将SQL Server数据库中的字符串解析为DateTime变量... exception“字符串未被识别为有效的DateTime”。

string str,a;
DateTime dat, aa;

using (con = new SqlConnection(@"Data Source=daniyal\sqlexpress;Initial Catalog=webassign;Integrated Security=True"))
{
    con.Open();

    using(com = new SqlCommand("select * from tasks",con))
    {
        rdr = com.ExecuteReader();

        while (rdr.Read())
        {
            str = rdr.GetValue(1).ToString();
            dat = DateTime.ParseExact(str, "dd/MM/yyyy", CultureInfo.InvariantCulture);
        }
    }
}
string str,a;
DateTime dat, aa;

using (con = n