How to convert an await async function to a function that takes a callback as parameter?

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Hi
I need to have my own C# function in both an awaitable async version and in a version that takes a callback function as a parameter and can be called from multiple threads at the same time. The callback version must be based on the async version.

My awaitable function is:

public async Task<MyClass> GetDocumentAsync(long ID)
{
//I already have this function.
}

GetDocumentAsync(...) must be turned into

public void GetDocumentWithCallback(Action<MyClass> callback, long ID)
{
//1. Can be called from multiple threads at the same time.
//2. Must be none-blocking.
//3. Must be based on a call to the awaitable version GetDocumentAsync(...)
//Please help me with writing this function??
}

I hope someone can help my writing GetDocumentWithCallback(...)

Posted 19-Jan-16 3:53am

Henrik R L

Updated 19-Jan-16 3:56am

v2

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Richard Deeming · Accepted Answer · 2016-01-19T05:08:00

Solution 1

How about something like this:

C#

public void GetDocumentWithCallback(Action<MyClass> callback, long ID)
{
    Task<MyClass> task = GetDocumentAsync(ID);
    task.ContinueWith(t => callback(t.Result));
}

NB: Unlike the async version, callback will not be executed in the same context as the caller. That means you won't be able to access the UI in a Windows Forms / WPF application, and HttpContext.Current will not be set in an ASP.NET application.

If that's a problem, you'll have to explicitly capture and restore the context:

C#

public void GetDocumentWithCallback(Action<MyClass> callback, long ID)
{
    Task<MyClass> task = GetDocumentAsync(ID);
    ExecutionContext context = ExecutionContext.Capture();
    ContextCallback contextCallback = state => callback((MyClass)state);
    task.ContinueWith(t => ExecutionContext.Run(context, contextCallback, t.Result));
}

Posted 19-Jan-16 5:08am

Richard Deeming

Updated 19-Jan-16 5:22am

v2

Comments

Henrik R L 19-Jan-16 12:33pm

Hi Richard Deeming,

Thanks for the answer. I will try that.

One more thing... Can two threads call GetDocumentWithCallback() below at the same time? Is it thread safe?

public void GetDocumentWithCallback(Action<myclass> callback, long ID)
{
Task<myclass> task = GetDocumentAsync(ID);
ExecutionContext context = ExecutionContext.Capture();
ContextCallback contextCallback = state => callback((MyClass)state);
task.ContinueWith(t => ExecutionContext.Run(context, contextCallback, t.Result));
}

Will there be any problem in calling GetDocumentWithCallback() above in a loop on the UI thread like shown below?

for(int ID=1;ID<100;ID++)
{
GetDocumentWithCallback(UpdateGUI, ID);
}

void UpdateGUI(MyClass docInfo)
{
//Update GUI for specific document on each callback
}

Richard Deeming 19-Jan-16 12:38pm

Assuming GetDocumentAsync is thread-safe, there shouldn't be any problems.

With the loop in the UI thread, the callbacks will be executed sequentially. If more than one task completes simultaneously, the callbacks will be queued up waiting to execute on the UI thread.

Henrik R L 19-Jan-16 12:41pm

Hi Richard

Thanks a lot :-)