Calling an Internal Method on a Structure

Question

I am resorting to a horrible hack in order to fill a locked-down data type in the XNA framework: there's an internal method in a structure that I wish to call without feeding the garbage collector.

If I keep said structure boxed in an object variable and use MethodInfo.Invoke(), that call would itself feed the garbage collector by boxing the parameters:

private object boxedTouchCollection;

void test() {
  MethodInfo addTouchLocationMethod = typeof(TouchCollection).GetMethod(
    "AddTouchLocation", BindingFlags.Instance | BindingFlags.NonPublic
  );
  addTouchLocationMethod.Invoke(
    this.boxedState, new object[] { /* parameters being boxed */ }
  );
}

I'm not sure whether Delegate.CreateDelegate() can be used here - can I just turn the first parameter into an object and it will work on the boxed structure? Or can I store my structure unboxed and declare the first parameter as ref TouchCollection?

delegate void AddTouchLocationDelegate(
  ref TouchCollection collection,
  int id,
  // ...more parameters...
);

private TouchCollection touchCollection;

void test() {
  Delegate.CreateDelegate(
    typeof(AddTouchLocationDelegate),
    typeof(ref TouchCollection), // doesn't compile
    addTouchLocationMethod
  );
}

Is there a way I can make Delegate.CreateDelegate() work? Or will I have to resort to dynamic IL generation?

Answer 1

Here's one way.

It relies on this overload of Delegate.CreateDelegate, which creates open instance-method delegates. The only tricky bit is that so you'll have to create the appropriate delegate-type to be able to pass the struct by reference.

I don't think there should be any boxing with this technique - either with the arguments to the method, or with the struct itself.

Example: (Apologies for simplifying the example-types)

public struct Foo
{
    // Internal method to be called. Takes a value-type parameter.
    internal void Test(int someParam)
    {
        Console.WriteLine(someParam);
    }

    // Custom delegate-type. Takes the Foo instance of interest 
    // by reference, as well as the argument to be passed on to Test.
    public delegate void MyDelegate(ref Foo foo, int someParam);

    // Creates type-safe delegate
    private static MyDelegate GetTestDelegate()
    {
        var flags = BindingFlags.Instance | BindingFlags.NonPublic;
        var methodInfo = typeof(Foo).GetMethod("Test", flags);

        return (MyDelegate) Delegate.CreateDelegate
                            (typeof(MyDelegate), methodInfo);       
    }

    static void Main()
    {
        Foo foo = new Foo();
        MyDelegate action = GetTestDelegate();

        // should dodge boxing
        action(ref foo, 42);
    }
}

Answer 2

Here's another solution using Linq Expression Trees that I found in the meantime:

private delegate void AddTouchLocationDelegate(
  ref TouchCollection touchCollection,
  int id,
  TouchLocationState state,
  float x,
  float y,
  TouchLocationState prevState,
  float prevX,
  float prevY
);

private static AddTouchLocationDelegate createAddTouchLocationDelegate() {
  MethodInfo addTouchLocationMethod = typeof(TouchCollection).GetMethod(
    "AddTouchLocation", BindingFlags.Instance | BindingFlags.NonPublic
  );
  Type byrefTouchCollection = typeof(TouchCollection).MakeByRefType();

  ParameterExpression instance = Expression.Parameter(byrefTouchCollection, "instance");
  ParameterExpression idValue = Expression.Parameter(typeof(int), "id");
  ParameterExpression stateValue = Expression.Parameter(
    typeof(TouchLocationState), "state"
  );
  ParameterExpression xValue = Expression.Parameter(typeof(float), "x");
  ParameterExpression yValue = Expression.Parameter(typeof(float), "y");
  ParameterExpression prevStateValue = Expression.Parameter(
    typeof(TouchLocationState), "prevState"
  );
  ParameterExpression prevXValue = Expression.Parameter(typeof(float), "prevX");
  ParameterExpression prevYValue = Expression.Parameter(typeof(float), "prevY");

  Expression<AddTouchLocationDelegate> expression =
    Expression.Lambda<AddTouchLocationDelegate>(
      Expression.Call(
        instance, addTouchLocationMethod,
        idValue, stateValue, xValue, yValue, prevStateValue, prevXValue, prevYValue
      ),
      instance,
      idValue, stateValue, xValue, yValue, prevStateValue, prevXValue, prevYValue
    );

  return expression.Compile();
}

Usage is straightforward:

var d = createAddTouchLocationDelegate();
d(
  ref this.touches,
  1, TouchLocationState.Pressed, 10, 10, TouchLocationState.Released, 0, 0
);