Three Class Issues in VS .NET

It's late at night, but today I've found the third weird issue in VS .NET 2003 with classes over the last year or so. I'm sure others have run into these, although the third one is a really odd case, but I wanted to publish them anyway so you can be aware of these problems.

The first one consists of abstract user controls:

namespace Viktor.ClassFirstTests
{
    public abstract class AbstractControl : UserControl {}
}

If you open this class in the Designer, you'll have no problems. However, you will run into problems once you make a concrete class use AbstractControl as its base class:

namespace Viktor.ClassFirstTests
{
    public abstract class ImplementingControl : AbstractControl {}
}

If you try to open ImplementingControl in the Designer, you'll get an error message:

The designer must create an instance of type 'Viktor.ClassFirstTests.AbstractControl' but it cannot because the type is declared as abstract.

There's an easy way around this: make a shim class that sits between AbstractControl and ImplementingControl

namespace Viktor.ClassFirstTests
{
    public abstract class ShimControl : AbstractControl {}

    public abstract class ImplementingControl : ShimControl {}
}

This isn't an ideal solution, because any abstract methods and properties in AbstractControl will need implemetations in ShimControl (most likely no-op implementations), and then ImplementingControl needs to override these no-op implementations.

Moral of the first story? Be careful if you make abstract controls.

Here's the second issue:

namespace Viktor.ClassFirstTests
{
    public class Interference {}

    public class NotTheFirstClass : System.Windows.Forms.UserControl {}
}

If you try to open NotTheFirstClass in the Designer, you'll get the following error message:

The class NotTheFirstClass can be designed, but is not the first class in the file. Visual Studio requires that designers use the first class in the file. Move the class code so that it is the first class in the file and try loading the designer again.

Again, a simple fix: put the Interference class after the definition of NotTheFirstClass, or (better yet in my book), in another code file.

Moral of the second story? Don't put class definitions ahead of control definitions in a file.

Now the last one is really subtle:

namespace ClassFirstTests
{
    public delegate void InterferingDelegate();

    public class ResourceInterference : System.Windows.Forms.UserControl {}
}

Note that the namespace has been shortened from Viktor.ClassFirstTests to ClassFirstTests. Also, the Default Namespace project property is Viktor.ClassFirstTests. So what's the problem here? Simple - set Localizable on ResourceInterference to true, compile, and try to put ResourceInterference on a form. Uh, oh...:

I had to trim the message box a bit, but you can see that there's something wrong with the resource naming in the assembly. First, check out the generated code in InitializeComponent() for ResourceInterference:

private void InitializeComponent()
{
    System.Resources.ResourceManager resources = 
        new System.Resources.ResourceManager(typeof(ResourceInterference));
    this.AccessibleDescription = 
        resources.GetString("$this.AccessibleDescription");
    // ...
}

Since the control is localizable, the generated code initializes the control based on resource values. Note that the ResourceManager instance is based off of a type, which in this case has a full name of ClassFirstTests.ResourceInterference. However, check out what the resource is really named (via a trip to Reflector):

For some reason, VS .NET will use the Default Namespace value if the class is not the first class in the code file. I know, it sounds weird, but if you remove InterferingDelegate from the picture and recompile, everything will work as expected.

Moral of the third story? It's just like the second, but even more limiting: Strongly consider only having one class definition per file.

If you're curious to see these issues in action, you can get the code here. Let me know if you have any questions!

* Posted at 12.16.2004 01:49:35 AM CST | Link *

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