Recipe 4.9. Making Read-Only Collections the Generic WayProblemYou have a collection of information that you want to expose from your class, but you don't want any users modifying the collection. SolutionUse 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 DiscussionThe 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 AlsoSee the "IList" and "Generic IList" topics in the MSDN documentation. |