Question

This is my test code:

class PassingInActionStatement
{
    static void Main(string[] args)
    {
        var dsufac = new DoSomethingUsefulForAChange();

        dsufac.Do(WriteToConsole);
        dsufac.Do2(s => WriteToConsoleWithSomethingExtra("Test"));
        dsufac.Do(WriteToConsoleWithSomethingExtra("Test")); // Does not compile
    }

    internal static void WriteToConsole()
    {
        Console.WriteLine("Done");
    }

    internal static void WriteToConsoleWithSomethingExtra(String input)
    {
        Console.WriteLine(input);
    }
}

internal class DoSomethingUsefulForAChange
{
    internal void Do(Action action)
    {
        action();
    }

    internal void Do2(Action<String> action)
    {
        action("");
    }
}

The first 2 calls work but I am wondering why the 3rd one does not. I do not fancy the code inside Do2 as it seems strange that I have type type action("") in there in order to get it to work.

Could someone please explain the 2 things I do not understand please?

1. Why I can not write the third line like that with calling Do
2. Why I have to write action(“”) in order get it to work in Do2

Answer 1

dsufac.Do(WriteToConsoleWithSomethingExtra("Test"));

actually calls the function first (WriteToConsoleWithSomethingExtra("Test")) and then attempts to pass the result into Do. Since there is no result (void), it’s not possible.

What you actually want is this:

dsufac.Do(() => WriteToConsoleWithSomethingExtra("Test"));

The inner part declares a function that takes nothing (the () => bit), which calls WriteToConsoleWithSomethingExtra("Test") when executed. Then your dsufac.Do call will receive an action, like it expects.

As for Do2 – you’ve declared it as taking Action<String>, which means that action is a function that takes one argument. You have to pass it a string. That string might be empty, like in your action("") example, or it might be passed in externally, as in something like this:

dsufac.Do3(WriteToConsole, "Test");

...

internal void Do3(Action<String> action, String str)
{
    action(str);
}