WebHostBuilderExtensions Configure can result in HostingStartupAssemblies being run twice

[k-s-s]

Describe the bug

Background

I am using IHostingStartup within my package library as it is used by external users to create the web host, targets NET 4.6 and NET Core 3.1. I set the builder setting HostingStartupAssembliesKey to package library assembly FullName
I have added a feature to add use an instanced startup class instead of using the normal UseStartup. It is a custom interface, not Microsoft.AspNetCore.Hosting.IStartup
The intention is for testing, to make an easier way of testing different services, rather than separate startup classes.

Bug

1. When using the Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.Configure method (I add the method of the instanced startup), the extension method sets WebHostBuilder setting ApplicationKey to startup assembly. This was not correct as it was setting to the package library, using assembly Name
   public static IWebHostBuilder Configure(this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, IApplicationBuilder> configureApp)
2. When I do host.Build(), the method:
   private IServiceCollection BuildCommonServices(out AggregationException hostingStartupErrors)
   create WebHostOptions, which registers 2x hosting startup assemblies because of Name and FullName

To Reproduce

1. Use IHostingStartup, using assembly FullName
2. Use WebHostBuilder.Configure
3. Run WebHost.Build()

Exceptions (if any)

I found the issue that Swashbuckle raised an exception that a document was already registered.

Further technical details

• NET 4.6.1, NET Core 3.1

[davidfowl]

@k-s-s do you have a sample application?

[davidfowl]

Trying to interpret what you put in the issue, this is what I came up with so far. I know I'm missing something 😄 .

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

[assembly: HostingStartup(typeof(WebApplication50.HostingStartup))]

namespace WebApplication50
{
    public class HostingStartup : IHostingStartup
    {
        public void Configure(IWebHostBuilder builder)
        {
            
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey, typeof(Program).Assembly.FullName)
                              .Configure(new Startup().Configure);
                });
    }
}

[k-s-s]

Hi David

That looks about right, if you put a breakpoint on the HostingStartup.Configure, I think it should call twice. In the HostingStartup.Configure method, to check, register singleton, that links to a static variable and will throw exception if already set.
If it doesn't call twice, then try as well to call this from a separate project, with a startup from that project.

I will make a sample this weekend.

[davidfowl]

No need, I can reproduce the problem and will have a fix out ASAP.

[davidfowl]

@Tratcher should we back port this?

[Tratcher]

@Tratcher should we back port this?

Why? This one report doesn't seem to justify that.

[davidfowl]

It's not about the number of reports, it's about the potential impact, and re-reading this issue again I don't think its common to set a different assembly name like this.