Skip to main content

Open Instance Delegate for Generic Interface Methods

Ran into a snag when developing the latest abstraction for Sasa. The problem is distilled to its simplest form in this Stack Overflow question.

In summary, while the CLR supports open instance delegates to interface methods, and supports delegates to generic interface methods, it does not seem to support the composition of the two, ie. an open instance delegate to a generic interface method.

The following trivial example fails when creating the delegate with a NotSupportedException:
interface IFoo
{
  void Bar<T>(T j);
}
class Foo : IFoo
{
  public void Bar<T>(T j)
  {
  }
}
static void Main(string[] args)
{
  var bar = typeof(IFoo).GetMethod("Bar")
                        .MakeGenericMethod(typeof(int));
  var x = Delegate.CreateDelegate(typeof(Action<IFoo, int>),
                                  null, bar);
}

However, the CLR does support open instance delegates to generic class methods. For some reason interfaces are singled out here, and it's not clear why.

If I'm doing something wrong, please let me know!

Edit: I submitted a bug to Microsoft Connect, since this now looks like a legit bug.

Edit 2: MS acknowledged the limitation, but said it wouldn't be solved in the current version of .NET. As I explain in that bug, this limitation just doesn't seem to make sense, but it must be due to the internals that don't reuse the existing CLR dispatching logic.
Labels:

Comments

Post a Comment

Popular posts from this blog

async.h - asynchronous, stackless subroutines in C

The async/await idiom is becoming increasingly popular. The first widely used language to include it was C#, and it has now spread into JavaScript and Rust. Now C/C++ programmers don't have to feel left out, because async.h is a header-only library that brings async/await to C! Features: It's 100% portable C. It requires very little state (2 bytes). It's not dependent on an OS. It's a bit simpler to understand than protothreads because the async state is caller-saved rather than callee-saved. #include "async.h" struct async pt; struct timer timer; async example(struct async *pt) { async_begin(pt); while(1) { if(initiate_io()) { timer_start(&timer); await(io_completed() || timer_expired(&timer)); read_data(); } } async_end; } This library is basically a modified version of the idioms found in the Protothreads library by Adam Dunkels, so it's not truly ground bre
1 comment
Read more

Simple, Extensible IoC in C#

I just committed the core of a simple dependency injection container to a standalone assembly, Sasa.IoC . The interface is pretty straightforward: public static class Dependency { // static, type-indexed operations public static T Resolve<T>(); public static void Register<T>(Func<T> create) public static void Register<TInterface, TRegistrant>() where TRegistrant : TInterface, new() // dynamic, runtime type operations public static object Resolve(Type registrant); public static void Register(Type publicInterface, Type registrant, params Type[] dependencies) } If you were ever curious about IoC, the Dependency class is only about 100 lines of code. You can even skip the dynamic operations and it's only ~50 lines of code. The dynamic operations then just use reflection to invoke the typed operations. Dependency uses static generic fields, so resolution is pretty much just a field access + invoking a
2 comments
Read more

Building a Query DSL in C#

I recently built a REST API prototype where one of the endpoints accepted a string representing a filter to apply to a set of results. For instance, for entities with named properties "Foo" and "Bar", a string like "(Foo = 'some string') or (Bar > 99)" would filter out the results where either Bar is less than or equal to 99, or Foo is not "some string". This would translate pretty straightforwardly into a SQL query, but as a masochist I was set on using Google Datastore as the backend, which unfortunately has a limited filtering API : It does not support disjunctions, ie. "OR" clauses. It does not support filtering using inequalities on more than one property. It does not support a not-equal operation. So in this post, I will describe the design which achieves the following goals: A backend-agnostic querying API supporting arbitrary clauses, conjunctions ("AND"), and disjunctions ("OR"). Implemen
Post a Comment
Read more