Friday, September 10, 2010


Ok, after taking a month off from any blogging activity, I guess is time to catch up and blog again. And I believe there is no better way of doing so than to write an article explaining a way to get a reference to the object that calls a specific method.

For the last two years or so, I have been working hard on a new Content Manager replacement API for the XNA Framework to use on my own “never-ending” game engine (I will blog about this later on). In some early stages of development I found a way to solve this referencing burden that have been creating headaches to many devs, globally (being me, one of them, for a long time).

Although I must admit that at some point on the development cycle of my API I decided to abandon this workaround, after reading one of the articles mentioned by Shawn here (guess which one), I believe it’s a great opportunity to talk about it.

A little history … before founding this solution I tried everything: from attempting to use –with no luck- the info held in StackFrame instances and even delegates to finally “quit” and use a naive approach as passing the caller as a parameter in the signature of an operation. But, then came .NET Framework 3 and extension methods to the rescue!

Please bear with me that the example presented in this article is just a simple one to show how to use it, but believe my words when I say that it can be extended to complex scenarios.

Having said that … if you want to know which object is calling a certain method of a class like (which may or may not be sealed):

  public sealed class MyCalledClass
    internal bool JustReturnTrue(int myParameter)
      return true;

Being the caller, say, a specification of the following class:

  public abstract class MyCallingClass
    public void Print()
      Console.WriteLine("I am the object which is just about to call
         the monitored operation named 'JustReturnTrue' …"

Then all you have to do is to implement the following generic extension method for the types you want (in my example, “MyCallingClass”):

  public static class MyExtensionMethod
    public static bool ExecuteOperation<T>
      (this T mycallingClass, int myParameter)
      where T: MyCallingClass
      mycallingClass.Print(); // Replace it with your own stuff.
      return new MyCalledClass().JustReturnTrue(myParameter);

The trick here is to design and split assemblies and namespaces in a way that all instances of “MyCallingClass” cannot directly execute the “JustReturnTrue” operation (I leave that task as an exercise to the reader).

But there is one catch to watch closely, though. By doing this you are actually adding one more call (you know that), which is not generally a problem on Windows, but for the XBox 360 and all devices using the Compact Framework, it could turn out to be expensive if used lots of times on intensive or heavy tasks/loops.

But if you really need it when speed is not an issue or for operations where -for example- you need to set owners to something “automagically” behind the scenes and later assess whether an owner calls a restricted operation before executing it, then there you have it!

Just use it wisely …

> Link to Spanish version.