Sample code demonstrating the basics of writing OWIN middleware. Associated blog article: http://json.codes/blog/basics-of-writing-owin-middleware/

BasicMiddleware.cs

using System.Threading.Tasks;
using Microsoft.Owin;

public class BasicMiddleware : OwinMiddleware
{
    public BasicMiddleware(OwinMiddleware next) : base(next)
    {
            
    }

    public async override Task Invoke(IOwinContext context)
    {
        await Next.Invoke(context);
    }
}

BasicMiddleware.impl.cs

using System;
using System.Net;
using System.Threading.Tasks;
using Microsoft.Owin;
using Newtonsoft.Json;

public class BasicMiddleware : OwinMiddleware
{
    private static readonly PathString Path = new PathString("/my-middleware-url");

    public BasicMiddleware(OwinMiddleware next) : base(next)
    {

    }

    public async override Task Invoke(IOwinContext context)
    {
        if (!context.Request.Path.Equals(Path))
        {
            await Next.Invoke(context);
            return;
        }

        var responseData = new { Date = DateTime.Now };

        context.Response.StatusCode = (int)HttpStatusCode.OK;
        context.Response.ContentType = "application/json";
        context.Response.Write(JsonConvert.SerializeObject(responseData));
    }
}

BasicMiddlewareAppBuilderExtensions.cs

using Owin;

public static class BasicMiddlewareAppBuilderExtensions
{
    public static void UseBasicMiddleware(this IAppBuilder app)
    {
        app.Use<BasicMiddleware>();
    }
}

Startup.Basic.cs

using Owin;

public partial class Startup
{
    private void ConfigureBasicMiddleware(IAppBuilder app)
    {
        app.Use<BasicMiddleware>();
    }
}