in (Generic Modifier) (C# Reference)

For generic type parameters, use the in keyword to allow contravariant type arguments. Use the in keyword in generic interfaces and delegates.

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The documentation identifies any feature first introduced in the last three versions of the language or in current public previews.

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Contravariance enables you to use a less derived type than the type specified by the generic parameter. This feature allows for implicit conversion of classes that implement contravariant interfaces and implicit conversion of delegate types. Reference types support covariance and contravariance in generic type parameters, but value types don't support these features.

You can declare a type as contravariant in a generic interface or delegate only if it defines the type of a method's parameters and not the method's return type. In, ref, and out parameters must be invariant, meaning they're neither covariant nor contravariant.

An interface that has a contravariant type parameter allows its methods to accept arguments of less derived types than those specified by the interface type parameter. For example, in the IComparer<T> interface, type T is contravariant. You can assign an object of the IComparer<Person> type to an object of the IComparer<Employee> type without using any special conversion methods if Employee inherits Person.

You can assign a contravariant delegate to another delegate of the same type, but with a less derived generic type parameter.

For more information, see Covariance and Contravariance.

Contravariant generic interface

The following example shows how to declare, extend, and implement a contravariant generic interface. It also shows how you can use implicit conversion for classes that implement this interface.

// Contravariant interface.
interface IContravariant<in A> { }

// Extending contravariant interface.
interface IExtContravariant<in A> : IContravariant<A> { }

// Implementing contravariant interface.
class Sample<A> : IContravariant<A> { }

class Program
{
    static void Test()
    {
        IContravariant<Object> iobj = new Sample<Object>();
        IContravariant<String> istr = new Sample<String>();

        // You can assign iobj to istr because
        // the IContravariant interface is contravariant.
        istr = iobj;
    }
}

Contravariant generic delegate

The following example shows how to declare, instantiate, and invoke a contravariant generic delegate. It also shows how you can implicitly convert a delegate type.

// Contravariant delegate.
public delegate void DContravariant<in A>(A argument);

// Methods that match the delegate signature.
public static void SampleControl(Control control)
{ }
public static void SampleButton(Button button)
{ }

public void Test()
{

    // Instantiating the delegates with the methods.
    DContravariant<Control> dControl = SampleControl;
    DContravariant<Button> dButton = SampleButton;

    // You can assign dControl to dButton
    // because the DContravariant delegate is contravariant.
    dButton = dControl;

    // Invoke the delegate.
    dButton(new Button());
}

C# language specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See also

• out
• Covariance and Contravariance
• Modifiers