Changing IHostingEnvironment.ApplicationName using ASPNETCORE_APPLICATIONNAME env variable

[Chakrygin]

Currently it not possible to change IHostingEnvironment.ApplicationName property via environment variables.

Although the variable is correctly read in the WebHostBuilder class, it is overwritten in the UseStartup method.

https://github.com/aspnet/AspNetCore/blob/d24ec01224726fa85dc7683d0f70a94d9172bc94/src/Hosting/Hosting/src/WebHostBuilder.cs#L45
https://github.com/aspnet/AspNetCore/blob/d24ec01224726fa85dc7683d0f70a94d9172bc94/src/Hosting/Hosting/src/WebHostBuilderExtensions.cs#L75

Describe the solution you'd like

Maybe this fix in UseStartup method will solve the problem:

public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType)
{
    if (string.IsNullOrEmpty(hostBuilder.GetSetting(WebHostDefaults.ApplicationKey)))
    {
        var startupAssemblyName = startupType.GetTypeInfo().Assembly.GetName().Name;

        hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
    }

    // ...
}

[analogrelay]

Thoughts @davidfowl @Tratcher ? It does seem odd that the assembly name would clobber the environment variable since it makes the env-var useless.

[Tratcher]

Why are you trying to change it? I'm told MVC stops working if you do. @rynowak

[davidfowl]

We should have resolved this with the introduction of the new interface but we dropped the ball here. We need another property here, EntryPoint, see #7315