[ODS-5622] Make the ODSStartup configurable

[semalaiappan]

Update EdFi.Ods.WebApi project Appsettings as:

"StartupClassName": "CustomStartup"

EDIT: @gmcelhanon changed the sample code to include "debug" trace statements for visibility in the debug output.

Add the CustomStartup class below for testing this change:

using Autofac;
using EdFi.Ods.Api.Startup;
using EdFi.Ods.Common.Configuration;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;
using System.Diagnostics;

namespace EdFi.Ods.WebApi
{
    public class CustomStartup : OdsStartupBase
    {
        public CustomStartup(IWebHostEnvironment env, IConfiguration configuration)
            : base(env, configuration) { }

        public override void ConfigureServices(IServiceCollection services)
        {
            Debug.Print("CustomStartup: ConfigureServices...");
            base.ConfigureServices(services);
        }

        public override void ConfigureContainer(ContainerBuilder builder)
        {
            Debug.Print("CustomStartup: ConfigureContainer...");
            base.ConfigureContainer(builder);
        }
        
        public override void Configure(
            IApplicationBuilder app,
            IWebHostEnvironment env,
            IOptions<ApiSettings> apiSettingsOptions,
            IApplicationConfigurationActivity[] configurationActivities)
        {
            Debug.Print("CustomStartup: Configure...");
            base.Configure(app, env, apiSettingsOptions, configurationActivities);
        }
    }
}

[gmcelhanon]

Use TypeLoadException (see documentation here) rather than MissingMemberException which does not match semantically with what is happening here.

[gmcelhanon]

Add a blank lines at the end of the files.

[gmcelhanon]

Remove the blank line here.

[gmcelhanon]

Remove the blank line here.

[gmcelhanon]

Treat "Startup" as one word (as .NET does), not two. This means that this setting should be named StartupClassName not StartUpClassName.

[gmcelhanon]

Look at Line 26 above. You could add a line after that like the one below to read the StartupClassName setting into a variable which is then used to drive the logic in this section:

string startupClassName = config.GetValue<string>("StartupClassName");

With a value of "Startup" or a call to string.IsNullOrEmpty returning true, you would then just use the original code that was here:

webBuilder.UseStartup<Startup>();

With any other value, you'd then proceed down the path of logic you have, but I'd suggest skipping the rebuild of the configuration there (currently at line 24 of StartupFactory.cs), preferring instead an approach making the StartupFactory class non-static, and passing the startup class name value to its constructor.

Then, the webBuilder.UseStartup method overload that accepts a factory method is passed the public instance method rather than a static method, and all the logic related to building the configuration and extracting the "StartupClassName" setting is removed from the factory.

Here's my suggestion for the first part of the Main method:

string startupClassName;

using (var hostBuilderInitial = Host.CreateDefaultBuilder(args).Build())
{
    var config = hostBuilderInitial.Services.GetService<IConfiguration>();
    startupClassName = config.GetValue<string>("StartupClassName");

    AssemblyLoaderHelper.LoadAssembliesFromFolder(
        AssemblyLoaderHelper.GetPluginFolder(
            config?.GetValue<string>("Plugin:Folder") ?? string.Empty));
}

var hostBuilder = Host.CreateDefaultBuilder(args)
    .ConfigureLogging(ConfigureLogging)
    .UseServiceProviderFactory(new AutofacServiceProviderFactory())
    .ConfigureWebHostDefaults(
        webBuilder =>
        {
            webBuilder.ConfigureKestrel(
                serverOptions => { serverOptions.AddServerHeader = false; });

            if (string.IsNullOrEmpty(startupClassName) || startupClassName.EqualsIgnoreCase("Startup"))
            {
                webBuilder.UseStartup<Startup>();
            }
            else
            {
                var startupFactory = new StartupFactory(startupClassName);
                webBuilder.UseStartup(context => startupFactory.Create(context));
            }
        });

And the instance-based startup factory:

public class StartupFactory
{
    private readonly string _startupClassName;

    public StartupFactory(string startupClassName)
    {
        _startupClassName = startupClassName;
    }

    public OdsStartupBase Create(WebHostBuilderContext arg)
    {
        // Scanning logic here based on _startupClassName
    }
}

[gmcelhanon]

Change this message to:
Could not find a startup class named '{0}' that derives from OdsStartupBase.

[gmcelhanon]

Rename this setting to "StartupClassName", and default it to an empty string (which should cause the existing Startup class to be used by default, as described above). This also reduces the chance of this change breaking someone who inadvertently misses the setting in their configuration.

[gmcelhanon]

Looks pretty good now ... just one more tweak.

[gmcelhanon]

In testing, this is missing an argument from the custom startup class' constructor, which we can easily pass along.

Updated code should appear as follows:

            var startup = (OdsStartupBase)Activator.CreateInstance(
                startupType,
                webHostBuilderContext.HostingEnvironment,
                webHostBuilderContext.Configuration);

[gmcelhanon]

While we're in this file, simplify the statement by replacing the use of Where with FirstOrDefault (rather than repeating it at the end) since FirstOrDefault also accepts a predicate (so the Where can be eliminated).

[gmcelhanon]

Remove this blank line.

[gmcelhanon]

Change the capitalization of the name in this setting to StartupClassName (Startup is treated as one word, not two).