Modify startup assemblies using the minimal hosting model

[nfplee]

Is there a way to modify the startup assemblies using the minimal hosting model?

In ASP.NET Core 5 I would say the following (where hostBuilder is an instance of IWebHostBuilder):

hostBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey, string.Join(";", assemblies));

With the new minimal hosting model I tried saying:

builder.WebHost.UseSetting(WebHostDefaults.HostingStartupAssembliesKey, string.Join(";", assemblies));

However this throws a NotSupportedException and that I should use WebApplication.CreateBuilder(WebApplicationOptions) instead, but I can't see an option to specify the hosting startup assemblies.

[davidfowl]

This only works via environment variables with the minimal host. We didn't add it as a first class option because it was deemed more obscure to do it in code than other options.

[nfplee]

Thanks but I can’t see how i can achieve this using environment variables since the list of assemblies is dynamic.

[davidfowl]

What's the scenario?

[nfplee]

I split the areas of all my sites into Razor class libraries. This allows me to modularize my site aswell as use the same libraries in other sites (as package references). Each library has its own dependencies and hosting startup classes allow me to register them for each one. Therefore I use the following to scan for any potential assemblies:

var assemblies = DependencyContext.Default.GetRuntimeAssemblyNames(Environment.OSVersion.Platform.ToString())
    .Select(n => n.Name);

Then I just set the hosting startup assemblies key as mentioned previously.

I have multiple sites with multiple modules and having to manually set this for each one is quite tiresome. Also I would have to remember to alter it every time I add a new package reference to another module. Therefore I find the dynamic approach a lot easier.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[halter73]

After reading this more, I'm moving it out of the backlog so we can triage again. At the very least, we should update the exception message to indicate you need to modify environment variables.

aspnetcore/src/DefaultBuilder/src/ConfigureWebHostBuilder.cs

Lines 149 to 150 in e72a193

	// Disallow changing any host configuration
	throw new NotSupportedException($"The hosting startup assemblies changed from \"{previousHostingStartupAssemblies}\" to \"{value}\". Changing the host configuration using WebApplicationBuilder.WebHost is not supported. Use WebApplication.CreateBuilder(WebApplicationOptions) instead.");

If it's common enough to use non-default hosting startup assemblies, we could consider adding a property to WebApplicationOptions though. For now, does putting something like the following before WebApplication.CreateBuilder(args) work for you @nfplee?

Environment.SetEnvironmentVariable($"ASPNET_{WebHostDefaults.HostingStartupAssembliesKey}", string.Join(";", assemblies));

EDIT:

As noted down thread, the environment variable should have been prefixed with ASPNETCORE_.

Environment.SetEnvironmentVariable($"ASPNETCORE_{WebHostDefaults.HostingStartupAssembliesKey}", string.Join(";", assemblies));

[nfplee]

Thanks @halter73 but this now throws the following error (the whole error is much longer):

AggregateException: Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Routing.Matching.DfaMatcherBuilder Lifetime: Transient ImplementationType: Microsoft.AspNetCore.Routing.Matching.DfaMatcherBuilder'...

This is when saying:

var assemblies = DependencyContext.Default.GetRuntimeAssemblyNames(Environment.OSVersion.Platform.ToString())
    .Select(n => n.Name);

Environment.SetEnvironmentVariable($"ASPNET_{WebHostDefaults.HostingStartupAssembliesKey}", string.Join(";", assemblies));

WebApplication.Create(args)
    .Run();

However the following works fine:

var assemblies = DependencyContext.Default.GetRuntimeAssemblyNames(Environment.OSVersion.Platform.ToString())
    .Select(n => n.Name);

Host.CreateDefaultBuilder(args)
    .ConfigureWebHostDefaults(builder => builder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey, string.Join(";", assemblies)))
    .Build()
    .Run();

[davidfowl]

Can you show more details? What does your hosting startup do? Seems like its trying to build the service provider while configuring services maybe?

[nfplee]

@halter73 @davidfowl it looks like the hosting startup files are not called. Here's a sample application which shows it working:

SampleApplication.zip

If you comment out the old code (in the Program.cs file) and un-comment the minimal API code it does not work.

[nfplee]

Changing the environment variable prefix to "ASPNETCORE" from "ASPNET" fixed this. I'll leave this open incase you wish to make this simpler or improve the error message as mentioned above by @halter73.