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Covariance and Contravariance

I am confused about how contravariant / covariance works with C #. I have the following pseudocode

public static void Main()
{
    Action<string> action = e => Console.WriteLine(e);
    Execute(action);
}

private static void Execute(Action<object> action)
{
    action("hello world");
}

who throws

CS1502: Best overloaded method match for [...] has some invalid arguments

and I'm not really sure why. also what would be the correct way to do this?

In my real-world scenario, I have a basic interface that I use instead of passing an object in specific cases.

Thank!

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2 answers


Action<in T>

is contravariant, that is, you can pass "larger" types to a method. Because it string

is smaller (more specific or derived) than object

, you get a compile-time error. If you reversed your example and created Action<object>

instead Action<string>

, your method would compile:



public static void Main()
{
    Action<object> action = e => Console.WriteLine(e);
    Execute(action);
}

private static void Execute(Action<string> action)
{
    action("hello world");
}
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Action

and Func

are contravariant in their argument types, which means that you can assign Action<T>

to Action<U>

if it U

is a subtype T

. In your case, object

is a supertype string

, so the assignment is invalid.



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