C# Generics Recipes—Constraining Type Arguments

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Contents 1 4.12 Constraining Type Arguments 1.1 Problem 1.2 Solution 1.3 Discussion 1.4 See Also

4.12 Constraining Type Arguments

Problem

Your generic type needs to be created with a type argument that must support the members of a particular interface such as the IDisposable interface.

Solution

Use constraints to force the type arguments of a generic type to be of a type that implements one or more particular interfaces:

public class DisposableList<T> : IList<T>
   where T : IDisposable
{
   private List<T> _items = new List<T>( );
 
   // Private method that will dispose of items in the list
   private void Delete(T item)
   {
      item.Dispose( );
   }
}

This DisposableList class allows only an object that implements IDisposable to be passed in as a type argument to this class. The reason for this is that whenever an object is removed from a DisposableList object, the Dispose method is always called on that object. This allows you to transparently handle the management of any object stored within this DisposableList object.

The following code exercises a DisposableList object:

public static void TestDisposableListCls( )
{
   DisposableList<StreamReader> dl = new DisposableList<StreamReader>( );
   // Create a few test objects.
   StreamReader tr1 = new StreamReader("c:\\boot.ini");
   StreamReader tr2 = new StreamReader("c:\\autoexec.bat");
   StreamReader tr3 = new StreamReader("c:\\config.sys");
 
   // Add the test object to the DisposableList.
   dl.Add(tr1);
   dl.Insert(0, tr2);
   dl.Add(tr3);
 
   foreach(StreamReader sr in dl)
   {
      Console.WriteLine("sr.ReadLine( ) == " + sr.ReadLine( ));
   }
 
   // Call Dispose before any of the disposable objects are
   // removed from the DisposableList.
   dl.RemoveAt(0);
   dl.Remove(tr1);
   dl.Clear( );
}

Discussion

The where keyword is used to constrain a type parameter to accept only arguments that satisfy the given constraint. For example, the DisposableList has the constraint that any type argument T must implement the IDisposable interface:

public class DisposableList<T> : IList<T>
   where T : IDisposable

This means that the following code will compile successfully:

DisposableList<StreamReader> dl = new DisposableList<StreamReader>( );

but the following code will not:

DisposableList<string> dl = new DisposableList<string>( );

This is because the string type does not implement the IDisposable interface, and the StreamReader type does.

Other constraints on the type argument are allowed, in addition to requiring one or more specific interfaces to be implemented. You can force a type argument to be inherited from a specific base class, such as the TextReader class:

public class DisposableList<T> : IList<T>
   where T : System.IO.TextReader, IDisposable

You can also determine if the type argument is narrowed down to only value types or only reference types. The following class declaration is constrained to using only value types:

public class DisposableList<T> : IList<T>
   where T : struct

This class declaration is constrained to only reference types:

public class DisposableList<T> : IList<T>
   where T : class

In addition, you can also require any type argument to implement a public default constructor:

public class DisposableList<T> : IList<T>
   where T : IDisposable, new( )

Using constraints allows you to write generic types that accept a narrower set of available type arguments. If the IDisposable constraint is omitted in the solution for this recipe, a compile-time error will occur. This is because not all of the types that can be used as the type argument for the DisposableList class will implement the IDisposable interface. If you skip this compile-time check, a DisposableList object may contain objects that do not have a public no-argument Dispose method. In this case, a runtime exception will occur. Generics and constraints in particular force strict type checking of the class-type arguments and allow you to catch these problems at compile time rather than at runtime.

See Also

See the "where Keyword" topic in the MSDN documentation.