原文地址：http://haacked.com/archive/2010/05/16/three-hidden-extensibility-gems-in-asp-net-4.aspx/。

ASP.NET 4 introduces a few new extensibility APIs that live the hermit lifestyle away from the public eye. They’re not exactly hidden - they are well documented on MSDN - but they aren’t well publicized. It’s about time we shine a spotlight on them.

PreApplicationStartMethodAttribute

This new attribute allows you to have code run way early in the ASP.NET pipeline as an application starts up. I mean way early, even beforeApplication_Start.

This happens to also be before code in your App_code folder (assuming you have any code in there) has been compiled.

To use this attribute, create a class library and add this attribute as an assembly level attribute. A common place to add this would be in theAssemblyInfo.cs class within the Properties folder.

Here’s an example:

[assembly: PreApplicationStartMethod(
  typeof(SomeClassLib.Initializer), "Initialize")]

Note that I specified a type and a method. That method needs to be a public static void method with no arguments. Now, any ASP.NET website that references this assembly will call the Initialize method when the application is about to start, giving this method a chance to do perform some early initialization.

public static class Initializer
{
  public static void Initialize() { 
    // Whatever can we do here?
  }
}

The primary use of this feature is to enable tasks that can’t be done withinApplication_Start because it’s too late. For example, registering build providers and adding assembly references.

Which leads us to…

BuildProvider.RegisterBuildProvider

As you might guess, if one of the key scenarios for the previously mentioned feature is to allow registering build providers, well ASP.NET better darn well allow you to register them programmatically.

Prior to ASP.NET 4, the only way to register a custom build provider was via the <buildproviders> node within web.config. But now, you can register them programmatically via a call to the new BuildProvider.RegisterBuildProvider method.

BuildProvider.RegisterBuildProvider(".foo", typeof(MyBuildProvider));

Combining the PreApplicationStartMethodAttribute with this method call means that installing a build provider can be done in one step -simply reference the assembly with the build provider and the assembly can register it for you. Whereas before, you would have to reference the assembly and then muck around with web.config.

I think I speak for us all when I say “Yay! Less junk in my web.config trunk!”

BuildManager.AddReferencedAssembly

Another new method added in ASP.NET 4 allows adding an assembly to the application’s list of referenced assemblies. This is equivalent to adding an assembly to the <assemblies> section of web.config.

As you might guess, this comes in handy when registering a custom build provider. It allows you to programmatically add references to assemblies that may be needed by your build provider.

Oh, and it’s yet another way to reduce the size of your web.config file. Who doesn’t love that? :)