There is such a delegate

delegate void Del(string z);

And it is assigned a method

 public void Foo(object z) { }

Assignment occurs normally, due to variation. However, if in the delegate parameter the string is changed to int then the compiler is already outraged. Why I believe that this is somehow related to the reference / structural type. But they all come from the object.

    1 answer 1

    You are right, the variance of delegates is limited to reference types.

    Here is a normative quote from C # language reference (lies in <Visual Studio install dir>\VC#\Specifications\1033\CSharp Language Specification.docx ), section 15.2 (my translation):

    15.2 Delegate Compatibility

    A method or delegate M is said to be compatible with delegate type D if the following conditions are true:

    • D and M same number of parameters, and each parameter in D has the same ref / out modifiers, as does the corresponding parameter in M
    • For each parameter that does not contain ref / out , there is an identity conversion (§6.1.1) or an implicit reference conversion (§6.1.6) from the type of the parameter in D to the type of the corresponding parameter in M
    • For each ref - / out parameter, the parameter type in D is exactly the same as in M
    • There is an identical or implicit reference conversion from the return type in M to the return type in D

    This means that the parameter types must either be the same (up to the difference between object and dynamic ), or the parameter type must be a reference subtype of the parameter type in D This excludes meaningful types.