HttpWebRequest.GetResponse throws WebException on HTTP 304

Question

When a web server responds to HttpWebRequest.GetResponse() with HTTP 304 (Not Modified), GetResponse() thows a WebException, which is so very weird to me. Is this by design or am I missing something obvious here?

Solution 1

Ok, this seems to be a by-design behavior and a perfect example of a vexing exception. This can be solved with this:

public static HttpWebResponse GetHttpResponse(this HttpWebRequest request)
{
    try
    {
        return (HttpWebResponse) request.GetResponse();
    }
    catch (WebException ex)
    {
        if(ex.Response == null || ex.Status != WebExceptionStatus.ProtocolError)
            throw; 

        return (HttpWebResponse)ex.Response;
    }
}

Solution 2

This is really a frustrating problem, and can be alternatively worked around by using the following extension method class and calling request.BetterGetResponse()

//-----------------------------------------------------------------------
//
//     Copyright (c) 2011 Garrett Serack. All rights reserved.
//
//
//     The software is licensed under the Apache 2.0 License (the "License")
//     You may not use the software except in compliance with the License.
//
//-----------------------------------------------------------------------

namespace CoApp.Toolkit.Extensions {
    using System;
    using System.Net;

    public static class WebRequestExtensions {
        public static WebResponse BetterEndGetResponse(this WebRequest request, IAsyncResult asyncResult) {
            try {
                return request.EndGetResponse(asyncResult);
            }
            catch (WebException wex) {
                if( wex.Response != null ) {
                    return wex.Response;
                }
                throw;
            }
        }

        public static WebResponse BetterGetResponse(this WebRequest request) {
            try {
                return request.GetResponse();
            }
            catch (WebException wex) {
                if( wex.Response != null ) {
                    return wex.Response;
                }
                throw;
            }
        }
    }
}

You read more about it in my blog post on this subject at http://fearthecowboy.com/2011/09/02/fixing-webrequests-desire-to-throw-exceptions-instead-of-returning-status/

Solution 3

The way to avoid this System.WebException is to set AllowAutoRedirect property to false. This disables the automatic redirection logic of the WebRequest. It seems to be broken for 304 redirection requests, as it is not a real redirection in the strictest sense. Of course that means that the other redirection requests 3xx have to be handled manually.

Solution 4

Just as an FYI, this is an update to Anton Gogolev's answer that uses the C#6 (VS2015) when clause. It's a little less annoying when using a debugger as it removes one catchpoint:

public static HttpWebResponse GetHttpResponse(this HttpWebRequest request)
{
    try
    {
        return (HttpWebResponse) request.GetResponse();
    }
    catch (WebException ex)
        when (ex.Status == WebExceptionStatus.ProtocolError && ex.Response != null)
    {
        return (HttpWebResponse) ex.Response;
    }
}

View more solutions

Comments

[comshak]

This works most of the cases, but some web servers could return a response body when returning a 404 error. In that case, the code above would treat a 404 as it treats a 304!

[roufamatic]

@comshak that's a "good to know." The calling code will have to be aware of what are acceptable response codes.

[Danny Beckett]

I also added || ((HttpWebResponse) ex.Response).StatusCode != HttpStatusCode.NotModified

[binki]

You should update this to use C#-6.0’s when clause!

[crokusek]

As a bonus, one used to be able to decorate a method with [DebuggerNonUserCode] and the Debugger would not stop in that method when an exception is thrown. In this manner poorly designed exceptions could be wrapped and ignored. But now a registry setting is required

[jsuddsjr]

Absolutely brilliant. Why should I pay for the heavy-handed exception machinery if I don't need it?