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I wanted to update a web application web.config via Powershell, and found this post really helpful.

I'm intrigued as to how to this bit of code works, particularly the $closedMethod / MakeGenericMethod stuff.

$method = [Microsoft.Sharepoint.Administration.SPServiceCollection].GetMethod("GetValue", [Type]::EmptyTypes)
$closedMethod = $method.MakeGenericMethod([Microsoft.SharePoint.Administration.SPWebService])
$webService = $closedMethod.Invoke($webApp.Farm.Services, [Type]::EmptyTypes)

Any tips?

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This uses the .NET feature of Reflection to get information about a method within a class (in this case, the GetValue method within the SPServiceCollection class, then the method is "Invoked".

The bit that might be slightly confusing is the MakeGenericMethod call, which returns a MethodInfo object which represents the method declaration of type SPWebService (with type-assigned parameters), instead of a generic declaration.

The reason this is used (I can only assume from the blog post you linked to) is that you're calling a generic method in the web service, and thus you need to pass an object of unknown type (unknown at runtime, but known at compile-time) - using GetGenericMethod lets you call the generic method with a specific type at compile time.

This page demonstrates quite clearly the difference between a generic method invoked normally, and one invoked where the type parameters are set at compile time:

http://msdn.microsoft.com/en-us/library/system.reflection.methodinfo.makegenericmethod(v=VS.90).aspx

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