public frmDiffString()
 {
     InitializeComponent();
     frmDiff = this;
 }
 private void tsbDiffString_Click(object sender, RoutedEventArgs e)
 {
     if(frmDiffString.frmDiff == null)
     {
         frmDiffString frmDiff = new frmDiffString();
         frmDiff.WindowState = WindowState.Maximized;
         frmDiff.Show();
     }
     else
     {
         frmDiffString.frmDiff.WindowState = WindowState.Maximized;
         frmDiffString.frmDiff.Show();
     }
 }