An Action Cache

Do you ever find yourself in a loop calling a method that expects an Action or a Func as an argument? Here’s an example from an EasyNetQ test method where I’m doing just that:

[Test, Explicit("Needs a Rabbit instance on localhost to work")]
public void Should_be_able_to_do_simple_request_response_lots()
{
    for (int i = 0; i < 1000; i++)
    {
        var request = new TestRequestMessage { Text = "Hello from the client! " + i.ToString() };
        bus.Request<TestRequestMessage, TestResponseMessage>(request, response =>
            Console.WriteLine("Got response: '{0}'", response.Text));
    }

    Thread.Sleep(1000);
}

My initial naive implementation of IBus.Request set up a new response subscription each time Request was called. Obviously this is inefficient. It would be much nicer if I could identify when Request is called more than once with the same callback and re-use the subscription.

The question I had was: how can I uniquely identify each callback? It turns out that action.Method.GetHashcode() reliably identifies a unique action. I can demonstrate this with the following code:

public class UniquelyIdentifyDelegate
{
    readonly IDictionary<int, Action> actionCache = new Dictionary<int, Action>();
    public void DemonstrateActionCache()
    {
        for (var i=0; i < 3; i++)
        {
            RunAction(() => Console.Out.WriteLine("Hello from A {0}", i));
            RunAction(() => Console.Out.WriteLine("Hello from B {0}", i));

            Console.Out.WriteLine("");
        }
    }

    public void RunAction(Action action)
    {
        Console.Out.WriteLine("Mehod = {0}, Cache Size = {1}", action.Method.GetHashCode(), actionCache.Count);
        if (!actionCache.ContainsKey(action.Method.GetHashCode()))
        {
            actionCache.Add(action.Method.GetHashCode(), action);
        }

        var actionFromCache = actionCache[action.Method.GetHashCode()];

        actionFromCache();
    }
}

Here, I’m creating an action cache keyed on the action method’s hashcode. Then I’m calling RunAction a few times with two distinct action delegates. Note that they also close over a variable, i, from the outer scope.

Running DemonstrateActionCache() outputs the expected result:

Mehod = 59022676, Cache Size = 0
Hello from A 0
Mehod = 62968415, Cache Size = 1
Hello from B 0

Mehod = 59022676, Cache Size = 2
Hello from A 1
Mehod = 62968415, Cache Size = 2
Hello from B 1

Mehod = 59022676, Cache Size = 2
Hello from A 2
Mehod = 62968415, Cache Size = 2
Hello from B 2

Rather nice I think :)