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How to: Create MDI child forms

MDI child forms are an essential element of Multiple-Document Interface (MDI) applications, as these forms are the center of user interaction.

In the following procedure, you'll use Visual Studio to create an MDI child form that displays a RichTextBox control, similar to most word-processing applications. By substituting the System.Windows.Forms control with other controls, such as the DataGridView control, or a mixture of controls, you can create MDI child windows (and, by extension, MDI applications) with diverse possibilities.

Create MDI child forms

  1. Create a new Windows Forms application project in Visual Studio. In the Properties window for the form, set its IsMdiContainer property to true and its WindowsState property to Maximized.

    This designates the form as an MDI container for child windows.

  2. From the Toolbox, drag a MenuStrip control to the form. Set its Text property to File.

  3. Click the ellipsis (…) next to the Items property, and click Add to add two child tool strip menu items. Set the Text property for these items to New and Window.

  4. In Solution Explorer, right-click the project, and then select Add > New Item.

  5. In the Add New Item dialog box, select Windows Form (in Visual Basic or in Visual C#) or Windows Forms Application (.NET) (in Visual C++) from the Templates pane. In the Name box, name the form Form2. Select Open to add the form to the project.


    The MDI child form you created in this step is a standard Windows Form. As such, it has an Opacity property, which enables you to control the transparency of the form. However, the Opacity property was designed for top-level windows. Do not use it with MDI child forms, as painting problems can occur.

    This form will be the template for your MDI child forms.

    The Windows Forms Designer opens, displaying Form2.

  6. From the Toolbox, drag a RichTextBox control to the form.

  7. In the Properties window, set the Anchor property to Top, Left and the Dock property to Fill.

    This causes the RichTextBox control to completely fill the area of the MDI child form, even when the form is resized.

  8. Double click the New menu item to create a Click event handler for it.

  9. Insert code similar to the following to create a new MDI child form when the user clicks the New menu item.


    In the following example, the event handler handles the Click event for MenuItem2. Be aware that, depending on the specifics of your application architecture, your New menu item may not be MenuItem2.

    Protected Sub MDIChildNew_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MenuItem2.Click
       Dim NewMDIChild As New Form2()
       'Set the Parent Form of the Child window.
       NewMDIChild.MdiParent = Me
       'Display the new form.
    End Sub
    protected void MDIChildNew_Click(object sender, System.EventArgs e){
       Form2 newMDIChild = new Form2();
       // Set the Parent Form of the Child window.
       newMDIChild.MdiParent = this;
       // Display the new form.
       void menuItem2_Click(System::Object ^ sender,
          System::EventArgs ^ e)
          Form2^ newMDIChild = gcnew Form2();
          // Set the Parent Form of the Child window.
          newMDIChild->MdiParent = this;
          // Display the new form.

    In C++, add the following #include directive at the top of Form1.h:

    #include "Form2.h"
  10. In the drop-down list at the top of the Properties window, select the menu strip that corresponds to the File menu strip and set the MdiWindowListItem property to the Window ToolStripMenuItem.

    This enables the Window menu to maintain a list of open MDI child windows with a check mark next to the active child window.

  11. Press F5 to run the application. By selecting New from the File menu, you can create new MDI child forms, which are kept track of in the Window menu item.


    When an MDI child form has a MainMenu component (with, usually, a menu structure of menu items) and it is opened within an MDI parent form that has a MainMenu component (with, usually, a menu structure of menu items), the menu items will merge automatically if you have set the MergeType property (and optionally, the MergeOrder property). Set the MergeType property of both MainMenu components and all of the menu items of the child form to MergeItems. Additionally, set the MergeOrder property so that the menu items from both menus appear in the desired order. Moreover, keep in mind that when you close an MDI parent form, each of the MDI child forms raises a Closing event before the Closing event for the MDI parent is raised. Canceling an MDI child's Closing event will not prevent the MDI parent's Closing event from being raised; however, the CancelEventArgs argument for the MDI parent's Closing event will now be set to true. You can force the MDI parent and all MDI child forms to close by setting the CancelEventArgs argument to false.

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