For modless form this is the solution
Set location by manually to set modless dialog (form)center of the parent form (parent)
form.StartPosition = FormStartPosition.Manual;
form.Location = new Point(parent.Location.X + (parent.Width - form.Width) / 2, parent.Location.Y + (parent.Height - form.Height) / 2);
form..Show(parent);
In dot net 4.0 - User remove control box and FormBorderStyle not sizable
along with the set location manually:
form.ControlBox = false;
form.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedDialog;
problem is:
In dot net version 4.0 it is problem of cutoff the dialog from bottom (lower part of dialog not show when run)
solution is
Set Localizable property of the form as true