ASP.NET Core Swagger uses incorrect json url when web application is hosted in a subdirectory - swashbuckle

Question

I followed these instructions to add swagger to my ASP.NET Core application.
It works fine when I host it as a root website but when I host the app as an alias on an existing website, say myserver.com/myapp, swagger will look for swagger.json at an incorrect URL and report: *Can't read swagger JSON from https://myserver.com/swagger/v1/swagger.json. It should instead use https://myserver.com/myapp/swagger/v1/swagger.json.
The message I get is:
Can't read swagger JSON from https://myserver.com/swagger/v1/swagger.json
How can I configure swashbuckle/swagger to use the application base path and look for the swagger.json file at the right place?
I'm hosting on IIS.
The version of swashbuckle is:
"Swashbuckle": "6.0.0-beta902"
I suspect that I'll have to add something to the app.UseSwaggerUi() in the Configure method in Startup.cs but I'm not sure what.
Startup Configure:
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
loggerFactory.AddConsole(Configuration.GetSection("Logging"));
loggerFactory.AddDebug();
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
app.UseBrowserLink();
}
else
{
app.UseExceptionHandler("/Home/Error");
}
app.UseStaticFiles();
app.UseMvc(routes =>
{
routes.MapRoute(
name: "default",
template: "{controller=Home}/{action=Index}/{id?}");
});
// Enable middleware to serve generated Swagger as a JSON endpoint
app.UseSwagger();
// Enable middleware to serve swagger-ui assets (HTML, JS, CSS etc.)
app.UseSwaggerUi();
}

Answer 1

You can use the ASPNETCORE_APPL_PATH environment variable to get the application basepath.
app.UseSwaggerUI(c =>
{
string basePath = Environment.GetEnvironmentVariable("ASPNETCORE_APPL_PATH");
if (basePath == null) basePath = "/";
c.SwaggerEndpoint($"{basePath}swagger/v1/swagger.json", "My API");
});

Answer 2

I ended up specifying the endpoint explicitly:
app.UseSwagger();
app.UseSwaggerUi(c =>
{
c.SwaggerEndpoint($"/swagger/v1/swagger.json", "MyAPI documentation");
//c.SwaggerEndpoint($"/myapi/swagger/v1/swagger.json", "MyAPI documentation");
});
I've been fooling around with hosting the Web API in IIS and in IIS Express, and I end up having to swap out the endpoint depending on where I am hosting the app.