Generic method with Func parameter, Func contravariance doesn't seem to work

Question

.NET's Func<T1, T2, TResult> is written so that T1 and T2 are contravariant and TResult is covariant.

This means that:

Func<object, object, bool> objectEquals = object.Equals;
Func<MyObject, MyObject, bool> myObjectEquals = objectEquals;

is a valid assignment.

Now, I have a class with methods along these lines:

public void DoSomething<T>(T value)
{
    DoSomethingCore(T, Object.Equals);
}

protected abstract void DoSomethingCore<T>(T value, Func<T, T, bool> equals);

The code would not compile, with the following error:

CS0123 No overload for 'object.Equals(object, object)' matches delegate 'Func<T, T, bool>'

Why does contravariance not seem to work in this case?

score 5 · Accepted Answer · answered Sep 04 '17 at 14:18

Contravariance, and variance in general, don't work with value types.

Hence, you must constrain the DoSomething method to only work with reference types if you want to use Object.Equals as the function for Func<T, T, bool>.

In other words, add where T : class in the method signature:

public void DoSomething<T>(T value) where T : class
{
    DoSomethingCore(T, Object.Equals);
}

Generic method with Func parameter, Func contravariance doesn't seem to work

1 Answers1