Short Walks - Using CompilerService Arguments in an Interface

September 30, 2017

Until today, I thought that the following code would work:



class Program
{
    static void Main(string[] args)
    {
        ITest test = new Test();
        test.Log("testing");
        Console.ReadLine();
    }
}
 
interface ITest
{
    void Log(string text, string function = "");
}
 
class Test : ITest
{
    public void Log(string text, [CallerMemberName] string function = "")
    {
        Console.WriteLine($"{function} : text");
    }
}

And, by work, I mean output something along the lines of:

Main : testing

However; it actually outputs:

: testing

CompilerServiceAttributes need to be on the Interface, and not on the implementation



class Program
{
    static void Main(string[] args)
    {
        ITest test = new Test();
        test.Log("testing");
        Console.ReadLine();
    }
}
 
interface ITest
{
    void Log(string text, [CallerMemberName] string function = "");
}
 
class Test : ITest
{
    public void Log(string text, string function = "")
    {
        Console.WriteLine($"{function} : text");
    }
}

Why?

When you think about it, it does kind of make sense. Because you’re calling against the interface, the compiler injected value needs to be there; if you took the interface out of the equation, then the attribute needs to be on the class.

You live and learn!