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c#.netcovariancecontravariance

Contravariance and method signature?

looking at this code

delegate void StringAction (string s);
class Test
{
static void Main()
{
    StringAction sa = new StringAction (ActOnObject);
    sa ("hello");
}
static void ActOnObject (object o)
{

   Console.WriteLine (o); // hello
 }
}

Is this code is working due to Contravariance ? ( MoreDeriverdRef <== LessDerivedRef )

or because

(unrelated to contravariance) - In c# I can execute a method like ActOnObject (object o) with ActOnObject ("lalala")

Solution

  • This code works because, as Eric Lippert says in this article:

    Method group to delegate conversions are contravariant in their argument types.

    The above is only true for reference types, but both string and object are reference types, so the requirements are satisfied.

    This kind of variance has been supported since C# 2.0, you do not need the additional support introduced in version 4 to rely on it.