Question

I have a method that takes a callback argument to execute asynchronously, but the catch block doesn’t seem to be catching any exceptions thrown by the synchronous call (this.Submit refers to a synchronous method).

public void Submit(FileInfo file, AnswerHandler callback)
{
    SubmitFileDelegate submitDelegate = new SubmitFileDelegate(this.Submit);
    submitDelegate.BeginInvoke(file, (IAsyncResult ar) =>
    {
        string result = submitDelegate.EndInvoke(ar);
        callback(result);
    }, null);
}

Is there a way to catch the exception thrown by the new thread and send it to the original thread? Also, is this the “proper” way to handle async exceptions? I wrote my code so it could be called like this (assuming the exception issue is fixed):

try
{
    target.Submit(file, (response) =>
    {
        // do stuff
    });
}
catch (Exception ex)
{
    // catch stuff
}

but is there a more proper or elegant way to do this?

Answer 1

This is not a ‘best practice’ solution, but I think it’s a simple one that should work.

Instead of having the delegate defined as

private delegate string SubmitFileDelegate(FileInfo file);

define it as

private delegate SubmitFileResult SubmitFileDelegate(FileInfo file);

and define the SubmitFileResult as follows:

public class SubmitFileResult
{
    public string Result;
    public Exception Exception;
}

Then, the method that actually does the file submission (not shown in the question) should be defined like this:

private static SubmitFileResult Submit(FileInfo file)
{
    try
    {
        var submissionResult = ComplexSubmitFileMethod();

        return new SubmitFileResult { Result = submissionResult };
    }
    catch (Exception ex)
    {
        return new SubmitFileResult {Exception = ex, Result = "ERROR"};
    }
}

This way, you’ll examine the result object, see if it has the Result or the Exception field set, and act accordingly.