Xamarin-android: Redirect to relative url doesn't work with AndroidClientHandler

[dominik-weber]

Steps to Reproduce

1. Execute this code:

var client = new HttpClient(new AndroidClientHandler());
var res = await client.GetStringAsync("https://nghttp2.org/httpbin/redirect-to?url=/httpbin/get");

Expected Behavior

The initial request returns a 302 redirect to relative url: /httpbin/get. This endpoint returns a JSON string.

Actual Behavior

Instead of following the redirect, an exception is thrown:
System.InvalidCastException: Unable to convert instance of type 'Java.Net.URLConnectionInvoker' to type 'Java.Net.HttpURLConnection'.
Please note that the code above works fine when using HttpClientHandler instead of AndroidClientHandler

Version Information

Microsoft Visual Studio Enterprise 2017
Version 15.7.4
VisualStudio.15.Release/15.7.4+27703.2035
Microsoft .NET Framework
Version 4.7.03056

Installed Version: Enterprise

Xamarin 4.10.10.2 (35a01d8dc)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer 4.12.1 (f3257e429)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin.Android SDK 8.3.3.2 (HEAD/dffc59120)
Xamarin.Android Reference Assemblies and MSBuild support.

Xamarin.iOS and Xamarin.Mac SDK 11.12.0.4 (64fece5)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

VS bug #742257

[dominik-weber]

Based on our testing, the issue seems to be caused by the handling of the redirect uri here: https://github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs#L570

Please note that according to the mentioned rfc https://tools.ietf.org/html/rfc3986#section-5 (example in section 5.4) relative uris (starting with /) should be appended to the base uri (ie. the original request uri) following multiple rules, only some of which are currently implemented in AndroidClientHandler.

Please see this reference source here: https://github.com/dotnet/corefx/blob/master/src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RedirectHandler.cs#L110
Note that in this implementation the Uri(Uri baseUri, string relativeUri) constructor is used to create the final redirect uri. This constructor seems to handle all of the cases in the RFC correctly (also including the already supported case where the redirect uri starts with //, ie. the protocol of the base uri should be used). See https://msdn.microsoft.com/en-us/library/9hst1w91(v=vs.110).aspx for more info.

Please also note that this issue causes real world problems with the following links not being able to load:

Dropbox share links:
Dropbox uses relative uris to redirect to for file download. You may use this link for testing:
https://www.dropbox.com/s/f1md5z13sgs351k/test-image.jpg?dl=1

Sharepoint online links:
Same as Dropbox, uses relative uris for redirect.
https://crmclient-my.sharepoint.com/:i:/g/personal/d_biecuszek_openasapp_net/ER0uRnZwruZDotjiEypDaN0Bodq_AKCLiUsH583vsE6TAg?download=1

[justintoth]

+1, seeing the same issue in our mobile app. It only started happening in our latest release, so it seems like something changed recently in the AndroidClientHandler code.

[scriptam]

+1, The issue appeared recently when working with services that redirects Json responses in my case. Issue seems to be related to AndroidClientHandler indeed and is observed consistently. Specifically at Xamarin.Android.Net.AndroidClientHandler.SetupRequestInternal

The issue persists on the latest preview release as well.

For anyone else who is experiencing the issue, changing the HttpClientHandler to Managed is a workaround. Not ideal, but a workaround until the issue is resolved.

If you are using the default constructor for HttpClient, you can change the behaviour throughout:
Project Options > Build > Android Build > General > HttpClient implementation = Managed (HttpClientHandler) instead of AndroidClientHandler.

Alternatively, you may switch to Managed handler explicitly, and only when required, instead of setting it project-wide: new HttpClient(new HttpClientHandler())

[dominik-weber]

@jonpryor @grendello anybody here?

[raspyweather]

I had a similar issue recently and was abled to resolve it by using the Managed version of HttpClient and using this nuget https://github.com/tranb3r/secure-httpclient to initialize it

[Nico04]

I've got the exact same issue with url "https://picsum.photos/400/200/?random".
I need to explicitly use HttpClientHandler but it's a workaround...