More About Generics - 4.8 Making Read-Only Collections the Generic Way
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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()
{
// pick the winning numbers
_numbers = new List<int>(5) { 17, 21, 32, 44, 58 };
}
public ReadOnlyCollection<int> Results
{
// return a wrapped copy of the results
get { return new ReadOnlyCollection<int>(_numbers); }
}
}
Lotteryhas aList<int>of winning numbers that it fills in the constructor. The interesting part is that it also exposes a property calledResults, which returns aReadOnlyCollectiontyped as<int>for seeing the winning numbers. Internally, a newReadOnlyCollection wrapper is created to hold theList<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 = {0, 1, 2 };
ReadOnlyCollection<int> readOnlyItems =
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
The “ReadOnlyCollection” topic in the MSDN documentation.
Please check back next week for the conclusion of this article.
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 This article is excerpted from chapter four of the C# 3.0 Cookbook, Third Edition, written by Jay Hilyard and Stephen Teilhet (O'Reilly, 2008; ISBN: 059651610X). Check it out today at your favorite bookstore. Buy this book now.
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