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A getter for dictionary elements in C# 4.0
public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    public new TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (!TryGetValue(key, out value))
            {
                value = Activator.CreateInstance<TValue>();
                Add(key, value);
            }
            return value;
        }
        set { base[key] = value; }
    }
}

public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    readonly Func<TKey, TValue> _activator;

    public MyDictionary(Func<TKey, TValue> activator)
    {
        _activator = activator;
    }

    public new TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (!TryGetValue(key, out value))
            {
                value = _activator(key);
                Add(key, value);
            }
            return value;
        }
        set { base[key] = value; }
    }
}

static void Main(string[] args)
{
    var dict = new MyDictionary<int, string>(x => string.Format("Automatically created Value for key {0}", x));
    dict[1] = "Value for key 1";
    for (int i = 0; i < 3; i++)
    {
        Console.WriteLine(dict[i]);
    }
    Console.Read();
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DictionaryElementsGetter_Test {

  // class inherits from Dictionary<int, string>
  public class MyDictionary : Dictionary<int, string> {
    // new: hide element of base class to redefine it in derived class.
    //      see http://msdn.microsoft.com/en-us/library/435f1dw2.aspx
    // string this[int key]: create an indexer
    //                       (actually: replace the indexer of base class, because of "new")
    //                       see http://msdn.microsoft.com/en-us/library/2549tw02.aspx
    new public string this[int key] {
      get {
        string value;
        // out: pass argument by reference instead of by value
        //      This is the standard definition of TryGetValue.
        //      Beside the bool result that indicates the existence of the key-value-pair,
        //      TryGetValue also returns the value itself into this reference parameter (if key is found).
        //      see http://msdn.microsoft.com/en-us/library/ee332485.aspx
        if( !TryGetValue( key, out value ) ) {
          value = "abc" + key + "def";
          Add( key, value );
          // just to see when the getter really did an Add():
          Console.Write( "ADDED!... " );
        }
        return value;
      }
      // base: access element of the base class Dictionary<int, string>
      //       see http://msdn.microsoft.com/en-us/library/hfw7t1ce(v=vs.100).aspx
      set { base[key] = value; }
    }
  }


  class Program {
    static void Main( string[] args ) {

      var dict = new MyDictionary();
      dict[1] = "EXTERNAL VALUE";

      for( int i = 0; i < 3; i++ ) {
        Console.WriteLine( i + ": " + dict[i] );
      }
      /*
      Output:
        ADDED!... 0: abc0def
        1: EXTERNAL VALUE
        ADDED!... 2: abc2def
      */
      for( int i = 0; i < 3; i++ ) {
        Console.WriteLine( i + ": " + dict[i] );
      }
      /*
      Output:
        0: abc0def
        1: EXTERNAL VALUE
        2: abc2def
      */

      Console.ReadKey();

    }
  }
}

Dictionary<int, string> _myDictionary = new Dictionary<int, string>();

_myDictionary[1] = "Hello";
_myDictionary[2] = "World!";