How to get the method actually called by the callvirt IL instruction within FxCop

Question

I'm still trying to get my FxCop rule working.

As part of this, i need to work out what methods a method calls. Previously i was using CallGraph.CallersFor() (doing it in reverse, which is my final aim anyway), however it appears to have the same issue i describe below.

As an alternative to using the CallGraph class i tried visiting all method calls to build a dictionary, based on this code:

public override void VisitMethodCall(MethodCall call)
{
    Method CalledMethod = (call.Callee as MemberBinding).BoundMember as Method;
    // ....
}

However, it turns out that if the called method is on a derived class that overrides a base class' method, then the BoundMember is the base class' method, not the child class' method (which is the one that will actually be called).

Question: How can i get the method that will be called in the case of a callvirt IL instruction in FxCop?

Answer 1

Turns out in my case that i don't actually need this exactly (which is good because i don't have an exact answer in this case).

Because FxCop is a static checker it can never know the type of the instance of the object pointed to by the variable, which could be declared as a base type. Therefore i believe what i'm asking for is impossible.

My solution here, is to, when building the call tree, i add in extra references for a base class 'calling' any derived classes. In this way if i call a method on the base class, and the derived class' method could be called i can follow the call tree in that way.

See below for my class used by FxCop rule classes:

public class CallGraphBuilder : BinaryReadOnlyVisitor
{
    public Dictionary<TypeNode, List<TypeNode>> ChildTypes;

    public Dictionary<Method, List<Method>> CallersOfMethod;

    private Method _CurrentMethod;

    public CallGraphBuilder()
        : base()
    {
        CallersOfMethod = new Dictionary<Method, List<Method>>();
        ChildTypes = new Dictionary<TypeNode, List<TypeNode>>();
    }

    public override void VisitMethod(Method method)
    {
        _CurrentMethod = method;

        base.VisitMethod(method);
    }

    public void CreateTypesTree(AssemblyNode Assy)
    {
        foreach (var Type in Assy.Types)
        {
            if (Type.FullName != "System.Object")
            {
                TypeNode BaseType = Type.BaseType;

                if (BaseType != null && BaseType.FullName != "System.Object")
                {
                    if (!ChildTypes.ContainsKey(BaseType))
                        ChildTypes.Add(BaseType, new List<TypeNode>());

                    if (!ChildTypes[BaseType].Contains(Type))
                        ChildTypes[BaseType].Add(Type);
                }
            }
        }
    }

    public override void VisitMethodCall(MethodCall call)
    {
        Method CalledMethod = (call.Callee as MemberBinding).BoundMember as Method;

        AddCallerOfMethod(CalledMethod, _CurrentMethod);

        Queue<Method> MethodsToCheck = new Queue<Method>();

        MethodsToCheck.Enqueue(CalledMethod);

        while (MethodsToCheck.Count != 0)
        {
            Method CurrentMethod = MethodsToCheck.Dequeue();

            if (ChildTypes.ContainsKey(CurrentMethod.DeclaringType))
            {
                foreach (var DerivedType in ChildTypes[CurrentMethod.DeclaringType])
                {
                    var DerivedCalledMethod = DerivedType.Members.OfType<Method>().Where(M => MethodHidesMethod(M, CurrentMethod)).SingleOrDefault();

                    if (DerivedCalledMethod != null)
                    {
                        AddCallerOfMethod(DerivedCalledMethod, CurrentMethod);

                        MethodsToCheck.Enqueue(DerivedCalledMethod);
                    }
                }
            }
        }

        base.VisitMethodCall(call);
    }

    private void AddCallerOfMethod(Method CalledMethod, Method CallingMethod)
    {
        if (!CallersOfMethod.ContainsKey(CalledMethod))
            CallersOfMethod.Add(CalledMethod, new List<Method>());

        if (!CallersOfMethod[CalledMethod].Contains(CallingMethod))
            CallersOfMethod[CalledMethod].Add(CallingMethod);
    }

    private bool MethodHidesMethod(Method ChildMethod, Method BaseMethod)
    {
        while (ChildMethod != null)
        {
            if (ChildMethod == BaseMethod)
                return true;

            ChildMethod = ChildMethod.OverriddenMethod ?? ChildMethod.HiddenMethod;
        }

        return false;
    }
}