Generic classes and methods

Generics introduces the concept of type parameters to .NET. Generics make it possible to design classes and methods that defer the specification of one or more type parameters until you use the class or method in your code. For example, by using a generic type parameter T, you can write a single class that other client code can use without incurring the cost or risk of runtime casts or boxing operations, as shown here:

// Declare the generic class.
public class GenericList<T>
{
    public void Add(T item) { }
}

public class ExampleClass { }

class TestGenericList
{
    static void Main()
    {
        // Create a list of type int.
        GenericList<int> list1 = new();
        list1.Add(1);

        // Create a list of type string.
        GenericList<string> list2 = new();
        list2.Add("");

        // Create a list of type ExampleClass.
        GenericList<ExampleClass> list3 = new();
        list3.Add(new ExampleClass());
    }
}

Generic classes and methods combine reusability, type safety, and efficiency in a way that their nongeneric counterparts can't. Generic type parameters are replaced with the type arguments during compilation. In the preceding example, the compiler replaces T with int. Generics are most frequently used with collections and the methods that operate on them. The System.Collections.Generic namespace contains several generic-based collection classes. The nongeneric collections, such as ArrayList aren't recommended and are maintained only for compatibility purposes. For more information, see Generics in .NET.

You can also create custom generic types and methods to provide your own generalized solutions and design patterns that are type-safe and efficient. The following code example shows a simple generic linked-list class for demonstration purposes. (In most cases, you should use the List<T> class provided by .NET instead of creating your own.) The type parameter T is used in several locations where a concrete type would ordinarily be used to indicate the type of the item stored in the list:

• As the type of a method parameter in the AddHead method.
• As the return type of the Data property in the nested Node class.
• As the type of the private member data in the nested class.

T is available to the nested Node class. When GenericList<T> is instantiated with a concrete type, for example as a GenericList<int>, each occurrence of T is replaced with int.

// Type parameter T in angle brackets.
public class GenericList<T>
{
    // The nested class is also generic, and
    // holds a data item of type T.
    private class Node(T t)
    {
        // T as property type.
        public T Data { get; set; } = t;

        public Node? Next { get; set; }
    }

    // First item in the linked list
    private Node? head;

    // T as parameter type.
    public void AddHead(T t)
    {
        Node n = new(t);
        n.Next = head;
        head = n;
    }

    // T in method return type.
    public IEnumerator<T> GetEnumerator()
    {
        Node? current = head;

        while (current is not null)
        {
            yield return current.Data;
            current = current.Next;
        }
    }
}

The following code example shows how client code uses the generic GenericList<T> class to create a list of integers. If you change the type argument, the following code creates lists of strings or any other custom type:

// A generic list of int.
GenericList<int> list = new();

// Add ten int values.
for (int x = 0; x < 10; x++)
{
    list.AddHead(x);
}

// Write them to the console.
foreach (int i in list)
{
    Console.WriteLine(i);
}

Console.WriteLine("Done");

• Use generic types to maximize code reuse, type safety, and performance.
• The most common use of generics is to create collection classes.
• The .NET class library contains several generic collection classes in the System.Collections.Generic namespace. The generic collections should be used whenever possible instead of classes such as ArrayList in the System.Collections namespace.
• You can create your own generic interfaces, classes, methods, events, and delegates.
• Generic classes can be constrained to enable access to methods on particular data types.
• You can obtain information at run time on the types that are used in a generic data type by using reflection.

For more information, see the C# Language Specification.

• Generics in .NET
• System.Collections.Generic