How can I get Visual Studio 2008 Windows Forms designer to render a Form that implements an abstract base class?_问答_开发者

I engaged a problem with inherited Controls in Windows Forms and need some advice on it.

I do use a base class for items in a List (selfmade GUI list made of a panel) and some inherited controls that are for each type of data that could be added to the list.

There was no problem with it, but I now found out, that it would be right, to make the base-cont开发者_如何学Crol an abstract class, since it has methods, that need to be implemented in all inherited controls, called from the code inside the base-control, but must not and can not be implemented in the base class.

When I mark the base-control as abstract, the Visual Studio 2008 Designer refuses to load the window.

Is there a way to get the Designer work with the base-control made abstract?

I KNEW there had to be a way to do this (and I found a way to do this cleanly). Sheng's solution is exactly what I came up with as a temporary workaround but after a friend pointed out that the Form class eventually inherited from an abstract class, we SHOULD be able to get this done. If they can do it, we can do it.

We went from this code to the problem

Form1 : Form

Problem

public class Form1 : BaseForm
...
public abstract class BaseForm : Form

This is where the initial question came into play. As said before, a friend pointed out that System.Windows.Forms.Form implements a base class that is abstract. We were able to find...

Proof of a better solution

Inheritance Hierarchy:
- System.Object
  - System.MarshalByRefObject (public **abstract** class MarshalByRefObject)
    - System.ComponentModel.Component
      - System.Windows.Forms.Control
        
        System.Windows.Forms.ScrollableControl
        
        System.Windows.Forms.ContainerControl
        
        System.Windows.Forms.Form

From this, we knew that it was possible for the designer to show a class that implemented a base abstract class, it just couldn't show a designer class that immediately implemented a base abstract class. There had to be at max 5 inbetween, but we tested 1 layer of abstraction and initially came up with this solution.

Initial Solution

public class Form1 : MiddleClass
...
public class MiddleClass : BaseForm
... 
public abstract class BaseForm : Form
...

This actually works and the designer renders it fine, problem solved.... except you have an extra level of inheritance in your production application that was only necessary because of an inadequacy in the winforms designer!

This isn't a 100% surefire solution but its pretty good. Basically you use #if DEBUG to come up with the refined solution.

Refined Solution

Form1.cs

public class Form1
#if DEBUG
    : MiddleClass
#else 
    : BaseForm
#endif
...

MiddleClass.cs

public class MiddleClass : BaseForm
...

BaseForm.cs

public abstract class BaseForm : Form
...

What this does is only use the solution outlined in "initial solution", if it is in debug mode. The idea is that you will never release production mode via a debug build and that you will always design in debug mode.

The designer will always run against the code built in the current mode, so you cannot use the designer in release mode. However, as long as you design in debug mode and release the code built in release mode, you are good to go.

The only surefire solution would be if you can test for design mode via a preprocessor directive.

@smelch, There is a better solution, without having to create a middle control, even for debug.

What we want

First, let's define the final class and the base abstract class.

public class MyControl : AbstractControl
...
public abstract class AbstractControl : UserControl // Also works for Form
...

Now all we need is a Description provider.

public class AbstractControlDescriptionProvider<TAbstract, TBase> : TypeDescriptionProvider
{
    public AbstractControlDescriptionProvider()
        : base(TypeDescriptor.GetProvider(typeof(TAbstract)))
    {
    }

    public override Type GetReflectionType(Type objectType, object instance)
    {
        if (objectType == typeof(TAbstract))
            return typeof(TBase);

        return base.GetReflectionType(objectType, instance);
    }

    public override object CreateInstance(IServiceProvider provider, Type objectType, Type[] argTypes, object[] args)
    {
        if (objectType == typeof(TAbstract))
            objectType = typeof(TBase);

        return base.CreateInstance(provider, objectType, argTypes, args);
    }
}

Finally we just apply a TypeDescriptionProvider attribute to the Abastract control.

[TypeDescriptionProvider(typeof(AbstractControlDescriptionProvider<AbstractControl, UserControl>))]
public abstract class AbstractControl : UserControl
...

