What does a Visual Studio 2013 Project makes it a katana Project?

Question

I just started my struggle to understand owin and katana. Following the Asp.Net tutorial I created a blank asp.net project in VS2013 and added a Nuget Package reference to Microsoft.Owin.Host.SystemWeb. The project I created is bear blank as shown.

This contains nothing except AssemblyInfo.cs, Web.config and packages.config. Now when I run(F5) this, it says

• No assembly found containing an OwinStartupAttribute.
• No assembly found containing a Startup or [AssemblyName].Startup class. To disable OWIN startup discovery, add the appSetting owin:AutomaticAppStartup with a value of "false" in your web.config. To specify the OWIN startup Assembly, Class, or Method, add the appSetting owin:AppStartup with the fully qualified startup class or configuration method name in your web.config.

Now the question is how come just by adding a Nuget reference to Microsoft.Owin.Host.SystemWeb, it started to look for something specific to Owin like Startup class and so on as indicated in the error message? I mean I ran a different project without that Nuget reference and the error message is totally different. Nothing seems to have changed at least in the two files AssemblyInfo.cs, Web.config by adding the Nuget reference. As I understand adding the Nuget added a packages.config file and added some project reference. Also I have compared the project properties for the two projects tab by tab and I did not find any difference! So I wonder what in the world is causing the Owin project look for a Startup class?

Answer 1

The secret is that Katana uses an ASP.NET feature called PreAppStart. You can see the source codes here:

https://katanaproject.codeplex.com/SourceControl/latest#src/Microsoft.Owin.Host.SystemWeb/PreApplicationStart.cs

If an assembly in an ASP.NET app has this assembly-level attribute:

[assembly: PreApplicationStartMethod(typeof(PreApplicationStart), "Initialize")]

Then ASP.NET will automatically run that code as the app starts. This code will run before "user" code will run, before even the Application_Start event. That's why it's called PreAppStart.

In the case of Katana, this code dynamically registers an ASP.NET HTTP Module (IHttpModule) that will eventually search for and attempt to call the app's startup/builder class. And if that fails, kablamo!

To disable the automatic behavior, add this line to web.config in the <appSettings> section:

<add key="owin:AutomaticAppStartup " value="false" />

More information on this behavior can be found on the www.asp.net site: http://www.asp.net/aspnet/overview/owin-and-katana/owin-startup-class-detection (same as the commenter mentioned).