Watch Out for this Web Form Designer issue

The problem

If you see the following error when trying to load your web form designer :-

The class file for this Web Form was not loaded. 
Close this view, correct any errors, then reopen the Web Form.

Then you might have to check the code behind for your web form carefully. VS.NET complains if you have declared a class in your aspx.cs file in the following manner :-

public class CustomDropDown : Control //yada yada
public class WebForm1 : System.Web.UI.Page
{
    //yada yada
}

In other words, you have created a class outside the web form's Page class. The weird thing is that it will compile just fine. But when you try to visit the web form designer for your page, you'll either get the error message I described, or your web designer will behave in a funny manner.

The quick solution

Remove the offending class and either place it within your Page class or in a separate class file (my preferred solution).

Thoughts

Some of you might know the technical reason for this behavior and as such might scoff at this "obvious" workaround, but my argument is that the code compiles fine. As such it shouldn't have such a drastic effect on the forms designer in my opinion. And the error message implies that your code is wrong, which I don't think it is. But if anyone can educate me, then please let me know what is the difference between declaring a class outside my Page class (but within the same namespace) and in a separate file altogether (again within the same namespace)

Another Workaround (update)

Jamie Nordemeyer informs me that placing the above class simply under the Page class also solves the problem.

public class WebForm1 : System.Web.UI.Page //yada yada
public class CustomDropDown : Control //yada yada

Thanks Jamie!!

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