How to await an async private method invoked using reflection in WinRT?

Question

I'm writing unit tests for a WinRT app, and I am able to invoke non-async private methods using this:

TheObjectClass theObject = new TheObjectClass();
Type objType = typeof(TheObjectClass);
objType.GetTypeInfo()
       .GetDeclaredMethod("ThePrivateMethod")
       .Invoke(theObject, null);

However, if the private method in question is async, the code will continue execution without waiting for it to finish.

How do I add await functionality to this?

Since this is a WinRT app, I have the feeling that reflection/invocation of private members is disallowed. I can't find official documentation of this on google right now, closest is: http://blogs.microsoft.co.il/blogs/sasha/archive/2011/09/17/metro-net-framework-profile-windows-tailored.aspx EDIT: These are unit tests though, so maybe it's a non-issue. :) — Chris Sinclair, Feb 05 '13 at 16:04
@Chris Sinclair Actually, the code I have above works perfectly fine for private methods. My issue is specifically with asynchronous ones. The issue would apply to public methods invoked via reflection as well. — jokeefe, Feb 05 '13 at 20:49

score 116 · Accepted Answer · answered Feb 05 '13 at 16:01

116

Well you need to use the value returned by the method. Do you know the type? For example, if it's always a Task, you could use:

await (Task) objType.GetTypeInfo()
                    .GetDeclaredMethod("ThePrivateMethod")
                    .Invoke(theObject, null);

If you don't know the return type but know it will be awaitable, you could use dynamic typing:

await (dynamic) objType.GetTypeInfo()
                       .GetDeclaredMethod("ThePrivateMethod")
                       .Invoke(theObject, null);

I would try to avoid having to call a private method by reflection in your unit tests in the first place though. Can you test it indirectly via the public (or internal) API? That's generally preferable.

answered Feb 05 '13 at 16:01

Jon Skeet

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But cast to `dynamic` won't work if `GetAwaiter()` is an extension method, which is exactly the case for WinRT `IAsyncAction`. – svick Feb 05 '13 at 16:21
@Jon Skeet: Thanks! This did it. It was a `Task` object being returned, so all I needed to do was cast it and add the `await`. I also found that you need to make the unit test method's return type `Task` as well when you make it `async` or it won't be executed by the unit test frame work. – jokeefe Feb 06 '13 at 16:14
@Jeff: It would help if you'd give more details, like the compiler error you're getting. It may well be worth creating a new question rather than having a comment discussion about it. – Jon Skeet Nov 01 '13 at 18:00
@Jeff: That sounds like you're trying to use `await` without being in an `async` method (or lambda) - that has nothing to do with *what* you're trying to await. – Jon Skeet Nov 01 '13 at 20:21
@JonSkeet I think you're right. I don't have the code in front of me, but I think it might have been inside an anonymous method that wasn't marked async. Sorry for the digression, I'll erase my comments. – Jeff Nov 04 '13 at 00:22
Is there a way to determine if that method is `async` or not? – Shimmy Weitzhandler Dec 08 '15 at 08:38
1

@Shimmy: No, and you shouldn't need to. That's just an implementation detail. You should know whether it will perform asynchronously or not, but a method can return a `Task` and perform asynchronously without being implemented via async/await. – Jon Skeet Dec 08 '15 at 08:40
Alight I got it now after experimenting with it a bit more. Thanks Jon! Do you ever `Thread.Sleep()`? – Shimmy Weitzhandler Dec 08 '15 at 09:38
It worked for me, though i added a slight modification into it for my need `var instance = MyFactory.Instance.Create(myClassName); // custom factory` `var method = instance.GetType().GetMethod(myMethodName);` `await (dynamic) method.Invoke(instance, null);` – Atta H. May 02 '17 at 10:56
When I try something like this, I get the compiler error: CS0656: Der vom Compiler angeforderte Member "Microsoft.CSharp.RuntimeBinder.CSharpArgumentInfo.Create" fehlt. – ygoe Jun 24 '18 at 18:47
@ygoe: It sounds like your project may be messed up. It's hard to say more without a lot more information. I suggest you ask a new question. – Jon Skeet Jun 24 '18 at 21:44
@JonSkeet Maybe this only works in WinRT and not NETStd. – ygoe Jun 25 '18 at 06:56
@ygoe: It works with .NET Standard projects running in regular .NET Core or .NET. Again, rather than trying to get the details in comments, I suggest you ask a new question with all those details. – Jon Skeet Jun 25 '18 at 07:44
In Xamarin.Forms for UWP and Android it doesn't works. – Petr Voborník Apr 09 '19 at 08:04
@PetrVoborník: I'm afraid with no more information than "it doesn't works" it's unlikely that anyone will be able to help you. I suggest you create a new question with a complete example, specific context (not just "Android" but which runtime you're using) and error message. – Jon Skeet Apr 09 '19 at 08:13

score 9 · Answer 2 · answered Feb 05 '13 at 16:00

9

Invoke should return an object convertible to Task. Just await that.

If your private method returns void, then you'll need a custom SynchronizationContext, which is messy. It's better to have your methods return Task/Task<T>.

answered Feb 05 '13 at 16:00

Stephen Cleary

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How to await an async private method invoked using reflection in WinRT?

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