C# Generics Recipes—Making Read-Only Collections the Generic Way

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C# Generics Recipes When and Where to Use Understand Generic Types Getting the Type Replacing the ArrayList Replace Stack, Queue Implement a Linked List Value Type to be Inited Reverse Sorted List Read-Only Collections Replace the Hashtable Using foreach Constrain Type Arguments Initialize Generic Variables

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Contents 1 4.9 Making Read-Only Collections the Generic Way 1.1 Problem 1.2 Solution 1.3 Discussion 1.4 See Also

4.9 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.