通用表单和VS设计器

Question

I have a base class

internal partial class View<T> : UserControl
  where T : class
{
    protected T t;
}

and I want to derive a child from View

internal partial class ViewChild<T> : View<T>
  where T : class
{
}

it works OK, but I cannot edit ViewChild in the VS designer. I know the problem is generic base class. However I do not understand how I can avoid that in this case. Is there any way to fix that?

Answer 1

There is another way, and it doesn't rely on compiler flags:

http://wonkitect.wordpress.com/2008/06/20/using-visual-studio-whidbey-to-design-abstract-forms/

I really wouldn't advise the use of conditional compilation. Much better to work with the framework, and not against it.

Basically, you can give VS a different class through the existing framework. You decorate your base class with a TypeDescriptionProvider attribute which tells VS to use a different class as a designer.

As mentioned in the original blog post, there may be caveats associated with this workaround, but I got it working neatly on a project with > 25 UserControls inheriting from a common base class.

Answer 2

Generics break the designer because it cannot instantiate the class without a type T . I explain a workaround in my blog post:

http://adamhouldsworth.blogspot.co.uk/2010/02/winforms-visual-inheritance-limitations.html

In short, you need to "resolve" the type with an intermediary class:

• BaseControl<T> : UserControl
• CustomerControl_Design : BaseControl<Customer>
• CustomerControl : CustomerControl_Design

You can then conditionally switch this class out of the code based on the DEBUG or RELEASE compiler switches:

#if DEBUG

namespace MyNamespace
{
    using System;


    public partial class CustomerEditorControl_Design : BaseEditorControl<Customer>
    {
        public CustomerEditorControl_Design()
            : base()
        {
            InitializeComponent();
        }
    }
}

#endif

    public partial class CustomerEditorControl
#if DEBUG
        : CustomerEditorControl_Design
#else
        : BaseEditorControl<Customer>
#endif
    {
    }

This will let you open the derived class of CustomerControl , unfortunately you will never be able to design a UI control with generics in the signature. My solution is only enabling the design of derived items.

I have no idea why CustomerControl : BaseControl<Customer> won't work as in this case the type T is defined, but it simply doesn't - I'm guessing because of the rules of generic usage.

To their defense, Microsoft do say that this isn't supported.