Startup.cs - The path in 'value' must start with '/' - asp.net-mvc

Question

I have created a new .NET Core MVC application in Visual Studio 2017 and enabled multi-tenant authentication.
I've completed the configuration (ClientId, Authority, etc) but when I debug the application there is an exception in the Startup.cs, specifically the app.useOpenIdConnectAuthentication method.
The exception given is
System.ArgumentException: The path in 'value' must start with '/'.
I'm a bit of a novice when it comes to C# and .NET Core, so I'm not sure whether I'm missing something obvious. The main sticking point is what the debugger is referring to with the parameter 'value' as I can't see any mention of it in the code. There are no changes beyond the default template generated by visual studio, other than adding the configuration items to the appsettings.json.

Answer 1

As there is no code in the question, so I will try to make a general answer as possible.
This exception appears when you use this overload PathString.FromUriComponent(string) and the string does not start with the / character
so,for example, the following code will throw an exception :
PathString.FromUriComponent("controllerName/actionName"); // throw exception
and to fix the previous exception you can write it like this
PathString.FromUriComponent("/controllerName/actionName"); // working, but as relative path
and of course, this will be a relative path.
In case you wanted to use an absolute path, (and not start your string with /), then you must use another overload of this method which takes Uri object as a parameter instead of string
here is an example
// use an absolute path
PathString.FromUriComponent(new Uri("https://localhost:8000/controller/action/"))

Answer 2

I also get the same exception, but with another method UseExceptionHandler:
The error was:
ArgumentException: The path in 'value' must start with '/'. Nombre del parámetro: value Microsoft.AspNetCore.Http.PathString..ctor(string value) Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler(IApplicationBuilder app, string errorHandlingPath)
Fixed by replacing
app.UseExceptionHandler("Home/Error");
with
app.UseExceptionHandler("/Home/Error");
notice the addition of the / char at the beginning of the string.