.NET Tip: Creating a Read-Only List

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Have you ever had the need to expose a read-only list? I’m not just talking about a List<T> property that could not be set directly, but one that could not have any elements added, removed, or modified? There is a simple way to do this by using the AsReadOnly() method of a List<T>. Here is an example class that uses a List<string> to store the names of the authors of Design Patterns.

public class Patterns
{
   List<string><string> _GangOfFourAuthors =
      new List<string> { "Gamma", "Helm", "Johnson", "Vlissides" };

   public IList<string><string> GangOfFourAuthors
   {
      get { return _GangOfFourAuthors.AsReadOnly(); }
   }

   public void AddMe()
   {
      _GangOfFourAuthors.Add("Miller");
   }
}

A List<string> is created and initialized with the names of the authors. The GangOfFourAuthors property returns a read-only IList<string>. Look at a quick example of using the class and property to print the names of the authors to the Output window.

Patterns p = new Patterns();
IList<string> Authors = p.GangOfFourAuthors;

foreach (string a in Authors)
    Debug.WriteLine(a);

Output:
Gamma
Helm
Johnson
Vlissides

This code creates a new Patterns object, retrieves the list of authors, and then prints them to the Output window. You can see that the output matches the initialization of the List<string> in the Patterns class. At this point, elements of Authors cannot be added, removed, or modified. The following code would throw an error:

Authors.Add("Miller"); 

Authors cannot be changed in any way without causing an error. The original list, however, can be modified and those changes will be reflected in the read-only version of the list. If you look at the AddMe() method in the Patterns class above, you will see that it adds an element to the internal List<string> of authors. Here is code that calls that method and then outputs the list of authors.

Patterns p = new Patterns();
IList<string> Authors = p.GangOfFourAuthors;

p.AddMe();

foreach (string a in Authors)
   Debug.WriteLine(a);

Output:
Gamma
Helm
Johnson
Vlissides
Miller

The AsReadOnly() method provides an easy way to restrict access to your classes’ data. The method is available for List<T> as well as arrays.

About the Author

Jay Miller is a Software Engineer with Electronic Tracking Systems, a company dedicated to robbery prevention, apprehension, and recovery based in Carrollton, Texas. Jay has been working with .NET since the release of the first beta and is co-author of Learn Microsoft Visual Basic.Net In a Weekend. Jay can be reached via email at jmiller@sm-ets.com.