Why MDI child form not show when I insert textbox C#

Question

I'm created Form1 and create MDI child form is Form2. Form2 window state is Maximized in Parent form.

My Some Code :

bool IsOpen = false;
foreach (Form f in Application.OpenForms)
{
    if (f.Text == "Manage Model")
    {
        IsOpen = true;
        f.Focus();
        break;
    }
}

if (IsOpen == false)
{
    var fmodel = new formModel();
    fmodel.MdiParent = this;
    fmodel.WindowState = FormWindowState.Minimized;
    fmodel.Show();
    fmodel.WindowState = FormWindowState.Maximized;
}

// Close another forms
foreach (Form frm in this.MdiChildren)
{
    if (!frm.Focused)
    {
        frm.Visible = false;
        frm.Dispose();
    }
}

Form2 is blank I tried the program click menu to open Form2 It's show successfully.

I inserted label1 in to Form2 and open Form2 It's show successfully.

but When I inserted textbox1 or button1 in to Form2 and open Form2 It's don't show. It's just blink.

Who can tell me Why It be like this? Thank you.

Answer 1

This is wrong:

f.Focus();

and so is this:

if (!frm.Focused)
{
    frm.Visible = false;
    frm.Dispose();
}

They should be:

f.Activate();

and:

if (frm != ActiveMdiChild)
{
    frm.Close();
}