How to open FormC as an MDI_Child of FormA, from FormB

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I am a beginner on C# .NET, trying to code out something. I have three Windows.Form:

1.	MainForm (MDI_FORM)
2.	Child1
3.	Child2

On MainForm, I commanded Child1.Show()
Then from Child1, I intend to assign and open Child2 as a child to MainForm via the codes below:

    On MainForm, I declared:

         public static Child2 child2;


    Then on btnOpen from Child1, I coded:

         private void btnOpen_Click(object sender, EventArgs e)
            {
                MainForm mainForm = new MainForm();
                mainForm.child2 = new Child2 ();
                mainForm.child2.MdiParent = fMainForm;
                mainForm.child2.Show();
            }

Someone help me on what to do please.
It's not working.
Thanks a lot!!!

Posted 8-Apr-14 23:26pm

Salisu Shaibu

Updated 9-Apr-14 12:34pm

v3

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johannesnestler · Accepted Answer · 2014-04-08T23:46:00

Hi Salisu Shaibu,

Your code doesn't work because you are creating a new instance of your MainForm. The trick is to use a reference to the MainForm and set it as MdiParent to your second Form. Have a look at this (runnable) example:

C#

using System;
using System.Windows.Forms;

namespace MdiOhMy
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Form formMain = new Form { IsMdiContainer = true };

            Form formChild1 = new Form { Text = "Child1", MdiParent = formMain, Visible = true };
            Button buttonShowChild2 = new Button { Text = "Open Child2", Parent = formChild1 };
            buttonShowChild2.Click += (object sender, EventArgs e) =>
                {
                    Form formChild2 = new Form { Text = "Child2", MdiParent = formMain, Visible = true };
                };

            Application.Run(formMain);
        }
    }
}

In this example I can reference the formMain instance because it's in the captured scope. In a more realistic Scenario you can obtain a reference via Child1's MdiParent property.

So your handler could look like this:

C#

private void btnOpen_Click(object sender, EventArgs e)
 {
     MainForm mainForm = this.MdiParent as MainForm; // "this" is Child1 which refernces it's MdiParent
     MainForm.child2 = new Child2();
     MainForm.child2.MdiParent = fMainForm;
     MainForm.child2.Show();
 }

Also think about if you want to create a new instance of Child2 every time you click your button, or just hide/reshow the same instance...

Happy coding

Johannes