My c# custom delegate behaves weird (No overload for <X> matches <my custom delegate>)

Question

Okay, so after reading into creating custom function delegates after Func<> didnt accept a ref keyword inside the type declaration, I ended up with this delegate:

private delegate TResult RefFunc<T, out TResult>(ref T arg);

inside my little test project. I also declared these two functions:

private static string FirstFunction(string arg) {
    return "";
}
private static string SecondFunction(string arg) {
    return "";
}

(Since I discovered the problem, this truly is the only function content!)
that adhere to the above delegate and are passed as parameters to this function:

private static void SampleFunction(RefFunc<String, String> argOne, RefFunc<String, String> argTwo) { ... }

Like so (simplified):

private static void Main(string[] args) {
    SampleFunction(FirstFunction, SecondFunction);
}

That function call wouldn't work out because "Conversion of 'method group' to '<class>.RefFunc<string, string>' isn't possible", which makes sense - I'm directly passing a function without turning it into a delegate first - although I'm pretty sure I've seen that same syntax working with the Action<> Delegate somewhere. Anyways, I then modified my caller code in the Main() function in many ways while looking into the issue, however none of the below approaches resolved the issue.

SampleFunction(new RefFunc<String, String>(FirstFunction), ...); // Causes "No overload for FirstFunction matches delegate RefFunc<string, string>" at the first function parameter
SampleFunction(RefFunc FirstFunction, ...); // Was worth a try
RefFunc handler = FirstFunction; SampleFunction(handler, ...); // As described in docs. Causes "The usage of type "RefFunc<T, TResult>" (generic) requires 2 type arguments" at keyword RefFunc
RefFunc<String, String> handler = FirstFunction; SampleFunction(handler, ...); // Didn't help it. Causes the same error as with the first attempt
// and a few others

Docs
I've finally decided to turn to stackoverflow. Since my functions clearly adhere to the delegate I created, I can't really understand why C# believes they do not. I appreciate any help!
Please note that while I'm only showing code relevant to the error caused, the project is very small, so I'm sure the problem doesn't lie elsewhere

Answer 1

Since my functions clearly adhere to the delegate I created

Unfortunately, neither of your FirstFunction and SecondFunction is of RefFunc type, just because the T is passed by ref in the delegate's defintion and in the same time you lack ref in your actual functions.

Either you modify both functions

public class Program
{
    private delegate TResult RefFunc<T, out TResult>(ref T arg);

    private static string FirstFunction(ref string arg) {
       return "";
    }
    private static string SecondFunction(ref string arg) {
       return "";
    }

    private static void SampleFunction(
        RefFunc<String, String> argOne, RefFunc<String, String> argTwo) 
    { 
    }

    public static void Main()
    {
        SampleFunction(FirstFunction, SecondFunction);
    }
}

or you drop ref

private delegate TResult RefFunc<T, out TResult>(T arg);

which now is a correct type for

private static string SecondFunction(string arg) {
    return "";
}