Exception handling in async methods in .NET4.5 MVC4 with C#

In this post we’ll take a look at how to handle exceptions that are thrown by actions that are awaited. My previous post already included some exception handling techniques in MVC4 but here we will concentrate on exceptions thrown by await actions. Check my previous 3 posts for the full story behind the code examples shown here.

We will simulate some problems by intentionally throwing an exception in GetResultAsync and GetDataAsync:

public async Task<String> GetDataAsync(CancellationToken ctk)
        {
            StringBuilder dataBuilder = new StringBuilder();
            dataBuilder.Append("Starting GetData on thread id ").Append(Thread.CurrentThread.ManagedThreadId)
                .Append(". ");
            ctk.ThrowIfCancellationRequested();
            await Task.Delay(2000);
            throw new Exception("Something terrible has happened!");
            dataBuilder.Append("Results from the database. ").Append(Environment.NewLine);
            dataBuilder.Append("Finishing GetData on thread id ").Append(Thread.CurrentThread.ManagedThreadId)
                .Append(".");
            return dataBuilder.ToString();
        }
public async Task<String> GetResultAsync(CancellationToken ctk)
        {
            StringBuilder resultBuilder = new StringBuilder();
            resultBuilder.Append("Starting GetResult on thread id ").Append(Thread.CurrentThread.ManagedThreadId)
                .Append(". ");
            ctk.ThrowIfCancellationRequested();
            await Task.Delay(2000);
            throw new Exception("The service is down!");
            resultBuilder.Append("This is the result of a long running calculation. ");
            resultBuilder.Append("Finishing GetResult on thread id ").Append(Thread.CurrentThread.ManagedThreadId)
                .Append(".");
            return resultBuilder.ToString();
        }

Also, increase the timeout value of the Index action so that we do not get a Timeout Exception:

[AsyncTimeout(4000)]
[HandleError(ExceptionType = typeof(TimeoutException), View = "Timeout")]
public async Task<ActionResult> Index(CancellationToken ctk)

Before you run the application change the ‘mode’ attribute of the customErrors element in the web.config to “Off” as we want to see the debug data.

It does not come as a surprise that we run into an exception:

Intentional exception YSOD

If you had worked with the TPL library before then you may have expected an AggregateException that wraps all exceptions encountered during the parallel calls. However, TPL behaves slightly differently in conjunction with the await keyword. It is still an AggregateException that is instantiated behind the scenes but the .NET runtime will only throw the first exception that was encountered during the method execution.

This is good news: we can set up our try-catch structures as usual; we don’t need to worry about inspecting an AggregateException anymore.

View the list of MVC and Web API related posts here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

One Response to Exception handling in async methods in .NET4.5 MVC4 with C#

  1. Pingback: C#: Snippets | RaSor's Tech Blog

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