Getting started with OWIN

Owin is an Open Web Interface for .NET, it describes how components in a HTTP pipeline should communicate, like NodeJS Connect or Rack.

Owin has one main interface, every components will communicate through this interface.

Func<IDictionary<string, object>,Task>

//Actual implementation
public Task Invoke(IDictionary<string, object> environment) 
{ 
   var someObject = environment[“some.key”] as SomeObject; 
   // etc… 
}

Owin specs contains details of IDictionary object.

Owin host will composes multiple middlewares into a application. It will implement the following interface

public interface IAppBuilder 
{ 
    IDictionary<string, object> Properties { get; }

    object Build(Type returnType); 
    IAppBuilder New(); 
    IAppBuilder Use(object middleware, params object[] args); 
}

We compose middlewares in Configuration method of Startup class. When OwinHost starts, it looks into Configuration method to build up middlewares of our app.

public class Startup
{
  public void Configuration(IAppBuilder builder)
  {
      builder.Use(typeof(Logger));      
  }
}

Let build a simple logger and a request handler middleware

Create new blank MVC4 (or MVC5) project OwinTest
Add the following packages:

Install-Package Owin
Install-Package Install-Package Microsoft.Owin.Host.HttpListener -Pre
Create a new class Logger and enter the following code

using System.Collections.Generic;
using System.Threading.Tasks;
using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;

public class Logger
{
    private readonly AppFunc _next;
    public Logger(AppFunc next)
    {
        if(next == null)
        {
          throw new ArgumentNullException("next");
        }
        _next = next;
    }
    public Task Invoke(IDictionary<string, object> environment)
    {
        System.Diagnostics.Trace.WriteLine(string.Format("{0} {1}",DateTime.Now, environment["owin.RequestPath"]));
        return _next(environment);
    }
}

Create a class Startup and enter the following code

public class Startup
{
  public void Configuration(IAppBuilder builder)
  {
      builder.Use(typeof(Logger));      
  }
}

Download Katana hosting, extract to C:\ (or any folder)
Open Project property, confiure Start external program
Press F5 and browse http://localhost:8080 the console will show like this
Add a class HelloRequestHandler and enter the following code

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Threading.Tasks;
using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;

public class HelloRequestHandler
{
    private readonly AppFunc _next;
    public HelloRequestHandler(AppFunc next)
    {
        if(next == null)
        {
            throw new ArgumentNullException("next");
        }
        _next = next;
    }
    public Task Invoke(IDictionary<string, object> environment)
    {
        IDictionary<string, string[]> responseHeaders =
                (IDictionary<string, string[]>)environment["owin.ResponseHeaders"];
        var responseBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(
                     string.Format("<h1>Hello world</h1>"));
        Stream responseStream = (Stream)environment["owin.ResponseBody"];

        responseHeaders["Content-Length"] = new string[] { responseBytes.Length.ToString(CultureInfo.InvariantCulture) };
        responseHeaders["Content-Type"] = new string[] { "text/html" };
        responseStream.Write(responseBytes, 0, responseBytes.Length);

        return _next(environment);
    }
}

Add HelloRequestHandler middleware to Startup class

public class Startup
{
  public void Configuration(IAppBuilder builder)
  {
      builder.Use(typeof(Logger));
      builder.Use(typeof(HelloRequestHandler));
  }
}

Press F5 and browse http://localhost:8080 we will see the result like this

ASP.NET MVC5, WEB API 2, SignalR are compatible with OWIN, so we can compose middlewares or develop middlewares for our application's need.