Fody.AsyncErrorHandler

What?

Fody.AsyncErrorHandler is a Fody extension for weaving exception handling code into applications which use async code.

Introduction to Fody

The nuget package

https://nuget.org/packages/AsyncErrorHandler.Fody/

PM> Install-Package AsyncErrorHandler.Fody

Why?

Because writing plumbing code is dumb and repetitive.

How?

IL-weaving after the code is compiled, bro.

For example, imagine you've got this code to serialize an object to the filesystem:

public class DataStorage
{
    public async Task WriteFile(string key, object value)
    {
        var jsonValue = JsonConvert.SerializeObject(value);
        using (var file = await folder.OpenStreamForWriteAsync(key, CreationCollisionOption.ReplaceExisting))
        using (var stream = new StreamWriter(file))
            await stream.WriteAsync(jsonValue);
    }
}

After the code builds, the weaver could scan your assembly looking for code which behaves a certain way, and rewrite it to include the necessary handling code:

public class DataStorage
{
    public async Task WriteFile(string key, object value)
    {
        try 
        {
            var jsonValue = JsonConvert.SerializeObject(value);
            using (var file = await folder.OpenStreamForWriteAsync(key, CreationCollisionOption.ReplaceExisting))
            using (var stream = new StreamWriter(file))
                await stream.WriteAsync(jsonValue);
        }
        catch (Exception exception)
        {
            AsyncErrorHandler.HandleException(exception);
        } 
    }
}

And your application could provide its own implementation of the error handling module:

public static class AsyncErrorHandler
{
    public static void HandleException(Exception exception)
    {
        Debug.WriteLine(exception);
    }
}

Which allows you to intercept the exceptions at runtime.

What it really does

So the above example is actually a little misleading. It shows "in effect" what is inject. In reality the injected code is a little more complicated.

What async actually produces

So given a method like this

public async Task Method()
{
    await Task.Delay(1);
}

The compile will produce this

[AsyncStateMachine(typeof(<Method>d__0)), DebuggerStepThrough]
public Task Method()
{
    <Method>d__0 d__;
    d__.<>4__this = this;
    d__.<>t__builder = AsyncTaskMethodBuilder.Create();
    d__.<>1__state = -1;
    d__.<>t__builder.Start<<Method>d__0>(ref d__);
    return d__.<>t__builder.Task;
}

So "Method" has become a stub that calls into a state machine.

The state machine will look like this

[CompilerGenerated]
struct <Method>d__0 : IAsyncStateMachine
{
    // Fields
    public int <>1__state;
    public Target <>4__this;
    public AsyncTaskMethodBuilder <>t__builder;
    private object <>t__stack;
    private TaskAwaiter <>u__$awaiter1;

    // Methods
    private void MoveNext();
    [DebuggerHidden]
    private void SetStateMachine(IAsyncStateMachine param0);
}

The method we care about is MoveNext. It will look something like this

void MoveNext()
{
    try
    {
        TaskAwaiter awaiter;
        bool flag = true;
        switch (this.<>1__state)
        {
            case -3:
                goto Label_009F;

            case 0:
                break;

            default:
                awaiter = Task.Delay(1).GetAwaiter();
                if (awaiter.IsCompleted)
                {
                    goto Label_006F;
                }
                this.<>1__state = 0;
                this.<>u__$awaiter1 = awaiter;
                this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Target.<Method>d__0>(ref awaiter, ref this);
                flag = false;
                return;
        }
        awaiter = this.<>u__$awaiter1;
        this.<>u__$awaiter1 = new TaskAwaiter();
        this.<>1__state = -1;
    Label_006F:
        awaiter.GetResult();
        awaiter = new TaskAwaiter();
    }
    catch (Exception exception)
    {
        this.<>1__state = -2;
        this.<>t__builder.SetException(exception);
        return;
    }
Label_009F:
    this.<>1__state = -2;
    this.<>t__builder.SetResult();
}

Most of that can be ignored. The important thing to note is that it is swallowing exceptions in a catch. And passing that exception to a SetException method.

So when AsyncErrorHandler does its weaving it searches for SetException(exception); and then modifies the catch to look like this.

catch (Exception exception)
{
    this.<>1__state = -2;
    AsyncErrorHandler.HandleException(exception);
    this.<>t__builder.SetException(exception);
    return;
}

Icon

Icon courtesy of The Noun Project