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Adding Type Constraints

The final element to discuss is constraints. Constraints are placed on classes or features and use the following syntax:

Where T : constraint_type

For example, anything we'd like to use in a using statement, such as a SqlDataReader, must implement IDisposable. This is because a using statement like this:

using(Form f = new Form()){...}

works just like a try..finally block by always cleaning up the created resource. This works quite simply by the CLR emitting a call to IDisposable.Dispose for the object created in the using statement. For instance, in the fragment a new Form is created, and before the using statement exits Form.Dispose is called.

To place a constraint on a generic class that ensures that the class implements IDisposable, we would add the predicate where T : IDisposable. Applied to our generic list in Listing 4, we would revise Listing 4 as shown in Listing 5.

Listing 5 Adding a constraint to the generic class to ensure that all values for T for our List<T> implement IDisposable.

using System;
using System.Collections;
using System.Text;

namespace Generics
    public class List<T> : CollectionBase where t : IDisposable
        public List(){ }

        public T this[int index]
            get { return (T)List[index]; }
            set { List[index] = value; }

        public int Add(T value)
            return List.Add(value);

Values for the where predicate can be classes, interfaces, structures, classes that implement a parameter-less public constructor, or classes that have a specific base class. Check out the help documentation for specifics.

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