Microsoft.OpenApi.Models.OpenApiDocument.SerializeAsV2 Invalid URI: The format of the URI could not be determined

Question

I am upgrading my API from .netcore 2.1 to 3.1

The API uses SwagerUI and Autorest to generate the client.

From this question about Autorest I found out that I need to use:

app.UseSwagger(c =>
{
    c.SerializeAsV2 = true;
});

However, the UI gives:

Fetch error
Internal Server Error v1/swagger.json

The JSON is:

{
  "StatusCode": 500,
  "ErrorMessage": "Internal server error: Invalid URI: The format of the URI could not be determined. nStack:
    at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)
    at Microsoft.OpenApi.Models.OpenApiDocument.WriteHostInfoV2(IOpenApiWriter writer, IList`1 servers)
    at Microsoft.OpenApi.Models.OpenApiDocument.SerializeAsV2(IOpenApiWriter writer)
    at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.RespondWithSwaggerJson(HttpResponse response, OpenApiDocument swagger)
    at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
    at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.<Invoke>g__Awaited|6_0(ExceptionHandlerMiddleware middleware, HttpContext context, Task task)",
  "SystemMessage": "Invalid URI: The format of the URI could not be determined."
}

[Update]

I found mention of it here:

c.PreSerializeFilters.Add((swaggerDoc, httpReq) =>
{
    swaggerDoc.Servers = new List<OpenApiServer> { new OpenApiServer { Url = $"{httpReq.Scheme}://{httpReq.Host.Value}{pathPrefix}" } };
});

Now the JSON shows:

{
  "StatusCode": 500,
  "ErrorMessage": "Internal server error: Invalid URI: Invalid port specified. Stack:
    at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)
    at Microsoft.OpenApi.Models.OpenApiDocument.WriteHostInfoV2(IOpenApiWriter writer, IList`1 servers)
    at Microsoft.OpenApi.Models.OpenApiDocument.SerializeAsV2(IOpenApiWriter writer)
    at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.RespondWithSwaggerJson(HttpResponse response, OpenApiDocument swagger)
    at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
    at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.<Invoke>g__Awaited|6_0(ExceptionHandlerMiddleware middleware, HttpContext context, Task task)",
  "SystemMessage": "Invalid URI: Invalid port specified."
}

Answer 1

The following worked

        var basePath = "/v1";
        app.UseSwagger(c =>
        {
            c.RouteTemplate = "swagger/{documentName}/swagger.json";
            c.SerializeAsV2 = true;
            c.PreSerializeFilters.Add((swaggerDoc, httpReq) =>
            {
                swaggerDoc.Servers = new List<OpenApiServer> { new OpenApiServer { Url = $"{httpReq.Scheme}://{httpReq.Host.Value}{basePath}" } };
            });

        });