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IDictionary Interface

Represents a nongeneric collection of key/value pairs.

Namespace:  System.Collections
Assemblies:   mscorlib (in mscorlib.dll)
  System.Runtime (in System.Runtime.dll)

[ComVisibleAttribute(true)]
public interface IDictionary : ICollection, 
	IEnumerable

The IDictionary type exposes the following members.

  NameDescription
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsCountGets the number of elements contained in the ICollection. (Inherited from ICollection.)
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIsFixedSizeGets a value indicating whether the IDictionary object has a fixed size.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIsReadOnlyGets a value indicating whether the IDictionary object is read-only.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIsSynchronizedGets a value indicating whether access to the ICollection is synchronized (thread safe). (Inherited from ICollection.)
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsItemGets or sets the element with the specified key.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsKeysGets an ICollection object containing the keys of the IDictionary object.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsSyncRootGets an object that can be used to synchronize access to the ICollection. (Inherited from ICollection.)
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsValuesGets an ICollection object containing the values in the IDictionary object.
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsAddAdds an element with the provided key and value to the IDictionary object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsClearRemoves all elements from the IDictionary object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsContainsDetermines whether the IDictionary object contains an element with the specified key.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsCopyToCopies the elements of the ICollection to an Array, starting at a particular Array index. (Inherited from ICollection.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetEnumeratorReturns an IDictionaryEnumerator object for the IDictionary object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsRemoveRemoves the element with the specified key from the IDictionary object.
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  NameDescription
Public Extension MethodSupported by Portable Class LibrarySupported in .NET for Windows Store appsAsParallel()Overloaded. Enables parallelization of a query. (Defined by ParallelEnumerable.)
Public Extension MethodSupported by Portable Class LibrarySupported in .NET for Windows Store appsAsParallel()Overloaded. (Defined by ParallelEnumerable.)
Public Extension MethodSupported by Portable Class LibrarySupported in .NET for Windows Store appsAsQueryable()Overloaded. Converts an IEnumerable to an IQueryable. (Defined by Queryable.)
Public Extension MethodSupported by Portable Class LibrarySupported in .NET for Windows Store appsAsQueryable()Overloaded. (Defined by Queryable.)
Public Extension MethodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsCast<TResult>()Overloaded. Casts the elements of an IEnumerable to the specified type. (Defined by Enumerable.)
Public Extension MethodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsCast<TResult>()Overloaded. (Defined by Enumerable.)
Public Extension MethodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsOfType<TResult>()Overloaded. Filters the elements of an IEnumerable based on a specified type. (Defined by Enumerable.)
Public Extension MethodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsOfType<TResult>()Overloaded. (Defined by Enumerable.)
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The IDictionary interface is the base interface for nongeneric collections of key/value pairs. For the generic version of this interface, see System.Collections.Generic.IDictionary<TKey, TValue>.

Each element is a key/value pair stored in a DictionaryEntry object.

Each pair must have a unique key. Implementations can vary in whether they allow the key to be null. The value can be null and does not have to be unique. The IDictionary interface allows the contained keys and values to be enumerated, but it does not imply any particular sort order.

IDictionary implementations fall into three categories: read-only, fixed-size, variable-size. A read-only IDictionary object cannot be modified. A fixed-size IDictionary object does not allow the addition or removal of elements, but does allow the modification of existing elements. A variable-size IDictionary object allows the addition, removal, and modification of elements.

The foreach statement of the C# language (For Each in Visual Basic) returns an object of the type of the elements in the collection. Since each element of the IDictionary object 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:

foreach (DictionaryEntry de in myDictionary)
{
    //...
}

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

Notes to Implementers

The implementing class must have a means to compare keys.

The following code example demonstrates how to define a simple dictionary class that implements the IDictionary interface.

using System;
using System.Collections;

// This class implements a simple dictionary using an array of DictionaryEntry objects (key/value pairs). 
public class SimpleDictionary : IDictionary
{
    // The array of items 
    private DictionaryEntry[] items;
    private Int32 ItemsInUse = 0;

    // Construct the SimpleDictionary with the desired number of items. 
    // The number of items cannot change for the life time of this SimpleDictionary. 
    public SimpleDictionary(Int32 numItems)
    {
        items = new DictionaryEntry[numItems];
    }


    #region IDictionary Members
    public bool IsReadOnly { get { return false; } }
    public bool Contains(object key)
    {
       Int32 index;
       return TryGetIndexOfKey(key, out index);
    }
    public bool IsFixedSize { get { return false; } }
    public void Remove(object key)
    {
        if (key == null) throw new ArgumentNullException("key");
        // Try to find the key in the DictionaryEntry array
        Int32 index;
        if (TryGetIndexOfKey(key, out index))
        {
            // If the key is found, slide all the items up.
            Array.Copy(items, index + 1, items, index, ItemsInUse - index - 1);
            ItemsInUse--;
        } 
        else
        {
            // If the key is not in the dictionary, just return. 
        }
    }
    public void Clear() { ItemsInUse = 0; }
    public void Add(object key, object value) 
    {
        // Add the new key/value pair even if this key already exists in the dictionary. 
        if (ItemsInUse == items.Length)
            throw new InvalidOperationException("The dictionary cannot hold any more items.");
        items[ItemsInUse++] = new DictionaryEntry(key, value);
    }
    public ICollection Keys
    {
        get
        {
            // Return an array where each item is a key.
            Object[] keys = new Object[ItemsInUse];
            for (Int32 n = 0; n < ItemsInUse; n++)
                keys[n] = items[n].Key;
            return keys;
        }
    }
    public ICollection Values
    {
        get
        {
            // Return an array where each item is a value.
            Object[] values = new Object[ItemsInUse];
            for (Int32 n = 0; n < ItemsInUse; n++)
                values[n] = items[n].Value;
            return values;
        }
    }
    public object this[object key]
    {
        get
        {   
            // If this key is in the dictionary, return its value.
            Int32 index;
            if (TryGetIndexOfKey(key, out index))
            {
                // The key was found; return its value. 
                return items[index].Value;
            } 
            else
            {
                // The key was not found; return null. 
                return null;
            }
        }

