c# - ExecuteReader- System.Data.dll 中出现类型为 'System.InvalidOperationException' 的异常，但未在用户代码中处理

Question

每次尝试运行此特定网页时，我都会收到上述错误。我正在添加一些代码和数据库结构的图像(作为链接给出)。在错误的附加信息行中，它表示在数据表为空时尝试读取数据。当我回去检查表数据时，有数据存在。请帮忙!

SqlConnection con = new SqlConnection(@"ConnectionString");
protected void Page_Load(object sender, EventArgs e)
{
    string checkdu = "select Top 1 Hour, Minute, Hour1, Minute1 from Clockin";
    SqlCommand cmddu = new SqlCommand(checkdu, con);
    con.Open();
    SqlDataReader dru = cmddu.ExecuteReader();
    int td1 = (Convert.ToInt16(dru["Hour"]) * 60) + (Convert.ToInt16(dru["Minute"]));      //error shown here[enter image description here][1]
    int td2 = (Convert.ToInt16(dru["Hour1"]) * 60) + (Convert.ToInt16(dru["Minute1"]));

Answer 1

你必须使用 SqlDataReader.Read使读者前进到下一条(第一条)记录:

using(var dru = cmddu.ExecuteReader())
{
    if(dru.Read())
    {
        int td1 = (Convert.ToInt16(dru["Hour"]) * 60) + (Convert.ToInt16(dru["Minute"]));      //error shown here[enter image description here][1]
        int td2 = (Convert.ToInt16(dru["Hour1"]) * 60) + (Convert.ToInt16(dru["Minute1"]));
    }
}

Remarks :

The default position of the SqlDataReader is before the first record. Therefore, you must call Read to begin accessing any data.