Properly handling HttpClient exceptions within async / await

问题

I was hoping somebody could enlighten me a little bit on an issue I am facing in regards to async/await exception handling with HttpClient. I have written some code to illustrate, and it is being excecuted on both a Windows Phone 8 device and the emulator:

    private async void SearchButton_Click(object sender, EventArgs e)
    {
        try
        {
            HttpClient client = new HttpClient();
            System.Diagnostics.Debug.WriteLine("BEGIN FAULTY REQUEST:");
            string response = await client.GetStringAsync("http://www.ajshdgasjhdgajdhgasjhdgasjdhgasjdhgas.tk/");
            System.Diagnostics.Debug.WriteLine("SUCCESS:");
            System.Diagnostics.Debug.WriteLine(response);
        }
        catch (Exception exception)
        {
            System.Diagnostics.Debug.WriteLine("CAUGHT EXCEPTION:");
            System.Diagnostics.Debug.WriteLine(exception);
        }
    }

Tapping the button that invokes this function, produces the following output in the debugger console, the most interesting being the ones in bold:

BEGIN FAULTY REQUEST:

An exception of type 'System.Net.WebException' occurred in System.Windows.ni.dll and wasn't handled before a managed/native boundary

An exception of type 'System.Net.WebException' occurred in System.Windows.ni.dll and wasn't handled before a managed/native boundary

A first chance exception of type 'System.Net.Http.HttpRequestException' occurred in mscorlib.ni.dll

An exception of type 'System.Net.Http.HttpRequestException' occurred in mscorlib.ni.dll and wasn't handled before a managed/native boundary

CAUGHT EXCEPTION:

(and here it prints out the HttpRequestException)

Of course I am expecting an error in this case since the URL I am calling is nonsense. What I am not understanding here, is why the debugger reports that the exceptions are not handled, when the output simultaneously reports that the exception is caught. Also, the UI side of the app becomes much less responsive while the output is being printed, indicating that something is probably amiss.

Is this not the way to handle exceptions when working with async and await? I appreciate any input! Thanks.

回答1:

This is an artifact of the debugger. It's determining that an exception is "uncaught" because it's not caught yet. In this case this is expected behavior.

You are handling the exceptions correctly.

回答2:

As you are using HttpClient, try to use response.EnsureSuccessStatusCode();

Now HttpClient will throw exception when response status is not a success code.

try
{
    HttpResponseMessage response = await client.GetAsync("http://www.ajshdgasjhdgajdhgasjhdgasjdhgasjdhgas.tk/");
    response.EnsureSuccessStatusCode();    // Throw if not a success code.

    // ...
}
catch (HttpRequestException e)
{
    // Handle exception.
}

ORIGINAL SOURCE OF THE CODE: http://www.asp.net/web-api/overview/advanced/calling-a-web-api-from-a-net-client

回答3:

The debugger is telling you that this exception is first chance. When a debugger is attached to your process it gets notified for every exception that is thrown and then based on how the debugger has been configured it will decide what to do with it. You can go through What is first chance exception? for more details.

On a side note, catch specific exceptions only so that you understand which exceptions you are expecting and why.

来源：https://stackoverflow.com/questions/19034199/properly-handling-httpclient-exceptions-within-async-await

标签

.net

asynchronous

windows-phone-8

async-await