OpenFileDialog openFileDialogModele = new OpenFileDialog();
openFileDialogModele.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*";
openFileDialogModele.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory);
 
DialogResult result = openFileDialogModele.ShowDialog();
if (result == DialogResult.OK) 
{
    textBoxModele.Text = openFileDialogModele.FileName;
}