Making Read-Only Collections the Generic Way

Making Read-Only Collections the Generic Way

Problem

You have a collection of information that you want to expose from your class, but you don't want any users modifying the collection.

Solution

Use the ReadOnlyCollection<T> wrapper to easily support collection classes that cannot be modified. For example, a Lottery class that contained the winning lottery numbers should make the winning numbers accessible, but not allow them to be changed:

	public class Lottery
	{
	    // Make a list.
	    List<int> _numbers = null;
	    
	    public Lottery()
	    {
	        // Make the internal list
	        _numbers = new List<int>(5);
	        // Add values
	        _numbers.Add(17);
	        _numbers.Add(21);
	        _numbers.Add(32);
	        _numbers.Add(44);
	        _numbers.Add(58);
	    }

	    public ReadOnlyCollection<int> Results
	    {
	        // Return a wrapped copy of the results.
	        get { return new ReadOnlyCollection<int>(_numbers); }
	    }
	}

Lottery has an internal List<int> of winning numbers that it fills in the constructor. The interesting part is that it also exposes a property called Results, which returns a ReadOnlyCollection typed as <int> for seeing the winning numbers. Internally, a new ReadOnlyCollection wrapper is created to hold the List<int> that has the numbers in it, and then this instance is returned for use by the user.

If users then attempt to set a value on the collection, they get a compile error:

	Lottery tryYourLuck = new Lottery();
	// Print out the results.
	for (int i = 0; i < tryYourLuck.Results.Count; i++)
	{
	    Console.WriteLine("Lottery Number " + i + " is " + tryYourLuck.Results[i]); 
	}

	// Change it so we win!
	tryYourLuck.Results[0]=29;

	//The above line gives // Error 26 // Property or indexer
	// 'System.Collections.ObjectModel.ReadOnlyCollection<int>.this[int]'
	// cannot be assigned to -- it is read only

Discussion

The main advantage ReadOnlyCollection provides is the flexibility to use it with any collection that supports IList or IList<T> as an interface. ReadOnlyCollection can be used to wrap a regular array like this:

	        int [] items = new int[3];
	        items[0]=0;
	        items[1]=1;
	        items[2]=2;
	            new ReadOnlyCollection<int>(items);

This provides a way to standardize the read-only properties on classes to make it easier for consumers of the class to recognize which properties are read-only simply by the return type.

See Also

See the "IList" and "Generic IList" topics in the MSDN documentation.