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ListDictionary Class

Implements IDictionary using a singly linked list. Recommended for collections that typically contain 10 items or less.

Namespace: System.Collections.Specialized
Assembly: System (in system.dll)

[SerializableAttribute] 
public ref class ListDictionary : IDictionary, ICollection, IEnumerable
/** @attribute SerializableAttribute() */ 
public class ListDictionary implements IDictionary, ICollection, 
	IEnumerable
SerializableAttribute 
public class ListDictionary implements IDictionary, ICollection, 
	IEnumerable
Not applicable.

This is a simple implementation of IDictionary using a singly linked list. It is smaller and faster than a Hashtable if the number of elements is 10 or less. This should not be used if performance is important for large numbers of elements.

Items in a ListDictionary are not in any guaranteed order; code should not depend on the current order. The ListDictionary is implemented for fast keyed retrieval; the actual internal order of items is implementation-dependent and could change in future versions of the product.

Members, such as Item, Add, Remove, and Contains are O(n) operations, where n is Count.

A key cannot be a null reference (Nothing in Visual Basic), but a value can.

The foreach statement of the C# language (for each in Visual Basic) requires the type of each element in the collection. Since each element of the ListDictionary is a key/value pair, the element type is not the type of the key or the type of the value. Instead, the element type is DictionaryEntry. For example:

No code example is currently available or this language may not be supported.

The foreach statement is a wrapper around the enumerator, which only allows reading from, not writing to, the collection.

The following code example demonstrates several of the properties and methods of ListDictionary.

#using <System.dll>

using namespace System;
using namespace System::Collections;
using namespace System::Collections::Specialized;

void PrintKeysAndValues1( IDictionary^ myCol );
void PrintKeysAndValues2( IDictionary^ myCol );
void PrintKeysAndValues3( ListDictionary^ myCol );

int main()
{
   // Creates and initializes a new ListDictionary.
   ListDictionary^ myCol = gcnew ListDictionary;
   myCol->Add( "Braeburn Apples", "1.49" );
   myCol->Add( "Fuji Apples", "1.29" );
   myCol->Add( "Gala Apples", "1.49" );
   myCol->Add( "Golden Delicious Apples", "1.29" );
   myCol->Add( "Granny Smith Apples", "0.89" );
   myCol->Add( "Red Delicious Apples", "0.99" );

   // Display the contents of the collection using for each. This is the preferred method.
   Console::WriteLine( "Displays the elements using for each:" );
   PrintKeysAndValues1( myCol );

   // Display the contents of the collection using the enumerator.
   Console::WriteLine( "Displays the elements using the IDictionaryEnumerator:" );
   PrintKeysAndValues2( myCol );

   // Display the contents of the collection using the Keys, Values, Count, and Item properties.
   Console::WriteLine( "Displays the elements using the Keys, Values, Count, and Item properties:" );
   PrintKeysAndValues3( myCol );

   // Copies the ListDictionary to an array with DictionaryEntry elements.
   array<DictionaryEntry>^myArr = gcnew array<DictionaryEntry>(myCol->Count);
   myCol->CopyTo( myArr, 0 );

   // Displays the values in the array.
   Console::WriteLine( "Displays the elements in the array:" );
   Console::WriteLine( "   KEY                       VALUE" );
   for ( int i = 0; i < myArr->Length; i++ )
      Console::WriteLine( "   {0,-25} {1}", myArr[ i ].Key, myArr[ i ].Value );
   Console::WriteLine();

   // Searches for a key.
   if ( myCol->Contains( "Kiwis" ) )
      Console::WriteLine( "The collection contains the key \"Kiwis\"." );
   else
      Console::WriteLine( "The collection does not contain the key \"Kiwis\"." );

   Console::WriteLine();

   // Deletes a key.
   myCol->Remove( "Plums" );
   Console::WriteLine( "The collection contains the following elements after removing \"Plums\":" );
   PrintKeysAndValues2( myCol );

