In order to support all of the features of the OWIN Framework, middleware components need to implement IMiddleware<T>. This is great going forward but what about existing middleware that wasn't written with OWIN Framework in mind?

The OWIN Framework contains a LegacyMiddlewareWrapper class that can be used to bridge the gap between earlier standards and the OWIN Framework.

In OWIN itself middleware is defined as a function taking IDictionary<string, object> and returning Task. Katana adds IAppBuilder extension methods to supprt 5 different ways of adding middleware to the pipeline see (http://benfoster.io/blog/how-to-write-owin-middleware-in-5-different-steps) and all 5 of these are supported by the LegacyMiddlewareWrapper class.

If you have existing code similar the sample below that uses Katana middleware or middleware that does not directly support the OWIN Framework:

public class Startup
{
  public static Configuration(IAppBuilder app)
  {
    app.UseWelcomePage("/");
  }
}

You can use it with the OWIN Framework using the LegacyMiddlewareWrapper class like this:

public class Startup
{
  public static Configuration(IAppBuilder app)
  {
    var builder = new Builder();
  
    builder.Register(new LegacyMiddlewareWrapper().UseWelcomePage("/"));
	  
    app.UseBuilder(builder);
  }
}

This code can be used in conjunction with the fluid syntax for setting up the pipeline:

public class Startup
{
  public static Configuration(IAppBuilder app)
  {
    var builder = new Builder();
	  
    builder.Register(new LegacyMiddlewareWrapper().UseWelcomePage("/"))
      .As("OWIN welcome page")
      .RunFirst();

    app.UseBuilder(builder);
  }
}

Note that legacy middleware will not play nicely with the OWIN Framework because the OWIN Framework doesn't know anything about this middleware. Legacy middleware is just a function with an AppFunc signature. This means for example that if you include authentication middleware in your pipeline the OWIN Framework will not know that it provides authentication and will not automatically ensure that the dependencies of other middleware components have been met.