Child form opens on first screen when user has dragged the main form to the second screen

Question

My Application's Main Form opens on the Laptop (main screen) when it starts. Then user drags it to the other screen (for big display), and opens a child form that is displayed on the Laptop screen rather than Application's Main Form (big display). I want the child form to open on the screen where Application's Main Form is open at the moment.

I tried following options, but they only worked in debug mode and did not work in production

ChildForm.ShowDialog((IWin32Window)this.MainForm);
ChildForm.ShowDialog(formMainInstance);
ChildForm.Show(formMainInstance);

I know about FormStartPosition.CenterParent, but it's not the right option for me. How can I do this?

When do you set the FormStartPosition to CenterParent? (in code or in the designer) — Steve, Jan 21 '22 at 14:53
I dont want to use FormStartPosition to CenterParent because child form opens on top left corner — Tim, Jan 21 '22 at 14:55
What is this.MainForm? What is this here? What is formMainInstance? What are these coming from? -- The new Form should open where the Owner is located. How should it open on top left corner? You're not setting anything that causes that, unless you have just set `StartPosition` to `Manual` without specifying *where* to start from. -- Could be useful: [Using SetWindowPos with multiple monitors](https://stackoverflow.com/a/53026765/7444103) — Jimi, Jan 21 '22 at 15:39
In other *words*, you could set: `var screen = Screen.FromControl(this); ChildForm.Location = screen.WorkingArea.Location; ChildForm.Show(this);` where `this` is the Owner Form and `StartPosition` is set to `Manual`. — Jimi, Jan 21 '22 at 15:42
@Jimi this.MainForm is Windows Form property ApplicationContext.MainForm that gets or sets the form to use as context. however formMainInstance is the instance of my mainform — Tim, Jan 21 '22 at 15:43

score 0 · Answer 1 · answered Jan 21 '22 at 15:20

As far as I know you have 3 options:

Use FormStartPosition.CenterScreen;

     private void button2_Click(object sender, EventArgs e)
 {          
     Form2 child = new Form2();
     child.StartPosition = FormStartPosition.CenterScreen;
     child.ShowDialog(this);
 }

Use FormStartPosition.CenterParent

private void button2_Click(object sender, EventArgs e)
 {          
     Form2 child = new Form2();
     child.StartPosition = FormStartPosition.CenterParent;
     child.ShowDialog(this);
 }

}

Use FormStartPosition.Manual and pass a point

    private void button2_Click(object sender, EventArgs e)
 {          
     Form2 child = new Form2();
     child.StartPosition = FormStartPosition.Manual;
     child.Location = new System.Drawing.Point(0, 0);
     child.ShowDialog(this);
 }

Tim · Accepted Answer · 2022-02-03T14:42:30.993

The following code worked for me.

var screen = Screen.FromControl(childFormInstance);
var MainFormScreen = Screen.FromControl(_formMainInstance);
ChildForm.Left = MainFormScreen.WorkingArea.Left + 120;
ChildForm.Top = MainFormScreen.WorkingArea.Top + 120;
ChildForm.ShowDialog();

First I query which screen the child form want to open
Then I query which screen my Main form is currently open
Then I overwrite the child form screen

Child form opens on first screen when user has dragged the main form to the second screen

2 Answers2