What are Covariance, Contravariance, and the 'in'/'out' Keywords in .NET Generics?

Detailed explanation of covariance, contravariance and "in/out" in .NET generics

Covariance and contravariance play a crucial role in the design of .NET generic interfaces and delegates. They determine how to safely replace generic types in different scenarios.

Covariance and contravariance

Covariance allows a more "derived" (more specific) type to be used in place of a primitive type, provided that the primitive type is only a "return value" (e.g., as a return value). In contrast, contravariance allows a more "base" (less specific) type to be used in place of the original type, provided that the original type is only used as "input" (for example, as a method parameter).

“in” and “out”

The "in" and "out" keywords are abbreviations for covariance and contravariance respectively. When a generic type appears only as a return value, use "out"; when a generic type only appears as a method parameter, use "in".

Relationship Clarification

The example in the article about the inability to treat List as List illustrates the concept of contravariance. Writing to a more specific (derived) type means using an "in" constraint (contravariance) because the type of data being written should be more specific.

In-depth explanation of using generic interfaces

For a deeper understanding of covariance and contravariance, consider the following two generic methods:

<code>public Base DoSomething(int variable)
public Derived DoSomethingElse(int variable)</code>

Assume an interface MyInterface, which has a method MyFunction(int variable), which returns a T.

• Use "out" to modify T: interface MyInterface<out t></out> indicates that T can only be used as a return type. Assigning MyInterface to MyInterface is valid because Derived is a derived type from Base.
• Use "in" to modify T: interface MyInterface<in t></in> indicates that T can only be used as a method parameter. Assigning MyInterface to MyInterface is invalid because it violates the contravariance rule.

Summary

Understanding the concepts of covariance, contravariance, and "in/out" is crucial to effectively using generics in .NET. It allows you to create type-safe code by ensuring that generic types are replaced correctly based on their usage.

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