39

I created a generic base class for a WinForm UserControl:

public partial class BaseUserControl<T> : UserControl
{
    public virtual void MyMethod<T>() 
    { 
        // some base stuff here 
    }
}

And a UserControl based on that:

public partial class MyControl : BaseUserControl<SomeClass>
{
    public override void MyMethod<SomeClass>() 
    { 
        // some specific stuff here 
        base.MyMethod<SomeClass>();
    }
}

It works fine, but MyControl cannot be edited in the VisualStudio Designer, because it says it cannot load the base class. I tried to define another class BaseUserControl, non generic, hoping it would load it, but the trick doesn't seem to work.

I already have a workaround: define an interface, IMyInterface<T>, and then create my control as 

public partial class MyControl : UserControl, IMyInterface<SomeClass>

But I lose my base virtual methods (not a big deal, but still...).

Is there a way to create a base generic class for a UserControl, with the possiblity to edit it in the VisualStudio Designer?

Reza Aghaei
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Filini
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  • you dont have to loose your base virtual methods, contain the Implementor class instead of inherit from it. – Robocide Sep 05 '10 at 22:59
  • Starting from *VS 2015.1*, Windows Forms Designer shows classes which have generic base class without any problem. [See example](https://stackoverflow.com/a/49477226/3110834). – Reza Aghaei Dec 25 '19 at 17:08

3 Answers3

36

We're doing the same thing and we work around by specializing a class first and derive from the specialized class. Using the code from your example this means something like:

public partial class UserControl : UserControlDesignable 
{

...
}
public class UserControlDesignable : BaseUserControl<Someclass> { }

The designer is still acting flaky sometimes - but most of the time it works.

bernhardrusch
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  • While a workaround, note that this must in a extra file, and if the base is abstract and one is using a custom type provider attribute (as it is mentioned in other posts on this site) it will have to go on each class – yoel halb Aug 13 '13 at 20:48
  • still the only working solution in VS2015. No native support added by MS... :-( – Tobias Knauss Feb 12 '18 at 14:02
  • @TobiasKnauss Starting from *VS 2015.1*, Windows Forms Designer shows classes which have generic base class without any problem. [See example](https://stackoverflow.com/a/49477226/3110834). – Reza Aghaei Dec 25 '19 at 17:13
14

You'll have to trick the designer by adding a 'regular' class that inherits from your generic base form. Your designable form should then inherit from this class. The following 2 class definitions are thus in the same file. You'll have to make sure that the class that inherits from the generic base user-control, is the last class in the file.

public MyForm : EditableCustomerForm
{}

public EditableCustomerForm : GenericForm<Customer>
{}

The designer will display the first class in the code file that it encounters.

Frederik Gheysels
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  • Great idea, but note that since a base class must already be compiled before the designer can use it, it means that for any changes to the EditableCustomerForm class it will have to be rebuild for the designer to take affect, also the designer will probably work only on the MyForm class – yoel halb Aug 13 '13 at 20:47
3

Well this appears to be a bug in Visual studio.

By digging into the framework (actually by adding a RootDesignerSerializer with a custom type derived from CodeDomSerializer and overriding the serialize method), I was able to prove that the VS Code Dom provider is actually parsing wrong the generic classes, and instead of considering it as a generic class it is considering it as a regular class with the name class<T>, which Type.GetType() can of course not find.

I am still searching for a way to work around it, but in the mean time one can use the solutions above.

There is a Microsoft.Connect bug report on it, please vote on it at https://connect.microsoft.com/VisualStudio/feedback/details/797279/win-forms-designer-error-when-inheriting-from-a-generic-form

yoel halb
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