Reading response of non-successful HTTP requests done via System.Net.Http.HttpClient (ASP.NET MVC)

Question

I'm working on an ASP.NET webapp made of two modules:

• An "API" module, offering web service endpoints
• An "UX" module, calling those endpoints

One of the endpoints in the API module looks like this when simplified:

public class ReportingApiController: System.Web.Mvc.Controller
{
    [HttpPost]
    public async Task<ActionResult> Upload()
    {
        [...]
        return new HttpStatusCodeResult(540, "Failure parsing the file");
    }
}

The UX module calls that endpoint with code that looks like this:

public class ReportingController : System.Web.Mvc.Controller
{
    private async Task Foo()
    {
        var requestContent = [...]

        HttpResponseMessage httpResponse = await httpClient.PostAsync("api/ReportingApi/Upload", requestContent); // `httpClient` is a System.Net.Http.HttpClient

        // How to read the response? Both the HTTP status code returned (540 in the example), and the corresponding message.
    }
}

I have noticed that the PostAsync() call throws an exception in the above example, because HTTP status codes 5XX are not successful. Using a try/catch is ok, as long as I can still read the response code & message. Which hasn't been the case in my tests (I don't see that info in the exception, and httpResponse is null inside the catch clause).

In my preferred scenario, I would like to avoid a try/catch , simply have the PostAsync() call complete normally, and read the code & message in the httpResponse variable.

What do you recommend?

Answer 1

catch the WebException, it will return the original response. Here an example using String Builder to output headers and body from an HttpWebResponse.

    catch (WebException e)
    {
        using (WebResponse response = e.Response)
        {
            HttpWebResponse httpResponse = (HttpWebResponse)response;
            var stringBuilder = new StringBuilder();
            stringBuilder.AppendLine($"Returned Status Code: {httpResponse.StatusCode.ToString()}");
            using (Stream data = httpResponse.GetResponseStream())
            {
                for (var i = 0; i < httpResponse.Headers.Count; ++i)
                {
                    stringBuilder.AppendLine($"{httpResponse.Headers.Keys[i]}: {httpResponse.Headers[i]}");
                }

                using (var reader = new StreamReader(data))
                {
                    string text = reader.ReadToEnd();
                    stringBuilder.AppendLine($"Body: {text}");
                }
            }
            // do whatever
        }
    }