Sunday, May 17, 2009

Tips to refactor Asp.Net codebehinds to a common base page.

[This was originally posted at]

In Asp.Net, it is common to have the aspx page inherit from a custom page as opposed to directly from System.Web.UI.Page. Usually the application architecture has at least one common "base page" that handles core architectural components. However, you may find that for larger apps, you want to extend your base-page hierarchy such that an entire sub system gets its own base page (which in turn inherits from the global base page). While inheritance with normal C# classes (like in a ClassLibrary project) works great, there are some common hurdles when trying to refactor to a base page in ASP.Net codeBehinds.

Perhaps the biggest issue is the dependencies, namely:

  1. Your base page cannot directly reference any user controls. Assuming you put the page page in the AppCode folder, this gets compiled to its own separate DLL, and that DLL has no reference to the UC dll (however, the UC has a reference to AppCode).
  2. Your base page cannot directly reference the html controls instantiated in the derived-page's aspx file. For example, if your derived aspx page contains a hidden field or textbox, your base page cannot reference that.

There are ways to break these dependencies.

For problem #1 with the User Control references, you can make an interface, have that user control implement the interface, and then let the base page have a member variable of the interface type. The derived page could populate that variable. In other words:

  • Say you have UserControl "Address".
  • Your base page needs to call the "LookupZipCode" method
  • Create an interface IAddress with member "LookupZipCode". Have the UC implement this interface.
  • Have your base page contain an instance field of type IAddress. Your base page can then call this.
  • Have the derived page pass the UC reference to the base page (perhaps in the OnInit method), for example set base.IAddress = MyUserControl.

Another approach, say if you're casting, is to pass in a generic. For example, create this kind of method in your base page: Note that "T' would be a user control, but your base page cannot reference user controls.

public T HeaderControl<T>() where T : class
    return this.Master.FindControl("Header") as T;

For problem #2, with html control references, you could solve this in several ways. Keep in mind that unlike user controls, Html (or WebControls) are predefined in an external assembly - System.Web - and can hence be referenced in the base page. So the question becomes how to pass the reference from the derived to the base class?

  • Pass in references via method signature - for example the method that sets a hidden control expects an HtmlInputHidden parameter, as opposed to referencing "this.MyHiddenControl".
  • Make the base class have its own separate protected instance, and have the derived class set this (such as in the derived class's OnInit). For example, the derived class OnInit method may contain a line like "base.BaseHiddenField = thisMyHiddenField". This is useful if that control is referenced in many base-class methods, and you don't want to update all the signatures.
  • FindControl - You can always use the "Page.FindControl(string)" method to get a control given the string ID, but this is brittle and slower. Because you pass in a string, you don't get compile-time type checking. It also must search the entire control collection instead of just having a direct reference to the control.

While there's a lot more ways to refactor an codeBehind, these two tricks are useful.

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