Making a WinRT component out of Membus – Pt.1, making Membus compile

Sep 19 2011

After last week’s //BUILD conference we have learned (amongst other things) of a new class of apps that are touch-centric and use a new .NET Framework profile in order to be run on top of the new Windows Runtime. A nice way to learn about what is different in this new stack is to port some existing code. Membus is a nice contender since I know the codebase(o rly?), it has a fair number of unit and integration tests, it is (apart from some tests) unaware of UI stuff, but I use it extensively in WPF UI Apps, so let’s move in.

I installed git on my new, um, PC to download the current version of Membus. First thing to note is that you can’t just open class library projects like that, you will need the correct project type. What I did is add a new sln file “Membus.Metro” that points to new csproj files “Membus.Metro” and “Membus.Metro.Tests” (from the “Unit Test” template). I then added all relevant files to the projects and compiled. Note that I am not touching the test project yet as it uses NUnit. Here’s a rundown of the issues I encountered

Some minor changes

The interface ICloneable just disappeared. Not a big deal for this codebase.
The Serializable attribute is gone. Interesting – I don’t know for now if it just moved or what that means, but I do imagine that this has implications for other code bases.
The class List<T> does not have a “ForEach” method anymore. The correct thing to do here is to use the Each extension method from System.Linq (fi-na-lly!)

Reflection changed more than moving mirrors around

The reflection API has changed considerably. Let’s face it, it’s a bit of a mess anyway, hence there was scope for improvement. Since Membus uses some reflection, a number of classes were affected. The one major change is that the Type-class has been streamlined to just a few essential features – reflection info like members, interfaces and the like are accessed by using the GetTypeInfo extension method that works e.g. on Types. It returns a TypeInfo instance that allows access to the information one usually needs when reflecting. MemberInfos, interfaces etc. are now returned as IEnumerable<T> and things have in general been cleaned up.

Membus already had a ReflectionExtensions static class which has been expanded to simplify access for other parts of the codebase. Other things could be deleted, e.g. there are now extension method like GetAttribute<T> available, so you don’t need to reintroduce them all the time.

Here’s an example of the change:

public static bool ImplementsInterface&lt;T&gt;(this Type type) where T : class
{
-         return Array.Exists(type.GetInterfaces(), t => t == typeof(T));
+        return type.GetTypeInfo().ImplementedInterfaces.Any(t => t == typeof(T));
}

Incidentally, the Array.Exists method does not, er, exist anymore.

Delegate.CreateDelegate is gone

I am not sure about the exact reason of this change, but it could be roughly the same direction as the fact that Reflection.Emit is not available. Membus uses this method to build delegates from suitable methods that can act as a target of messages.

In this case I had to move the code creation to what the Expression-class makes available to us. The correct way to create an arbitrary Action<T> delegate from a suitable MethodInfo looks like this:

public static ISubscription ConstructSubscription(this MethodInfo info, object target)
{
  var parameterType = info.GetParameters()[0].ParameterType;
  var fittingDelegateType = typeof(Action<>).MakeGenericType(parameterType);
  var p = Expression.Parameter(parameterType);
  var call = Expression.Call(p, info);
  var @delegate = Expression.Lambda(fittingDelegateType, call, p);

  var fittingMethodSubscription = typeof(MethodInvocation&lt;&gt;).MakeGenericType(parameterType);
  var sub = Activator.CreateInstance(fittingMethodSubscription, @delegate.Compile());

  return (ISubscription)sub;
}

All in all the necessary changes to make Membus compile again were done in roughly 2 hours. The commit of the changes documented in this post can be seen here on github as a diff.