.NET Tip: Creating a Read-Only List

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.



Comments

  • There are no comments yet. Be the first to comment!

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Java developers know that testing code changes can be a huge pain, and waiting for an application to redeploy after a code fix can take an eternity. Wouldn't it be great if you could see your code changes immediately, fine-tune, debug, explore and deploy code without waiting for ages? In this white paper, find out how that's possible with a Java plugin that drastically changes the way you develop, test and run Java applications. Discover the advantages of this plugin, and the changes you can expect to see …

  • If you need new tools and tricks to make your meetings profitable and productive, then 5 Tips in 5 Minutes: A Quick Guide for More Profitable Sales Meetings is for you. Timely, practical tips that you can incorporate in just seconds will save you literally hours in travel and meeting time, not to mention help you to focus on what your sales prospects really want to know and how you can meet their needs. Get 5in5: A Quick Guide for More Profitable Sales Meetings and start building your sales the smarter, faster …

Most Popular Programming Stories

More for Developers

Latest Developer Headlines

RSS Feeds