This appendix summarizes generic classes and methods. The final section in this appendix describes items that you cannot make generic.

Generic Classes

To define a generic class, make a class declaration as usual. After the class name, add one or more type names for data types surrounded by brackets. The following code defines a generic BinaryNode class with the generic type highlighted in bold.

public class BinaryNode<T>
{
    public T Value;
    public BinaryNode<T> LeftChild, RightChild;
}

The class’s declaration includes one type parameter T. The class’s code declares a Value field of type T. It also uses the type to declare the LeftChild and RightChild fields of type BinaryNode<T>.

You can add constraints on the generic types, as in the following code.

public class SortedBinaryNode<T> where T : IComparable<T>
{
    ...
}

The code where T : IComparable<T> indicates that the generic type T must implement the interface IComparable<T>.

A generic type’s where clause can include one or more of the elements shown in the following table.

The following code defines the StrangeGeneric class where type T1 must implement IComparable<T1> and must provide a parameterless constructor, type T3 must inherit from the Control class, and type T2 must inherit from type T3.

public class StrangeGeneric<T1, T2, T3>
    where T1 : IComparable<T1>, new()
    where T3 : Control
    where T2 : T3
{

}

Generic Methods

You can also give a class (generic or otherwise) a generic method. Just as a generic class is not tied to a particular data type, the parameters of a generic method are not tied to a specific data type.

To make a generic method, include type parameters similar to those you would use for a generic class. The following code defines a generic Switch method inside the static Switcher class.

public static class Switcher
{
    // Switch two values.
    public static void Switch<T>(ref T value1, ref T value2)
    {
        T temp = value1;
        value1 = value2;
        value2 = temp;
    }
}