How to extend a method at runtime?

Question

Here is the class:

class Foo
{
    private void Boo()
    {
        // Body...
    }

    // Other members...
}

What I need is:

• Create a Foo2 class at runtime which has a copy of all Foo class members.
• In Foo2 class replace Boo method by Boo2 method that has its content alternated to some extent.
• Create an instance of Foo2 and invoke Boo2.

Thank you for help.

Answer 1

You can do it at runtime using a .NET AOP Framework event if it is not the the main purpose of this kind of framework.

I actively work on a new one which can handle it event if your method is not virtual.

You can take a look on NConcern .NET runtime AOP Framework

The monkey patch "aspect" :

public class MonkeyPatch : IAspect
{
    static public void Patch(MethodInfo oldMethod, MethodInfo newMethod)
    {
        //update monkey patch dictionary
        MonkeyPatch.m_Dictionary[oldMethod] = newMethod;

        //release previous monkey patch for target method.
        Aspect.Release<MonkeyPatch>(oldMethod);

        //weave monkey patch for target method.
        Aspect.Weave<MonkeyPatch>(oldMethod);
    }

    static private Dictionary<MethodInfo, MethodInfo> m_Dictionary = new Dictionary<MethodInfo, MethodInfo>();

    public IEnumerable<IAdvice> Advise(MethodInfo method)
    {
        if (MonkeyPatch.m_Dictionary.ContainsKey(_Method))
        {
            yield return Advice(MonkeyPatch.m_Dictionary[_Method]);
        }
    }
}

Patch :

static public void main(string[] args)
{
    //create Boo2, a dynamic method with Boo signature.
    var boo2 = new DynamicMethod("Boo2", typeof(void), new Type[] { typeof(Foo) }, typeof(Foo), true);
    var body = boo2.GetILGenerator();

    //Fill your ILGenerator...
    body.Emit(OpCodes.Ret);

    //Apply the patch
    MonkeyPatch.Patch(typeof(Foo).GetMethod("Boo"), boo2);
}

in the second hand, if you just need to call something after the original call, you are in AOP aim and you can do it like that...

Observation Aspect :

public class Observation : IAspect
{
    static public void Observe(MethodInfo method, Action action)
    {
        //update observation dictionary
        Observation.m_Dictionary[method] = action;

        //release observation aspect for target method
        Aspect.Release<Observation>(method);

        //weave observation aspect for target method.
        Aspect.Weave<Observation>(method);
    }

    static private Dictionary<MethodInfo, Action> m_Dictionary = new Dictionary<MethodInfo, Action>;

    public IEnumerable<IAdvice> Advice(MethodInfo method)
    {
        if (Observation.m_Dictionary.ContainsKey(method))
        {
            yield return Advice.Basic.After(Observation.m_Dictionary[method]);
        }
    }
}

Use case :

static public void main(string[] args)
{
    Observation.Observe(typeof(Foo).GetMethod("Boo"), () => { /* paste here your notification code... */ });
}