Null Reference when looping ReturnUrlParsers - identityserver4

Question

I've created a custom return url parser to determine which tenant is calling the login page, but when loading the login page I get a null reference on one of the returnUrlparsers.
at IdentityServer4.Services.ReturnUrlParser.ParseAsync(String returnUrl) in C:\local\identity\server4\IdentityServer4\src\IdentityServer4\Services\ReturnUrlParser.cs:line 38
at IdentityServer4.Services.DefaultIdentityServerInteractionService.GetAuthorizationContextAsync(String returnUrl) in C:\local\identity\server4\IdentityServer4\src\IdentityServer4\Services\DefaultIdentityServerInteractionService.cs:line 54 at IdentityServer4.Quickstart.UI.AccountController.BuildLoginViewModelAsync(String returnUrl) in D:\git\identity-server\IdentityServer4\src\Fifthplay.IdentityServer4\Quickstart\Account\AccountController.cs:line 346
I've created a custom implementation of the IReturnUrlParser
public class MyReturnUrlParser: IReturnUrlParser
{
private IdentityServerConfigurationContext _identityServicecontext { get; set; }
private ILogger<MyReturnUrlParser> _logger;
public MyReturnUrlParser(IdentityServerConfigurationContext context, ILogger<MyReturnUrlParser> logger)
{
_identityServicecontext = context;
_logger = logger;
}
public bool IsValidReturnUrl(string returnUrl)
{
return true;
}
public Task<AuthorizationRequest> ParseAsync(string returnUrl)
{
if(string.IsNullOrEmpty(returnUrl))
return null;
... doing some custom stuff
return Task.FromResult(authorizationRequest);
}
registered it to DI
services.AddTransient<IReturnUrlParser, MyReturnUrlParser>();
When the Login page is loaded I see that the "MyReturnUrlparser" is hit, it return a null because it is just browsing, to the login page but then afterwards a null reference is thrown.
https://github.com/IdentityServer/IdentityServer4/blob/c2255fae51ecac3048f7281fc7590bee085f30f2/src/Services/Default/ReturnUrlParser.cs
My next guess is that for some reason the default URL parser is null when looping through all available ReturnUrl parsers. Did anybody experience this behavior before? Or am I missing something obvious?

Answer 1

The default URL parser can't be null because you would have got a NullReferenceException on line 36 in ReturnUrlParser.cs:
if (parser.IsValidReturnUrl(returnUrl))
Have you pasted the whole stack trace? Is the exception thrown inside the `ParseAsync' method?
One possibility is that a URL parser is returning true for a URL that it shouldn't process and is falling over inside the ParseAsync because it is encountering something unexpected. I notice that you always return true from IsValidReturnUrl which means that your URL parser will be parsing all URLs.
You could try moving your register line:
services.AddTransient<IReturnUrlParser, MyReturnUrlParser>();
before the call services.AddIdentityServer() so that your parser is first in the collection of parser: