Thursday, June 2, 2011

WebActivator: How to control the order of execution?

One of the libraries we use during web development is WebActivator. WebActivator was introduced with NuGet to solve the following problem. 

With the current NuGet version it is not possible to alter existing code. This means that if you have a NuGet package that requires some extra coding executed (for example in the global.asax), that you need to add this extra code yourself, breaking the nice experience that NuGet gives you. To solve this issue, David Ebbo introduced WebActivator. This library allows you to add startup code in separate files instead of having to add it to your global.asax. This overcomes the current limitation of NuGet.

To use this library create an App_Start folder. Add a class file to this folder and include the following code:

using System;
[assembly: WebActivator.PreApplicationStartMethod(typeof(App_Start.MyApp), "PreStart")]
namespace App_Start 
{    
public static class MySuperPackage 
{
public static void PreStart() 
{            
// Add your start logic here
}    
}
}

And that's it! Now your code will be called early on when the app starts.

After adding a few of this Start classes, we hit a problem. Some of these start scripts had a dependency on each other, meaning that the order of execution did matter. It took us some time to discover the following feature:
Ability to run code after Application_Start

When you use a WebActivator PreApplicationStartMethod attribute, the method it points to runs before your global.asax’s.  There is an alternative attribute that runs after the very first HttpModule get initialized, called PostApplicationStartMethod!

[assembly: WebActivator.PostApplicationStartMethod(typeof(App_Start.MyStartupCode), "CallMeAfterAppStart")]

After changing some of the classes to the PostApplicationStartMethod,  everything worked.

1 comment:

odinserj said...

> Some of these start scripts had a dependency on each other, meaning that the order of execution did matter.

There is yet another parameter for classes Pre and PostApplicationStartMethodAttribute, called Order (its default value is 0).

But order is matters only for startup methods in the same Assembly. Order of execution of startup methods in multiple assemblies is not determined. So, you can't place one method with Order 100 into one assembly and another method with order 200 into another assembly and expect, that everything will be ok.

Btw, there is ApplicationShutdownMethodAttribute, which runs at shutdown.

Btw, comment window is not usable at all. It is small and captcha is terrible :)