add(42, new Example());
        catch (ArgumentException ex)
            Console.WriteLine("An exception was caught for " + 
                "IDictionary.Add. Exception message:\n\t{0}\n",

        // The Add method throws an exception if the new key is  
        // already in the dictionary. 
            openWith.Add("txt", "winword.exe");
        catch (ArgumentException)
            Console.WriteLine("An element with Key = \"txt\" already exists.");

        // The Item property is another name for the indexer, so you  
        // can omit its name when accessing elements. 
        Console.WriteLine("For key = \"rtf\", value = {0}.", 

        // The indexer can be used to change the value associated 
        // with a key.
        openWith["rtf"] = "winword.exe";
        Console.WriteLine("For key = \"rtf\", value = {0}.", 

        // If a key does not exist, setting the indexer for that key 
        // adds a new key/value pair.
        openWith["doc"] = "winword.exe";

        // The indexer returns null if the key is of the wrong data  
        // type.
        Console.WriteLine("The indexer returns null" 
            + " if the key is of the wrong type:");
        Console.WriteLine("For key = 2, value = {0}.", 

        // The indexer throws an exception when setting a value 
        // if the key is of the wrong data type. 
            openWith[2] = "This does not get added.";
        catch (ArgumentException)
            Console.WriteLine("A key of the wrong type was specified" 
                + " when assigning to the indexer.");

        // Unlike the default Item property on the Dictionary class 
        // itself, IDictionary.Item does not throw an exception 
        // if the requested key is not in the dictionary.
        Console.WriteLine("For key = \"tif\", value = {0}.", 

        // Contains can be used to test keys before inserting  
        // them. 
        if (!openWith.Contains("ht"))
            openWith.Add("ht", "hypertrm.exe");
            Console.WriteLine("Value added for key = \"ht\": {0}", 

        // IDictionary.Contains returns false if the wrong data 
        // type is supplied.
        Console.WriteLine("openWith.Contains(29.7) returns {0}",

        // When you use foreach to enumerate dictionary elements 
        // with the IDictionary interface, the elements are retrieved 
        // as DictionaryEntry objects instead of KeyValuePair objects.
        foreach( DictionaryEntry de in openWith )
            Console.WriteLine("Key = {0}, Value = {1}", 
                de.Key, de.Value);

        // To get the values alone, use the Values property.
        ICollection icoll = openWith.Values;

        // The elements of the collection are strongly typed 
        // with the type that was specified for dictionary values, 
        // even though the ICollection interface is not strongly 
        // typed.
        foreach( string s in icoll )
            Console.WriteLine("Value = {0}", s);

        // To get the keys alone, use the Keys property.
        icoll = openWith.Keys;

        // The elements of the collection are strongly typed 
        // with the type that was specified for dictionary keys, 
        // even though the ICollection interface is not strongly 
        // typed.
        foreach( string s in icoll )
            Console.WriteLine("Key = {0}", s);

        // Use the Remove method to remove a key/value pair. No 
        // exception is thrown if the wrong data type is supplied.

        if (!openWith.Contains("dib"))
            Console.WriteLine("Key \"dib\" is not found.");

/* This code example produces the following output:

An exception was caught for IDictionary.Add. Exception message:
        The value "42" is not of type "System.String" and cannot be used in this generic collection.
Parameter name: key

An element with Key = "txt" already exists.
For key = "rtf", value = wordpad.exe.
For key = "rtf", value = winword.exe.
The indexer returns null if the key is of the wrong type:
For key = 2, value = .
A key of the wrong type was specified when assigning to the indexer.
For key = "tif", value = .
Value added for key = "ht": hypertrm.exe
openWith.Contains(29.7) returns False

Key = txt, Value = notepad.exe
Key = bmp, Value = paint.exe
Key = dib, Value = paint.exe
Key = rtf, Value = winword.exe
Key = doc, Value = winword.exe
Key = ht, Value = hypertrm.exe

Value = notepad.exe
Value = paint.exe
Value = paint.exe
Value = winword.exe
Value = winword.exe
Value = hypertrm.exe

Key = txt
Key = bmp
Key = dib
Key = rtf
Key = doc
Key = ht

Key "dib" is not found.

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

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0