ReadOnlyDictionary in .net 4.5

Oh yea, .net 4.5 is out (although it’s still Developer Preview).

Some time back, I blogged about a ReadOnlyCollection and .net 4.5 has introduced a similar concept with a Dictionary object – a ReadOnlyDictionary type in the System.Collections.ObjectModel namespace. (same place where ReadOnlyCollection typs is declared)

   1: // using System.Collections.ObjectModel;

   2: static void Main()

   3: {

   4:     IDictionary<string, int> myDictionary = new Dictionary<string, int>();

   5:     myDictionary.Add("a", 1);

   6:     myDictionary.Add("b", 2);

   7:     myDictionary.Add("c", 3);

   8:     myDictionary.Add("d", 4);

   9:  

  10:     ReadOnlyDictionary<string, int> readOnlyDictionary = new ReadOnlyDictionary<string, int>(myDictionary);

  11:  

  12:     foreach (KeyValuePair<string, int> item in readOnlyDictionary)

  13:     {

  14:         Console.WriteLine("{0} - {1}", item.Key, item.Value);

  15:     }

  16: }

That’s how a ReadOnlyDictionary is instantiated. As expected the output of the above is:

As you see in the image on the right, there are (at least) three methods “missing” from the ReadOnlyDictionary instance – the Add(), Clear() and Remove() methods. Of course, this is a read-only dictionary so doesn’t make sense implementing them on this type.

But what if I cast this back to an IDictionary instance and try adding items to it?

   1: IDictionary<string, int> castReadOnlyDictionary = (IDictionary<string, int>)readOnlyDictionary;

   2:  

   3: castReadOnlyDictionary.Add("e", 5);

The compiler does allow execution of line 1, but when it tries to run the command in line 3, you get a NotSupportedException exception thrown:

Also in the Watch window towards the bottom of the image, you’ll see a IsReadOnly property showing true. So to make sure I can add items to a dictionary, I need to check the IsReadOnly property first.