Understanding C# Covariance And Contravariance (7) CLR

Understanding C# Covariance And Conreavariance:

Variances in CLR

Unlike C# 3.0 features are mostly C# level syntactical sugars, the variance feature of C# 4.0 is not just a CLR level feature.

Take a look at the definition of System.Func<in T, out TResult>:

.class public auto ansi sealed System.Func`2<- T,+ TResult>
       extends System.MulticastDelegate

and the definition of System.IComparable<in T>:

.class interface public abstract auto ansi System.IComparable`1<- T>

CLR introduces the “-” operator to express the same meaning of “in” in C#, while “+” for “out”. The “in” and “out” keywords are introduced in part three.

Obviously, without the CLR supporting, those implicit type conversions in C# can never happen.

The + and – operators

Once again, take a look at these guys:

internal class Base

internal class Derived : Base

internal class AnotherDerived : Base

Many materials from Internet start variances from the relationship of type A and type B:

  • A is bigger than B: for example, Base is bigger than Derived;
  • A is smaller than B: for example, Derived is smaller than Base;
  • A is equal to B: for example, Derived is equal to Derived, which is very simple;
  • A is not related to B: for example, Derived is not related to AnotherDerived.

Think about the first two relationships. For the interfaces in part 2:

  • Covariance of output: Derived is a Base => for IOut<out T>, we have IOut<Derived> "is a" IOut<Base>;
  • Contravariance of input: Derived is a Base => for IIn<in T>, we have IIn<Base> "is a" IIn<Derived>.

Now with the “bigger and smaller concepts:

  • Covariance of output: Base is bigger than Derived => for IOut<+ T>, we have IOut<Derived> "is a" IOut<Base>;
  • Contravariance of input: Derived is smaller than Base => for IIn<- T>, we have IIn<Base> "is a" IIn<Derived>.

So for the + and - operators:

  • For covariance, + is used, which means can be bigger;
  • For contravariance, – is used, which means can be smaller.

They look confusing, and it is hard to remember which is which, even for the members of the C# design committee.


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