        set
        {
            // If this key is in the dictionary, change its value. 
            Int32 index;
            if (TryGetIndexOfKey(key, out index))
            {
                // The key was found; change its value.
                items[index].Value = value;
            } 
            else
            {
                // This key is not in the dictionary; add this key/value pair.
                Add(key, value);
            }
        }
    }
    private Boolean TryGetIndexOfKey(Object key, out Int32 index)
    {
        for (index = 0; index < ItemsInUse; index++)
        {
            // If the key is found, return true (the index is also returned). 
            if (items[index].Key.Equals(key)) return true;
        }

        // Key not found, return false (index should be ignored by the caller). 
        return false;
    }
    private class SimpleDictionaryEnumerator : IDictionaryEnumerator
    {
        // A copy of the SimpleDictionary object's key/value pairs.
        DictionaryEntry[] items;
        Int32 index = -1;

        public SimpleDictionaryEnumerator(SimpleDictionary sd)
        {
            // Make a copy of the dictionary entries currently in the SimpleDictionary object.
            items = new DictionaryEntry[sd.Count];
            Array.Copy(sd.items, 0, items, 0, sd.Count);
        }

        // Return the current item. 
        public Object Current { get { ValidateIndex(); return items[index]; } }

        // Return the current dictionary entry. 
        public DictionaryEntry Entry
        {
            get { return (DictionaryEntry) Current; }
        }

        // Return the key of the current item. 
        public Object Key { get { ValidateIndex();  return items[index].Key; } }

        // Return the value of the current item. 
        public Object Value { get { ValidateIndex();  return items[index].Value; } }

        // Advance to the next item. 
        public Boolean MoveNext()
        {
            if (index < items.Length - 1) { index++; return true; }
            return false;
        }

        // Validate the enumeration index and throw an exception if the index is out of range. 
        private void ValidateIndex()
        {
            if (index < 0 || index >= items.Length)
            throw new InvalidOperationException("Enumerator is before or after the collection.");
        }

        // Reset the index to restart the enumeration. 
        public void Reset()
        {
            index = -1;
        }
    }
    public IDictionaryEnumerator GetEnumerator()
    {
        // Construct and return an enumerator. 
        return new SimpleDictionaryEnumerator(this);
    }
    #endregion

    #region ICollection Members
    public bool IsSynchronized { get { return false; } }
    public object SyncRoot { get { throw new NotImplementedException(); } }
    public int Count { get { return ItemsInUse; } }
    public void CopyTo(Array array, int index) { throw new NotImplementedException(); }
    #endregion

    #region IEnumerable Members
    IEnumerator IEnumerable.GetEnumerator() 
    {
        // Construct and return an enumerator. 
        return ((IDictionary)this).GetEnumerator();
    }
    #endregion
}

public sealed class App
{
    static void Main()
    {
        // Create a dictionary that contains no more than three entries.
        IDictionary d = new SimpleDictionary(3);

        // Add three people and their ages to the dictionary.
        d.Add("Jeff", 40);
        d.Add("Kristin", 34);
        d.Add("Aidan", 1);

        Console.WriteLine("Number of elements in dictionary = {0}", d.Count);

        Console.WriteLine("Does dictionary contain 'Jeff'? {0}", d.Contains("Jeff"));
        Console.WriteLine("Jeff's age is {0}", d["Jeff"]);

        // Display every entry's key and value. 
        foreach (DictionaryEntry de in d)
        {
            Console.WriteLine("{0} is {1} years old.", de.Key, de.Value);
        }

        // Remove an entry that exists.
        d.Remove("Jeff");

        // Remove an entry that does not exist, but do not throw an exception.
        d.Remove("Max");

        // Show the names (keys) of the people in the dictionary. 
        foreach (String s in d.Keys)
            Console.WriteLine(s);

        // Show the ages (values) of the people in the dictionary. 
        foreach (Int32 age in d.Values)
            Console.WriteLine(age);
    }
}

// This code produces the following output. 
// 
// Number of elements in dictionary = 3 
// Does dictionary contain 'Jeff'? True 
// Jeff's age is 40 
// Jeff is 40 years old. 
// Kristin is 34 years old. 
// Aidan is 1 years old. 
// Kristin 
// Aidan 
// 34 
// 1

.NET Framework

Supported in: 4.5.3, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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