private void returnButton_Click(object sender, EventArgs e)//还书，完成
{
Book book = lendBindingSource.Current as Book;
if (book == null)
{
MessageBox.Show("请选择一本书进行操作！");
return;
}
BookShelfService.ReturnBooks(book, currentClient);//问题
lendBooks = new List <Book>();
lendBindingSource.DataSource = lendBooks;
lendBooks = BookShelfService.GetAllLentBooks(currentClient);
lendBindingSource.ResetBindings(false);
lendBindingSource.DataSource = lendBooks;
Books = new List <Book>();
booksBindingSource.DataSource = Books;
Books = BookShelfService.AllBooks();
booksBindingSource.ResetBindings(false);
booksBindingSource.DataSource = Books;
Query(1);
}