TextBox get LineNumber and Input Text Handle

private void Text_body_KeyDown(object sender, KeyEventArgs e)
{

var myTextBox = (TextBox)sender;
int caret = myTextBox.CaretIndex;
int iLine = myTextBox.GetLineIndexFromCharacterIndex(caret);

if (iLine < 11)
{
e.Handled = true;
return;
}

}