Hosting Windows Forms Designers

Extender Services

The host environment needs to maintain a list of extender providers, and provide one of its own. You could probably get away without implementing these but they're pretty trivial and for completeness should definitely be included. We'll make a class called ExtenderServices which will implement IExtenderListService and IExtenderProviderService.

IExtenderListService has just one method, GetExtenderProviders. This is easily implemented with a simple ArrayList.

IExtenderProviderService exposes the same as IExtenderListService only it includes methods to add and remove the extenderproviders to and from the list. That's all we will put in this class. We instantiate it in the designer host's constructor and add its services to the servicecontainer along with the rest.

Our DesignerHost class has to implement IExtenderProvider itself. We need to do this because we want to have that (name) entry in the propertygrid, and for that, code has to be written. That said, there isn't much to it - we just want to extend all objects of type IComponent, with a new property "name" which is parenthesized. We have already written to code to wrap getting and setting component names in our DesignSite class, so the rest is simple.

internal class ExtenderServices : IExtenderListService, IExtenderProviderService
    ArrayList extenderProviders = new ArrayList();
    public ArrayList ExtenderProviders
            return extenderProviders;
    public ExtenderServices()
    public IExtenderProvider[] GetExtenderProviders()
        IExtenderProvider[] e = new IExtenderProvider[extenderProviders.Count];
        extenderProviders.CopyTo(e, 0);
        return e;
    public void RemoveExtenderProvider(System.ComponentModel.IExtenderProvider provider)
    public void AddExtenderProvider(System.ComponentModel.IExtenderProvider provider)

Implementing ISelectionService

While this service is fairly simple in what it does, the implementation is important to get right. Selected components are kept track of internally with a simple ArrayList, and the only method of note is the SetSelectedComponents method. This method has to actually look at the state of the control and shift keys to cater for the various standard selecting operations.

This class also subscribes to the IComponentChangeService, because when a component is removed the selectionservice needs to know, if it is selected. Once the deleted component has been removed from the list of selected components, if there are none left the root component is selected.

The behaviour of this class is copied from that of Visual Studio so the shift and control keys work in the same way. Clicking on an already selected component makes it the primary selection.

Implementing ITypeDescriptorFilterService

This is an easy one. When the designers add/remove/shadow properties on the components they're designing, they do so because this method tells them to. When the propertygrid requests info on the members of the component, it queries for this interface on the component's ServiceProvider. If it finds it, it uses the simple members on the interface to alter any of the properties before they are displayed.

All we have do to when we implement this interface is (for each of the three methods) get the designer for the component being passed (easy, since we've already implemented the GetDesigner method on IDesignerHost) and call the designer's PreFilterProperties and PostFilterProperties (or equivalent) methods with the attributes we were passed.

Implementing INameCreationService

This service is implemented on a different level to the rest of the services we have added so far. This example is very simple so it will actually end up being added to the same servicecontainer as the rest, but let's consider the example of Visual Studio again.

The INameCreationService interface is used by the code we've written already to come up with a name for a component being added to the design container, if none was supplied. When you add a textbox to a form in Visual Studio, it gets the name "TextBox1" if you're writing in VB, and "textBox1" if you're writing in C#. That's because this service is implemented on a project level. Remember how servicecontainers are linked together in a tree? The request is made to the ServiceContainer at the document view level but cascades up until it finds one.

It's a very simple interface to implement. You have the CreateName function, in which we will use the same naming algorithm as Visual Studio uses when you're writing in VB. We will just increment an integer counter until we find a name that isn't already in use.

The IsValidName function ensures that a name is valid. For this example we'll assume the name is going to be persisted to code at some point, and apply standard rules such as no spaces, and only alphanumeric characters are allowed. The ValidateName function just calls IsValidName, and if that returns false it throws an exception.

Implementing IUIService

This service is implemented only once and added to the top-level service container. It is how designers show messages, errors, popups and other windows. The documentation on this interface is pretty good so I won't explain what all the methods do here.

Implementing IToolboxService and IMenuCommandService

This is actually a fairly complicated interface to implement. It is the gateway between the toolbox user interface in the development environment and the designers. The designers constantly query the toolbox when the cursor is over over them to get feedback about the selected control.

Properly implementing IToolboxService is beyond the scope of this article so I will write only a skeleton implementation. It will be enough so that you can select a tool (or the pointer) and create and manipulate components on the design surface.

IMenuCommandService is looked for by some controls and components and they don't fail gracefully when it isn't found. This interface is responsible for managing standard menu commands, which the designers add to it. We will use it to execute the Delete command that has been added to it when the user hits the delete key. This is the correct way of letting the user delete components from the design surface.

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