ASP.NET Core–Swagger error when using a virtual directory

After changing our ASP.NET Core application to no longer run under the root site(e.g. https://localhost/) but under a virtual directory instead (e.g. https://localhost/mysampleapp/) , loading the API definitions in Swagger no longer worked and failed with the following error message:

Fetch error undefined swagger/v1/swagger.json

To fix the problem, I had to do 2 things:

First I updated the Swagger json generation to set the SwaggerDoc to a specific path. Although the documentation mentions the first parameter of SwaggerDoc as the name, it becomes part of the URI of your swagger.json file.

In this case I set the name to ‘v1’:

	services.AddSwaggerGen(options =>
	{
	options.SwaggerDoc("v1", new Info
	{
	Version = "v1",
	Title = "Sample API",
	Description = "API Sample",
	TermsOfService = "None"
	});

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As a second step, I had to update the Swagger UI configuration to generate the swagger endpoint on a relative path. Important here is that the name of the swagger doc matches part of the URI before the swagger.json; in our case ../swagger/v1/swagger.json

	app.UseSwaggerUI(c =>
	{
	c.SwaggerEndpoint("../swagger/v1/swagger.json", "V1 Docs");
	});

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