C# recursive delegate declaration problem

Question

* PLEASE SEE END FOR IMPORTANT EDIT *

For various reasons I have something like:

delegate void Task(QueueTask queueTask); delegate void QueueTask(Task task);

Using Visual Studio Express 2008 this works more or less fine apart from in lambda expressions where it can't deduce if I try call and call queueTask() in the function without explicit help:

 void MyTask(QueueTask queueTask) { queueTask(qt => {}); // fails with queueTask doesn't take 1 parameter? }

This works though:

 void MyTask(QueueTask queueTask) { queueTask((QueueTask qt) => {}); }

Its not a huge problem and I know I could also probably get away by passing in another object that contained the delegate instead but I was curious if there was a way to make it work as its stands?

Thanks.

* IMPORTANT EDIT BELOW *

It seems only if I put the class 'TaskPool2' in a separate assembly I get the compile error!?

 namespace Test { public class TaskPool2 { public delegate void QueueTask(Task task); public delegate void Task(QueueTask queueTask); public void Queue(Task task) { } } } namespace MyApp { class Program { static void SomeFunc() { Test.TaskPool2 taskPool = new Test.TaskPool2(); taskPool.Queue(queue => { Console.WriteLine("A"); queue(q => Console.WriteLine("C")); }); } } }

Granted its somewhat convoluted!

Sorry about that please see my edit! That makes me even more curious! — iam
– iam, Commented Jul 6, 2010 at 13:32

Neowizard · Accepted Answer · 2010-10-17 12:13:21Z

After using several ways to break\fix the code, I've found a small bit of information.

When the compiler reaches the line

taskPool.Queue(queue => { Console.WriteLine("A"); queue(q => Console.WriteLine("C")); });

it'll fall compilation if (and I think only if) it never encountered QueueTask before. For instance, declaring the next line in the class (before or after the SomeFunc method)

Public Delegate Test.TaskPool2.QueueTask x;

would fix the compilation. Also, adding the next line above the errornouse line will also fix it

Test.TaskPool2.QueueTask x; // declare x to be QueueTask

I've tried a few more lines using QueueTask, and they worked (fix the compilation).

I can only assume that the compiler doesn't load the definition for QueueTask, and this leads to this strangment.

Seems even like a bug.

Hope I managed to explain this well.

Jon Skeet · Accepted Answer · 2010-07-05 18:33:52Z

(This would have been a comment, but it's not really feasible to give full code in comments.)

Your code already works as it stands:

delegate void Task(QueueTask queueTask); delegate void QueueTask(Task task); class Test { void MyTask(QueueTask queueTask) { queueTask(qt => {}); // fails with queueTask doesn't take 1 parameter? } }

Compiles with no problems.

Please create a similarly short but complete program which doesn't compile.

Sorry about that please see my edit. Amazing that I got something wrong with something that does compile!

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