Generics in .NET

A generic type definition is a class, structure, or interface declaration that functions as a template, with placeholders for the types that it can contain or use. For example, the System.Collections.Generic.Dictionary<TKey,TValue> class can contain two types: keys and values. Because a generic type definition is only a template, you cannot create instances of a class, structure, or interface that is a generic type definition.

Generic type parameters, or type parameters, are the placeholders in a generic type or method definition. The System.Collections.Generic.Dictionary<TKey,TValue> generic type has two type parameters, TKey and TValue, that represent the types of its keys and values.

A constructed generic type, or constructed type, is the result of specifying types for the generic type parameters of a generic type definition.

A generic type argument is any type that is substituted for a generic type parameter.

The general term generic type includes both constructed types and generic type definitions.

Covariance and contravariance of generic type parameters enable you to use constructed generic types whose type arguments are more derived (covariance) or less derived (contravariance) than a target constructed type. Covariance and contravariance are collectively referred to as variance.

Constraints are limits placed on generic type parameters. For example, you might limit a type parameter to types that implement the System.Collections.Generic.IComparer<T> generic interface, to ensure that instances of the type can be ordered. You can also constrain type parameters to types that have a particular base class, that have a parameterless constructor, or that are reference types or value types. Users of the generic type cannot substitute type arguments that do not satisfy the constraints.

A generic method definition is a method with two parameter lists: a list of generic type parameters and a list of formal parameters. Type parameters can appear as the return type or as the types of the formal parameters, as the following code shows.

T MyGenericMethod<T>(T arg)
{
    T temp = arg;
    //...
    return temp;
}

class A
{
    T G<T>(T arg)
    {
        T temp = arg;
        //...
        return temp;
    }
}

class MyGenericClass<T>
{
    T M(T arg)
    {
        T temp = arg;
        //...
        return temp;
    }
}

Type safety. Generics shift the burden of type safety from you to the compiler. There is no need to write code to test for the correct data type because it is enforced at compile time. The need for type casting and the possibility of run-time errors are reduced.

Less code and code is more easily reused. There is no need to inherit from a base type and override members. For example, the LinkedList<T> is ready for immediate use. For example, you can create a linked list of strings with the following variable declaration:

LinkedList<string> llist = new LinkedList<string>();

Better performance. Generic collection types generally perform better for storing and manipulating value types because there is no need to box the value types.

Generic delegates enable type-safe callbacks without the need to create multiple delegate classes. For example, the Predicate<T> generic delegate allows you to create a method that implements your own search criteria for a particular type and to use your method with methods of the Array type such as Find, FindLast, and FindAll.

Generics streamline dynamically generated code. When you use generics with dynamically generated code you do not need to generate the type. This increases the number of scenarios in which you can use lightweight dynamic methods instead of generating entire assemblies.

Generic types can be derived from most base classes, such as MarshalByRefObject (and constraints can be used to require that generic type parameters derive from base classes like MarshalByRefObject). However, .NET does not support context-bound generic types. A generic type can be derived from ContextBoundObject, but trying to create an instance of that type causes a TypeLoadException.

Enumerations cannot have generic type parameters. An enumeration can be generic only incidentally (for example, because it is nested in a generic type that is defined using Visual Basic, C#, or C++).

Lightweight dynamic methods cannot be generic.

Covariance

Contravariance

Invariance

Variant type parameters are restricted to generic interface and generic delegate types.

A generic interface or generic delegate type can have both covariant and contravariant type parameters.

Variance applies only to reference types; if you specify a value type for a variant type parameter, that type parameter is invariant for the resulting constructed type.

Variance does not apply to delegate combination. That is, given two delegates of types Action<Derived> and Action<Base> (Action(Of Derived) and Action(Of Base) in Visual Basic), you cannot combine the second delegate with the first although the result would be type safe. Variance allows the second delegate to be assigned to a variable of type Action<Derived>, but delegates can combine only if their types match exactly.

Starting in C# 9, covariant return types are supported. An overriding method can declare a more derived return type the method it overrides, and an overriding, read-only property can declare a more derived type.

abstract class Animal
{
    public abstract Food GetFood();
    ...
}
class Tiger : Animal
{
    public override Meat GetFood() => ...;
}

You can use a covariant type parameter as the return value of a method that belongs to an interface, or as the return type of a delegate.

interface ICovariant<out R>
{
    void DoSomething(Action<R> callback);
}

You cannot use a covariant type parameter as a generic type constraint for interface methods. [4]

interface ICovariant<out R>
{
    // The following statement generates a compiler error
    // because you can use only contravariant or invariant types
    // in generic constraints.
    // void DoSomething<T>() where T : R;
}

You can use a contravariant type parameter as the type of a parameter of a method that belongs to an interface, or as the type of a parameter of a delegate.

You can use a contravariant type parameter as a generic type constraint for an interface method.

interface IContravariant<in A>
{
    void SetSomething(A sampleArg);
    void DoSomething<T>() where T : A;
    // The following statement generates a compiler error.
    // A GetSomething();
}

The type parameters of generic type definitions and generic method definitions are represented by instances of the Type class.

If an instance of Type represents a generic type, then it includes an array of types that represent the type parameters (for generic type definitions) or the type arguments (for constructed types). The same is true of an instance of the MethodInfo class that represents a generic method.