34. Windows Forms MDI

34.1 How can I create an MDI application in the .NET framework with C#?

One solution is to use a panel that has a picturebox placed on it with DockStyle.Fill. This will make the picturebox assume the size of the panel. In addition, set the DockPadding.All property to the width of the desired border. Then in the Panel's OnPaint method, call the baseclass and then paint the desired borders.

Here are both VB and C# projects that illustrate how you might go about this. The derived PicturePanel class has properties that allow you to set the bordersize and color as well as the image that is to be displayed. This sample retrieves the image from an embedded resource. It also uses double buffering to minimize flashing as you resize the control.

34.2 I have an MDI application with several child forms. The child form's Activated event is not being fired consistently as different child forms are activated. What's wrong?

In .Net 1.0, the child forms do not get the Form.Activated event (only the parent MDI). To catch MDI children being activated, listen to the Enter/Leave events of that child Form or listen to the Form.MdiChildActivate event in the parent Form.

In 1.1 the child Forms do get the Activated event.

34.3 In an MDI application, the MDI child's MaximumSize and MinimumSize properties don't seem to take effect. How can I restrict the size of my MDI child?

It appears that this behavior is a bug that will be corrected in a future .NET release.

You can control the size of your child form by adding a Layout event handler for it. Here is a code snippet that imposes the minimum size that you set in its properties. You can also handle it by overriding the form's WndProc method as explained in this Microsoft KB article.

34.4 How do I check to see if a child form is already displayed so I don't have two instances showing?

Here are two ways you can do this.

i) Within the parent MDI form, use code such as this:

ii) Here is a second solution suggested by John Conwell that implements a singleton pattern on the child form.

In the MDI Child form put this code in (where frmChildForm is the MDI child form you want to control)

In the Parent MDI form use the following code to call the child MDI form

The first time this code is called, the static GetInstance method will create and return an instance of the child form. Every other time this code is called, the GetInstance method will return the existing instance of the child from, stored in the static field childForm. If the child form instance is ever destroyed, the next time you call GetInstance, a new instance will be created and then used for its lifetime.

Also, if you need constructors or even overloaded constructors for your MDI Child Form, just add the needed parameters to the GetInstance function and pass them along to the class constructor.

34.5 I need to perform certain custom processing whenever an MDI child form is added to/removed from the MDIContainer form. How do I determine this?

MDIContainer forms have an MDIClient child window and it is to this MDIClient window that MDI child forms are parented. The MDIClient's ControlAdded/ControlRemoved events will be fired whenever a child form is added or removed. You can subscribe to these events and add the required processing code from within the handlers.

34.6 How do I paint in my mdi container, a logo, for example?

You should not try listening to your MDI container Form's Paint event, instead listen to the Paint event of the MDIClient control that is a child of the mdi container form. This article provides you a detailed example: Painting in the MDI Client Area

34.7 How do I make my child Form fill the entire mdi client without being maximized?

Here is how it can be done. This takes into account all docked controls (including menus) in the mdi parent form.

34.8 How can I change the background of my MDI Client container?

The default behavior is to make the client container use the Control color from the Control panel. You can change this behavior by making the MDI Client container use the form's BackColor and Image. To do this, after the call to InitializeComponents(), add the code below. You can also download a working MDI Client project that has this code in it.