Register startup filter to insert PathBase middleware at the earliest opportunity

PathBaseRegistration.cs

using Microsoft.Extensions.Options;

internal sealed class PathBaseSettings
{
    /// <summary>
    /// The path prefix, if any, which will be stripped from the request path and added to the request path base.
    /// During development, this might be empty, but then set when the application is deployed behind a reverse proxy.
    /// Example: "/my-api"
    /// </summary>
    public string PathBase { get; set; } = string.Empty;
}

internal static class PathBaseStartupFilterExtensions
{
    /// <summary>
    /// Adds a startup filter to register the path base middleware at the earliest possible point in the pipeline.
    /// This means that path base will be applied before most other middleware (including routing).
    /// </summary>
    /// <remarks>
    /// Based on: https://andrewlock.net/using-pathbase-with-dotnet-6-webapplicationbuilder/
    /// </remarks>
    public static WebApplicationBuilder AddPathBaseStartupFilter(this WebApplicationBuilder builder)
    {
        builder.Services.Configure<PathBaseSettings>(builder.Configuration);
        builder.Services.AddTransient<IStartupFilter, PathBaseStartupFilter>();
        return builder;
    }
}

file sealed class PathBaseStartupFilter : IStartupFilter
{
    private readonly string _pathBase;

    public PathBaseStartupFilter(IOptions<PathBaseSettings> settings)
    {
        _pathBase = settings.Value.PathBase;
    }

    public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
    {
        if (string.IsNullOrEmpty(_pathBase)) return next;

        return builder =>
        {
            builder.UsePathBase(_pathBase);
            next(builder);
        };
    }
}