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.