从Toolbox拖动控件时，ControlDesigner不活动

Question

I'm trying to add designer support for my custom WinForms control, but it looks designer is only active when I already have instance created, not during drag'n'drop from Toolbox.

To show what I mean I've created simple control with designer:

[Designer(typeof(MyButtonDesigner))]
public class MyButton:Button
{
    public MyButton()
    {
        base.Size= new Size(50,50);
    }
}
class MyButtonDesigner : ControlDesigner
{
    public override bool CanBeParentedTo(IDesigner parentDesigner)
    {
        return parentDesigner != null && parentDesigner.Component is Form;
    }
}

I'd like my control to by hosted only by form (user will be able to add control only to form, not to groupbox). When I drag control from Toolbox my validation logic is skipped, but when I try to move my control instance from form to groupbox I can see that drop is validated (as shown below)

I'm quite new to ControlDesigner so I'm not sure if this behavior is by design or can I change this so that my validation will work when dragging from Toolbox.

I'm using Visual Studio 2013, but I don't think this should be an issue.

Answer 1

CanBeParentedTo method is not called when an item is dragged from the Toolbox onto the design surface.
You can create a custom ToolBoxItem for your control and then override CreateComponentsCore method to prevent creating control when the parent is not Form . The ToolBoxItem will be used when dragging control from ToolBox and the Designer will be used when dragging control from design-surface.

//Add reference to  System.Drawing.dll then using
using System.Drawing.Design;
using System.Windows.Forms.Design;
public class MyButtonToolBoxItem:ToolboxItem
{
    protected override IComponent[] CreateComponentsCore(IDesignerHost host,
        System.Collections.IDictionary defaultValues)
    {
        if(defaultValues.Contains("Parent"))
        {
            var parent = defaultValues["Parent"] as Control;
            if(parent!=null && parent is Form)
                return base.CreateComponentsCore(host, defaultValues);
        }

        var svc = (IUIService)host.GetService(typeof(IUIService));
        if (svc != null) svc.ShowError("Can not host MyButton in this control");

        return null;
    }
}

To register the toolbox item for your control:

[ToolboxItem(typeof(MyButtonToolBoxItem))]
[Designer(typeof(MyButtonDesigner))]
public class MyButton : Button

As result, when you drag your control from toolbox and drop it on any container other than form, it shows a message to the user and nothing happens and it will not added to that parent. If you like, you can remove the messagebox.