wraps an action in an asynchronous Post on the current synchronization context

MarshalAction.cs

    // usage:
    //    // In a UI event handler:
    //    BarAsync(result => label.Text = result);
    //
    //    void BarAsync(Action<int> completed) {
    //       completed = completed.Marshal();
    //       ThreadPool.QueueUserWorkItem(() => completed(42));
    //    }
    //
    // Here's what it replaces. You'd need one of these for each delegate type
    // you want to Marshal (Action, Action<T1,T2>, WaitCallback, etc).
    // public static Action<T> Marshal<T>(this Action<T> self)
    // {
    //     var sync = SynchronizationContext.Current;
    //     if (sync == null)
    //     {
    //         return self;
    //     }
    //     return t => sync.Post(state => self(t), null);
    // }

    public static class ActionExt
    {
        /// <summary>
        /// Returns a delegate that invokes action in a Post on the current syncronization context.
        /// </summary>
        public static T MarshalAction<T>(this T action) // the compiler doesn't allow "where T:Delegate"
        {
            Debug.Assert(action is Delegate);
            var invoke = action.GetType().GetMethod("Invoke");
            Debug.Assert(invoke.ReturnType == typeof(void));

            var sync = SynchronizationContext.Current;
            if (sync == null)
            {
                return action;
            }

            var args = invoke.GetParameters().Select(p => Expression.Parameter(p.ParameterType, p.Name)).
                ToArray();
            var poster = Expression.Lambda<T>(
                Expression.Call(
                    Expression.Constant(sync),
                    typeof(SynchronizationContext).GetMethod("Post"),
                    Expression.Lambda<SendOrPostCallback>(
                        Expression.Invoke(Expression.Constant(action), args),
                        Expression.Parameter(typeof(object), "ignore")),
                    Expression.Constant(null)),
                args);
            return poster.Compile();
        }
    }