   // Clears the entire collection.
   myCol->Clear();
   Console::WriteLine( "The collection contains the following elements after it is cleared:" );
   PrintKeysAndValues2( myCol );
}

// Uses the for each statement which hides the complexity of the enumerator.
// NOTE: The for each statement is the preferred way of enumerating the contents of a collection.
void PrintKeysAndValues1( IDictionary^ myCol )  {
   Console::WriteLine( "   KEY                       VALUE" );
   for each ( DictionaryEntry^ de in myCol )
      Console::WriteLine( "   {0,-25} {1}", de->Key, de->Value );
   Console::WriteLine();
}

// Uses the enumerator. 
void PrintKeysAndValues2( IDictionary^ myCol )
{
   IDictionaryEnumerator^ myEnumerator = myCol->GetEnumerator();
   Console::WriteLine( "   KEY                       VALUE" );
   while ( myEnumerator->MoveNext() )
      Console::WriteLine( "   {0,-25} {1}", myEnumerator->Key, myEnumerator->Value );
   Console::WriteLine();
}

// Uses the Keys, Values, Count, and Item properties.
void PrintKeysAndValues3( ListDictionary^ myCol )
{
   array<String^>^myKeys = gcnew array<String^>(myCol->Count);
   myCol->Keys->CopyTo( myKeys, 0 );
   Console::WriteLine( "   INDEX KEY                       VALUE" );
   for ( int i = 0; i < myCol->Count; i++ )
      Console::WriteLine( "   {0,-5} {1,-25} {2}", i, myKeys[ i ], myCol[ myKeys[ i ] ] );
   Console::WriteLine();
}

/*
This code produces the following output.

Displays the elements using for each:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

Displays the elements using the IDictionaryEnumerator:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

Displays the elements using the Keys, Values, Count, and Item properties:
   INDEX KEY                       VALUE
   0     Braeburn Apples           1.49
   1     Fuji Apples               1.29
   2     Gala Apples               1.49
   3     Golden Delicious Apples   1.29
   4     Granny Smith Apples       0.89
   5     Red Delicious Apples      0.99

Displays the elements in the array:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

The collection does not contain the key "Kiwis".

The collection contains the following elements after removing "Plums":
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

The collection contains the following elements after it is cleared:
   KEY                       VALUE


*/

import System.*;
import System.Collections.*;
import System.Collections.Specialized.*;

public class SamplesListDictionary
{
    public static void main(String[] args)
    {
        // Creates and initializes a new ListDictionary.
        ListDictionary myCol =  new ListDictionary();
        myCol.Add("Braeburn Apples", "1.49");
        myCol.Add("Fuji Apples", "1.29");
        myCol.Add("Gala Apples", "1.49");
        myCol.Add("Golden Delicious Apples", "1.29");
        myCol.Add("Granny Smith Apples", "0.89");
        myCol.Add("Red Delicious Apples", "0.99");

        // Display the contents of the collection using for. This is the
        // preferred method.
        Console.WriteLine("Displays the elements using for:");
        PrintKeysAndValues1(myCol);
          
        // Display the contents of the collection using the enumerator.
        Console.WriteLine("Displays the elements using the"
            + " IDictionaryEnumerator:");
        PrintKeysAndValues2(myCol);
          
        // Display the contents of the collection using the Keys, Values,
        // Count, and Item properties.
        Console.WriteLine("Displays the elements using the Keys, Values,"
            + " Count, and Item properties:");
        PrintKeysAndValues3(myCol);
          
        // Copies the ListDictionary to an array with DictionaryEntry elements.
        DictionaryEntry myArr[] = new DictionaryEntry[myCol.get_Count()];
        myCol.CopyTo(myArr, 0);
          
        // Displays the values in the array.
        Console.WriteLine("Displays the elements in the array:");
        Console.WriteLine("   KEY                       VALUE");
        for(int i = 0; i < myArr.length; i++) {
            Console.WriteLine("   {0,-25} {1}", myArr[i].get_Key(), 
                myArr[i].get_Value());
        } 
        Console.WriteLine();
          
