How can I use Cecil to find the type passed to a generic method?

Question

I'm trying to use Cecil to find instances of calls to a generic method using an interface for a convention test. I'm having trouble identifying the generic type from the MethodReference.

I've set up a basic test:

private interface IAnimal
{
}

private class Duck : IAnimal
{
}

private class Farm
{
    private readonly ICollection<string> _animals = new List<string>();

    public void Add<T>()
    {
        _animals.Add(typeof(T).Name);
    }

    public override string ToString()
    {
        return string.Join(", ", _animals);
    }
}

static Farm FarmFactory()
{
    var farm = new Farm();
    farm.Add<Duck>();
    farm.Add<Duck>();
    farm.Add<IAnimal>();    // whoops
    farm.Add<Duck>();
    return farm;
}

private static void Main(string[] args)
{
    var farm = FarmFactory();
    Console.WriteLine("Farm:");
    Console.WriteLine(farm);

    // Use Cecil to find the call to farm.Add<IAnimal>():
    Console.WriteLine("Errors:");
    FindErrors();
    Console.Read();
}

So I want to find the call to farm.Add<IAnimal>(), which won't give a compile time error or even a run-time error, until the farm conceivable tried to create an instance of the type through reflection. My actual use case is a convention test for a DI container.

Cecil comes in to it in the FindErrors() method:

private static void FindErrors()
{
    var methods = AssemblyDefinition.ReadAssembly(typeof (Farm).Assembly.Location)
                                    .Modules
                                    .SelectMany(module => module.Types)
                                    .SelectMany(type => type.Methods)
                                    .Where(method => method.HasBody)
                                    .ToArray()
        ;
    var callsToFarmDotAdd = methods
        .Select(method => new
            {
                Name = method.Name,
                MethodReferences = GetCallsToFarmDotAdd(method)
            })
        .Where(x => x.MethodReferences.Any())
        .ToArray()
        ;
    var testCases = callsToFarmDotAdd
        .SelectMany(x => x.MethodReferences)
        ;
    var callsInError = testCases
        .Where(test => !test.GenericParameters[0].Resolve().IsClass)
        ;

    foreach (var error in callsInError)
    {
        Console.WriteLine(error.FullName);
    }
}

private static IEnumerable<MethodReference> GetCallsToFarmDotAdd(MethodDefinition method)
{
    return method.Body.Instructions
                 .Where(instruction => instruction.OpCode == OpCodes.Callvirt)
                 .Select(instruction => (MethodReference) instruction.Operand)
                 .Where(methodReference => methodReference.FullName.Contains("Farm::Add"))
        ;
}

The callsInError part is where I am failing to identify the generic type used when calling Farm::Add. Specifically, the GenericParameters property of the MethodReference is empty, so GenericParameters[0] gives an ArgumentOutOfRangeException. I've explored the MethodReference, and I'm definitely getting the calls to Farm::Add, but I can't see anywhere that relates to the generic type being used except for the FullName property, which isn't useful.

How can I get Cecil to identify the generic type used in the call?

Answer 1

If I cast the MethodReference to a GenericInstanceMethod the GenericArguments parameter does what I need:

var callsInError = testCases
    .Where(test => !((GenericInstanceMethod)test).GenericArguments[0].Resolve().IsClass)
    ;