And that's it. No middle control required.

And the provider class can be applied to as many Abstract bases as we want in the same solution.

* EDIT * Also the following is needed in the app.config

<appSettings>
    <add key="EnableOptimizedDesignerReloading" value="false" />
</appSettings>

Thanks @user3057544 for the suggestion.

@Smelch, thanks for the helpful answer, as I was running into the same issue recently.

Following is a minor change to your post to prevent compilation warnings (by putting the base class within the #if DEBUG pre-processor directive):

public class Form1
#if DEBUG  
 : MiddleClass 
#else  
 : BaseForm 
#endif

I had a similar problem but found a way to refactor things to use an interface in place of an abstract base class:

interface Base {....}

public class MyUserControl<T> : UserControl, Base
     where T : /constraint/
{ ... }

This may not be applicable to every situation, but when possible it results in a cleaner solution than conditional compilation.

I've got some tips for people who say the TypeDescriptionProvider by Juan Carlos Diaz is not working and don't like the conditional compilation neither:

First of all, you may have to restart Visual Studio for the changes in your code to work in the form designer (I had to, simple rebuild didn't work - or not every time).

I will present my solution of this problem for the case of abstract base Form. Let's say you have a BaseForm class and you want any forms based on it to be designable (this will be Form1). The TypeDescriptionProvider as presented by Juan Carlos Diaz didn't work for me also. Here is how I made it work, by joining it with the MiddleClass solution (by smelch), but without the #if DEBUG conditional compiling and with some corrections:

[TypeDescriptionProvider(typeof(AbstractControlDescriptionProvider<BaseForm, BaseFormMiddle2>))]   // BaseFormMiddle2 explained below
public abstract class BaseForm : Form
{
    public BaseForm()
    {
        InitializeComponent();
    }

    public abstract void SomeAbstractMethod();
}


public class Form1 : BaseForm   // Form1 is the form to be designed. As you see it's clean and you do NOTHING special here (only the the normal abstract method(s) implementation!). The developer of such form(s) doesn't have to know anything about the abstract base form problem. He just writes his form as usual.
{
    public Form1()
    {
        InitializeComponent();
    }

    public override void SomeAbstractMethod()
    {
        // implementation of BaseForm's abstract method
    }
}

Notice the attribute on the BaseForm class. Then you just have to declare the TypeDescriptionProvider and two middle classes, but don't worry, they are invisible and irrelevant for the developer of Form1. The first one implements the abstract members (and makes the base class non abstract). The second one is empty - it's just required for the VS form designer to work. Then you assign the second middle class to the TypeDescriptionProvider of BaseForm. No conditional compilation.

I was having two more problems:

Problem 1: After changing Form1 in designer (or some code) it was giving the error again (when trying to open it in designer again).
Problem 2: BaseForm's controls was placed incorrectly when the Form1's size was changed in designer and the form was closed and reopened again in the form designer.

The first problem (you may not have it because it's something that haunts me in my project in few another places and usually produces a "Can't convert type X to type X" exception). I solved it in the TypeDescriptionProvider by comparing the type names (FullName) instead of comparing the types (see below).

The second problem. I don't really know why the base form's controls are not designable in Form1 class and their positions are lost after resize, but I've worked it around (not a nice solution - if you know any better, please write). I just manually move the BaseForm's buttons (which should be in bottom-right corner) to their correct positions in a method invoked asynchronously from Load event of the BaseForm: BeginInvoke(new Action(CorrectLayout)); My base class has only the "OK" and "Cancel" buttons, so the case is simple.

class BaseFormMiddle1 : BaseForm
{
    protected BaseFormMiddle1()
    {
    }

    public override void SomeAbstractMethod()
    {
        throw new NotImplementedException();  // this method will never be called in design mode anyway
    }
}


class BaseFormMiddle2 : BaseFormMiddle1  // empty class, just to make the VS designer working
{
}