        // Searches for a key.
        if (myCol.Contains("Kiwis")) {
            Console.WriteLine("The collection contains the key \"Kiwis\".");
        }
        else {
            Console.WriteLine("The collection does not contain the key"
                + " \"Kiwis\".");
        }
        Console.WriteLine();
          
        // Deletes a key.
        myCol.Remove("Plums");
        Console.WriteLine("The collection contains the following elements"
            + " after removing \"Plums\":");
        PrintKeysAndValues1(myCol);
          
        // Clears the entire collection.
        myCol.Clear();
        Console.WriteLine("The collection contains the following elements"
            + " after it is cleared:");
        PrintKeysAndValues1(myCol);
    } //main
       
    // Uses the for statement which hides the complexity of the enumerator.
    // NOTE: The for statement is the preferred way of enumerating the
    // contents of a collection.
    public static void PrintKeysAndValues1(IDictionary myCol) 
    {
        String strKeys[] = new String[myCol.get_Count()];
        myCol.get_Keys().CopyTo(strKeys,0);
        Console.WriteLine("   KEY                       VALUE");
        for (int iCtr = 0; iCtr < myCol.get_Count(); iCtr++) {
            Console.WriteLine("   {0,-25} {1}", strKeys[iCtr],
                myCol.get_Item(strKeys[iCtr]));
        }
        Console.WriteLine();
    } //PrintKeysAndValues1
      
    // Uses the enumerator. 
    // NOTE: The for statement is the preferred way of enumerating the
    // contents of a collection.
    public static void PrintKeysAndValues2(IDictionary myCol) 
    {
        IDictionaryEnumerator myEnumerator = myCol.GetEnumerator();
        Console.WriteLine("   KEY                       VALUE");
        while(myEnumerator.MoveNext()) {
            Console.WriteLine("   {0,-25} {1}", myEnumerator.get_Key(),
            myEnumerator.get_Value());
        }
        Console.WriteLine();
    } //PrintKeysAndValues2
   
    // Uses the Keys, Values, Count, and Item properties.
    public static void PrintKeysAndValues3(ListDictionary myCol) 
    {
        String myKeys[] = new String[myCol.get_Count()];
        myCol.get_Keys().CopyTo(myKeys, 0);
      
        Console.WriteLine("   INDEX KEY                       VALUE");
        for(int i=0; i < myCol.get_Count(); i++) {
            System.Console.WriteLine("   {0,-5} {1,-25} {2}",(Int32)i, 
                myKeys[i],myCol.get_Item(myKeys[i]));
        } 
        Console.WriteLine();
    } //PrintKeysAndValues3
} //SamplesListDictionary
 
/*
Displays the elements using for:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

Displays the elements using the IDictionaryEnumerator:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

Displays the elements using the Keys, Values, Count, and Item properties:
   INDEX KEY                       VALUE
   0     Braeburn Apples           1.49
   1     Fuji Apples               1.29
   2     Gala Apples               1.49
   3     Golden Delicious Apples   1.29
   4     Granny Smith Apples       0.89
   5     Red Delicious Apples      0.99

Displays the elements in the array:
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

The collection does not contain the key "Kiwis".

The collection contains the following elements after removing "Plums":
   KEY                       VALUE
   Braeburn Apples           1.49
   Fuji Apples               1.29
   Gala Apples               1.49
   Golden Delicious Apples   1.29
   Granny Smith Apples       0.89
   Red Delicious Apples      0.99

The collection contains the following elements after it is cleared:
   KEY                       VALUE

*/

System.Object
  System.Collections.Specialized.ListDictionary

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

This implementation does not provide a synchronized (thread safe) wrapper for a ListDictionary, but derived classes can create their own synchronized versions of the ListDictionary using the SyncRoot property.

Enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0

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