Out (and ref) parameters in Actions, Funcs, delegates and lambdas

I ran into a couple of wrinkles I wasn’t aware of before when coding up a facade for the ThreadPool for tests today, along the lines of the Path, Directory etc facades I’ve done before.

I wanted to insert a replacement ThreadPool so I could test the GetAvailableThreads method, which takes two out parameters.

You can’t quite specify the lambda as you might expect for the method implementation. This initial attempt, for example, doesn’t work:

public static class ThreadPool
{
    public static Action<int, int> GetAvailableThreadsImpl =
        System.Threading.ThreadPool.GetAvailableThreads;

    public static void GetAvailableThreads(out int workerThreads, out int ioThreads)
    {
        GetAvailableThreadsImpl(out workerThreads, out ioThreads);
    }
}

I suspect this is to do with the calling style here -- but need to understand why it happens in more detail. You can work around it by just creating your own delegate class:

public delegate void GetAvailableThreadsDel(out int x, out int y);
 
public static class ThreadPool
{
    public static GetAvailableThreadsDel GetAvailableThreadsImpl =
        System.Threading.ThreadPool.GetAvailableThreads;

    public static void GetAvailableThreads(out int workerThreads, out int ioThreads)
    {
        GetAvailableThreadsImpl(out workerThreads, out ioThreads);
    }
}

Then follows a small wrinkle when defining a replacement behaviour in your test. You can, of course, create your own method with the right signature:

[Test]
public void Blah()
{
    int callCount = 0;
    ThreadPool.GetAvailableThreadsImpl = GetAvailableThreadsStub;
}

private int callCount = 0;
private void GetAvailableThreadsStub(out int x, out int y)
{
    if (callCount == 0)
    {
        x = 5;
        y = 5;
    }
    else
    {
        x = 10;
        y = 10;
    }

    callCount++;
}

It turns out you can define out parameters in a lambda however -- but you must also explicitly state the parameter type as well, unlike normal:

[Test]
public void Blah()
{
    int callCount = 0;
    ThreadPool.GetAvailableThreadsImpl = (out int x, out int y) =>
        {
            if (callCount == 0)
            {
                x = 5;
                y = 5;
            }
            else
            {
                x = 10;
                y = 10;
            }

            callCount++;
        };
    }
}

Again, I'm not entirely sure how it works here, and need to investigate further. For now, just a wrinkle to remember.