How to Get a Reference to Parent Form from a Component

Introduction

I have been working on a custom component that inherits from and extends the BindingSource component (more detailed article in the works). I needed to get a reference to the instance of the parent form the component was on. This proved to be very difficult.

Background

Components don't have a .Parent property like controls do. They do have a container property, but it does not have a parent property either. Sometimes casting the container as a ContainerControl works (it does have a parent property), but in this case it won't cast correctly.

All Windows Designer forms add a container called "components" and all components are added to this container and show at the bottom of the screen in design mode. From inside the component, it is easy to get a reference to the container, but I could not get a reference to the parent form of the container?

I got some useful help from "nobugz" (Hans Passant – Microsoft MVP) on the Windows Form Forum on MSDN who found that the Error Provider component manages to get a reference to the parent form and exposes it in a property called "ContainerControl". The secret the Error Provider uses is to override the Site function of the component and capture IDesignerHost service.

The Code

It turns out that the solution is even simpler. All that is needed is to reference the Site property of the base component and get the designer host information from it. Here are the steps to achieve the goal:

1. Create a new item in your Windows Project and use the "Component" template.
2. Switch to "Code View" and add the following code:
   VB.NET

   Imports System.ComponentModel.Design 
   … 
   Private _form As Form
   Private Sub GetFormInstance() ' called from constructor 
      Dim _host As IDesignerHost = Nothing 
      If MyBase.Site IsNot Nothing Then _host = _ 
      CType(MyBase.Site.GetService(GetType(IDesignerHost)), IDesignerHost) 
      If _host IsNot Nothing Then _form = CType(_host.RootComponent, Form) 
   End Sub

3. Open the Designer code (you may have to toggle the "Show All Files" button at the top of the Solution Explorer and then expand the files under the component to see the file with the .Designer extension).
4. At the end of the default constructor, add a call to the GetFormInstance function. The code should look like this:
   VB.NET

   <System.Diagnostics.DebuggerNonUserCode()> _ 
   Public Sub New() 
   
      MyBase.New() 
   
      'This call is required by the Component Designer. 
      InitializeComponent() 
   
      'Added by developer 
      GetFormInstance() 
   
   End Sub

5. Now switch back to the user partial class for the component and add whatever code is needed to manipulate the parent form.
6. Create a Form in your Windows Application and add the custom component (it should be listed at the top of the Toolbox after you build).

That's it. It is fairly simple once you understand the magic that happens in the ISite service interface that allows applications to interact with the designer (Visual Studio in this case).

History

• 19^th October, 2007: Initial post