How to throw an exception in an async method (Task.FromException)

Question

I just discovered that, since .NET 4.6, there is a new method FromException on the Task object, and I was wondering what was the best way to throw exceptions in an async method.

Here are two examples:

internal class Program
{
    public static void Main(string[] args)
    {
        MainAsync().Wait();
    }

    private static async Task MainAsync()
    {
        try
        {
            Program p = new Program();
            string x = await p.GetTest1(@"C:\temp1");
        }
        catch (Exception e)
        {
            // Do something here
        }
    }

    // Using the new FromException method
    private Task<string> GetTest1(string filePath)
    {
        if (!Directory.Exists(filePath))
        {
            return Task.FromException<string>(new DirectoryNotFoundException("Invalid directory name."));
        }
        return Task.FromResult(filePath);
    }

    // Using the normal throw keyword
    private Task<string> GetTest2(string filePath)
    {
        if (!Directory.Exists(filePath))
        {
             throw new DirectoryNotFoundException("Invalid directory name.");
        }
        return Task.FromResult(filePath);
    }
}

The best way to throw exceptions in an async method is throw. However, neither of your examples are async methods. — Stephen Cleary
– Stephen Cleary, Commented Jan 19, 2017 at 14:42

Stewart_R · Accepted Answer · 2017-01-19 14:46:02Z

25

There is a difference in behaviour between GetTest1() and GetTest2.

GetTest1() will not throw the exception when the method is called. Instead it returns a Task<string>. The exception will not be thrown until that task is awaited (we could also have chosen to inspect the task to see if succeeded without ever throwing the exception).

In contrast GetTest2() throws the exception immediately when called without ever returning the Task<string>

I guess which one you use would depend on the desired behaviour. If I had a bunch of GetTest() tasks I wanted to run in parallel and I wanted execution to continue for those tasks that succeeded then I would use Task.FromException which allows me to inspect the results of each Task and act accordingly. In contrast, if any Exception in the list meant I didn't want execution to continue I would likely just throw the Exception.

edited Jan 19, 2017 at 14:46

answered Jan 19, 2017 at 7:45

Stewart_R

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5 Comments

Stephen Cleary Over a year ago

That last paragraph doesn't make sense. Task.FromException shouldn't be used in async methods at all.

Stewart_R Over a year ago

@StephenCleary removed the offending paragraph - thanks for pointing it out

nicolas Over a year ago

@Stephen Cleary Why not? For example:

private async Task<string> GetTest3(string filePath)         {             if (!Directory.Exists(filePath))             {                 return await Task.FromException<string>(new DirectoryNotFoundException("Invalid directory name."));             }  ... }

Stephen Cleary Over a year ago

@Bidou: Because await Task.FromException<string>(new DirectoryNotFoundException("Invalid directory name.")); (the return won't compile) is just an overly verbose way of saying throw new DirectoryNotFoundException("Invalid directory name.");

nicolas Over a year ago

@Stephen Cleary Ok, thank you, this is what I was guessing. By the way, it compiles fine with the return, but this is a detail...

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