Optional arguments in a generic Func<>

Question

I have the following method in an assembly:

public string dostuff(string foo, object bar = null) { /* ... */ }

I use it as a callback, so a reference to it is passed to another assembly as such:

Func<string, object, string> dostuff

Now in the original form, I can call it without specifying that second argument, which defaults to null. But when I use it as a callback in that second assembly, I must specify that second argument.

What syntax allows me to ignore that second argument?

Answer 1

You'll need to create a new method that accepts only one argument, and that passes the default value for the second argument. You could do this with a lambda, rather than creating a new named method, if you wanted:

Func<string, string> doStuffDelegate = s => dostuff(s);

The other option would be to use a delegate who's signature has an optional second argument, instead of using Func, in which case your method's signature would match:

public delegate string Foo(string foo, object bar = null);

You could assign dostuff to a delegate of type Foo directly, and you would be able to specify only a single parameter when invoking that delegate.

Answer 2

You can't do this, simply because optional arguments are syntactic sugars and can be only used if you are calling the method directly. When you call the method like this:

dostuff(foo);

Compiler translates it into:

dostuff(foo, null);

In other cases such as using a delegate that doesn't accept an optional argument or when calling this method using reflection, you have to provide the optional argument.