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

I've got a custom class, which derives from UserControl. The code:

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

This classworks like it's supposed to work. But, when I try to enter design mode of the form which contains these Gallery classes, it gives me errors:

• Could not find type 'PresentrBuilder.Forms.Gallery'. Please make sure that the assembly that contains this type is referenced. If this type is a part of your development project, make sure that the project has been successfully built.

• The variable 'pictureGallery' is either undeclared or was never assigned.

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

How can solve this? This way, I can't work in design mode which makes creating my userinterface quite hard.

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The designer hates (i.e. doesn't support) generic controls, and that isn't going to change any time soon, so don't do that. Instead, consider having a property (or similar) that accepts a Type, and do some work at runtime (reflection etc) - or: don't use the designer.

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 currently have (new, IElement, etc) - but it just can't do any validation at compile-time.

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Sometimes the easiest thing to do in this case is to make an empty subclass that qualifies the generic parameter.

This is often done with the ObservableCollection:

public class SomeItemCollection : ObservableCollection<SomeItem>{

}

It is kind of irritating, but it may solve your problems.

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Like the others have stated, the Visual Studio Designer has a lot of trouble handling generics in controls. I've run into this myself when trying to implement something like a generic 'property viewer' class.

The solution that worked for me was defining an intermediary class, like Egor said. If I understand your question correctly, for your situation, that should be something like this:

public class PictureElementGallery : Gallery<PictureElement>

Then use the PictureElementGallery on your form, instead of Gallery < PictureElement >. The designer should have no trouble with that.

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Instead of having a generic control, have the control interact with a generic class that is separate from the control itself. Then pass this class into the control.