C # typed <T> usercontrol in development mode gives error

Question

I have a custom class that comes from UserControl . The code:

 public partial class Gallery<T> : UserControl where T : class, IElement, new() 

This class is how it should work. But when I try to enter the design mode of the form containing these Gallery classes, it gives me errors:

• Could not find type 'PresentrBuilder.Forms.Gallery'. Make sure the assembly that contains this type. If this type is part of your development, make sure that the project is successfully embedded.

• The variable 'pictureGallery' is either not declared or has never been assigned.

Note: ( pictureGallery actually has a Gallery<PictureElement> ).

How can this be solved? Thus, I cannot work in development mode, which makes it difficult to create my user interface.

Answer 1

The designer hates (i.e. does not support) the common controls, and this will not change soon, so do not do this. Instead, consider having a property (or the like) that accepts Type , and do some work at runtime (reflection, etc.) - or: don't use a constructor.

For example, if you have:

 public Type ControlType {get;set;} // comparable to T in the original 

You can use:

 IElement el = (IElement) Activator.CreateInstance(ControlType); 

This will give you everything you have ( new , IElement , etc.), but it just cannot perform the check at compile time.

Answer 2

Sometimes the easiest way to do this is to make an empty subclass that qualifies the general parameter.

This is often done using an ObservableCollection:

 public class SomeItemCollection : ObservableCollection<SomeItem>{ } 

This is annoying, but it can solve your problems.

Answer 3

Like others, the Visual Studio developer has many problems handling generics in controls. I came across this myself, trying to implement something like a generic property view class.

The solution that worked for me determined the intermediate class, as Egor said. If I understand your question correctly, for your situation it should be something like this:

 public class PictureElementGallery : Gallery<PictureElement> 

Then use the PictureElementGallery in your form, not the <PictureElement> gallery. The designer does not have to worry about this.

Answer 4

Instead of having a common control, control its interaction with a common class that is separate from the control itself. Then pass this class to the control.