And here you have the slightly modified version of TypeDescriptionProvider:

public class AbstractControlDescriptionProvider<TAbstract, TBase> : TypeDescriptionProvider
{
    public AbstractControlDescriptionProvider()
        : base(TypeDescriptor.GetProvider(typeof(TAbstract)))
    {
    }

    public override Type GetReflectionType(Type objectType, object instance)
    {
        if (objectType.FullName == typeof(TAbstract).FullName)  // corrected condition here (original condition was incorrectly giving false in my case sometimes)
            return typeof(TBase);

        return base.GetReflectionType(objectType, instance);
    }

    public override object CreateInstance(IServiceProvider provider, Type objectType, Type[] argTypes, object[] args)
    {
        if (objectType.FullName == typeof(TAbstract).FullName)  // corrected condition here (original condition was incorrectly giving false in my case sometimes)
            objectType = typeof(TBase);

        return base.CreateInstance(provider, objectType, argTypes, args);
    }
}

And that's it!

You don't have to explain anything to the future developers of forms based on your BaseForm and they don't have to do any tricks to design their forms! I think it's the most clean solution it can be (except for the controls repositioning).

One more tip:

If for some reason the designer still refuses to work for you, you can always do the simple trick of changing the public class Form1 : BaseForm to public class Form1 : BaseFormMiddle1 (or BaseFormMiddle2) in the code file, editing it in the VS form designer and then changing it back again. I prefer this trick over the conditional compilation because it's less likely to forget and release the wrong version.

I'm using the solution in this answer to another question, which links this article. The article recommends using a custom TypeDescriptionProvider and concrete implementation of the abstract class. The designer will ask the custom provider which types to use, and your code can return the concrete class so that the designer is happy while you have complete control over how the abstract class appears as a concrete class.

Update: I included a documented code sample in my answer to that other question. The code there does work, but sometimes I have to go through a clean/build cycle as noted in my answer to get it to work.

I have a tip for Juan Carlos Diaz solution. It works great for me, but was some problem with it. When I start VS and enter designer everything works fine. But after run the solution, then stop and exit it and then try to enter designer the exception appears again and again until restarting VS. But I found the solution for it - everything to do is add below to your app.config

  <appSettings>
   <add key="EnableOptimizedDesignerReloading" value="false" />
  </appSettings>

Since the abstract class public abstract class BaseForm: Form gives an error and avoid the use of the designer, I came with the use of virtual members. Basically, instead of declaring abstract methods, I declared virtual methods with the minimum body as possible. Here's what I've done :

public class DataForm : Form {
    protected virtual void displayFields() {}
}

public partial class Form1 : DataForm {
    protected override void displayFields() { /* Do the stuff needed for Form1. */ }
    ...
}

public partial class Form2 : DataForm {
    protected override void displayFields() { /* Do the stuff needed for Form2. */ }
    ...
}

/* Do this for all classes that inherit from DataForm. */

Since DataForm was supposed to be an abstract class with the abstract member displayFields, I "fake" this behavior with virtual members to avoid the abstraction. The designer doesn't complain anymore and everything works fine for me.

It's a workaround more readable, but since it's not abstract, I have to make sure that all child classes of DataForm have their implementation of displayFields. Thus, be careful when using this technique.

The Windows Forms Designer is creating an instance of the base class of your form/control and applies the parse result of InitializeComponent. That's why you can design the form created by the project wizard without even building the project. Because of this behavior you also can not design a control derived from an abstract class.

You can implement those abstract methods and throw an exception when it is not running in the designer. The programmer who derive from the control must provide an implementation that does not call your base class implementation. Otherwise the program would crash.

You could just conditionally compile in the abstract keyword without interposing a separate class:

#if DEBUG
  // Visual Studio 2008 designer complains when a form inherits from an 
  // abstract base class
  public class BaseForm: Form {
#else
  // For production do it the *RIGHT* way.
  public abstract class BaseForm: Form {
#endif

    // Body of BaseForm goes here
  }

This works provided that BaseForm doesn't have any abstract methods (the abstract keyword therefore only prevents the runtime instantiation of the class).