如何解决以下行中发生的输入字符串错误:
string body = String.Format(
"The following leave request was made by user {o}: \n Date: {1} \t Hours: {2} \t Reason: {3}",
username.ToString(),
TxtBoxDate.Text,
DDHours.SelectedValue+DDTimeWindow.SelectedValue,
TxtNotes.Text);