我的以下代码中出现以下错误:
读取器关闭时无效尝试调用Read。
我测试了我的代码时间,但我没有发现代码中的错误。谁能告诉我我的代码在哪里不正确或者为什么会出现这个错误?
/*start calculate grade of SA2 */
SqlConnection con = new SqlConnection();
con.ConnectionString = ConfigurationManager.ConnectionStrings["con1"].ConnectionString;
if (Convert.ToString(ddlexam.SelectedItem) == "SA2")
{
int SA2;
string grade = null;
SqlCommand comm;
SqlDataReader dr;
con.Open();
comm = new SqlCommand("select SA2 from terms_marks where admission_no=@admission_no and sub_id=@sub_id", con);
comm.Parameters.AddWithValue("@sub_id", sub_id);
comm.Parameters.AddWithValue("@admission_no", admission_no);
dr = comm.ExecuteReader();
while (dr.Read())
{
SA2 = Convert.ToInt32(dr["SA2"].ToString());
markspercentage = (SA2 * 100) / maxmarks;
if (markspercentage >= 91 && markspercentage <= 100)
{
grade = "A1";
}
else if ((markspercentage >= 81) && (markspercentage <= 90))
{
grade = "A2";
}
else if (markspercentage >= 71 && markspercentage <= 80)
{
grade = "B1";
}
else if (markspercentage >= 61 && markspercentage <= 70)
{
grade = "B2";
}
else if (markspercentage >= 51 && markspercentage <= 60)
{
grade = "C1";
}
else if (markspercentage >= 41 && markspercentage <= 50)
{
grade = "C2";
}
else if (markspercentage >= 33 && markspercentage <= 40)
{
grade = "D";
}
else if (markspercentage >= 21 && markspercentage <= 32)
{
grade = "E1";
}
else if (markspercentage >= 0 && markspercentage <= 20)
{
grade = "E2";
}
}
dr.Close();
cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "CreategradeDetails";
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@st_id", st_id);
cmd.Parameters.AddWithValue("@roll_no", roll_no);
cmd.Parameters.AddWithValue("@admission_no", admission_no);
cmd.Parameters.AddWithValue("@sub_id", sub_id);
cmd.Parameters.AddWithValue("@FA1", DBNull.Value);
cmd.Parameters.AddWithValue("@FA2", DBNull.Value);
cmd.Parameters.AddWithValue("@SA1", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_FirstTerm", DBNull.Value);
cmd.Parameters.AddWithValue("@FA3", DBNull.Value);
cmd.Parameters.AddWithValue("@FA4", DBNull.Value);
cmd.Parameters.AddWithValue("@SA2", grade);
cmd.Parameters.AddWithValue("@Total_SecondTerm", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_FA", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_SA", DBNull.Value);
cmd.Parameters.AddWithValue("@GrandTotal", DBNull.Value);
result = cmd.ExecuteNonQuery();
con.Close();
}
/*end calculate SA2 grade */
/* start calculate Total_SecondTerm grade;*/
if (Convert.ToString(ddlexam.SelectedItem) == "SA2")
{
int TotalST;
string grade = null;
SqlCommand comm;
SqlDataReader dr;
con.Open();
comm = new SqlCommand("select Total_SecondTerm from terms_marks where admission_no=@admission_no and sub_id=@sub_id", con);
comm.Parameters.AddWithValue("@sub_id", sub_id);
comm.Parameters.AddWithValue("@admission_no", admission_no);
dr = comm.ExecuteReader();
if (dr.Read())
{
TotalST = Convert.ToInt32(dr["Total_SecondTerm"].ToString());
dr.Close();
comm = new SqlCommand("select Total_SecondTerm from max_marks where admission_no=@admission_no and sub_id=@sub_id", con);
comm.Parameters.AddWithValue("@sub_id", sub_id);
comm.Parameters.AddWithValue("@admission_no", admission_no);
if (dr.Read())
{
total_STmaxmarks = Convert.ToInt32(dr["Total_SecondTerm"].ToString());
secondterm_percentage = (TotalST * 100) /total_STmaxmarks;
}
dr.Close();
if (secondterm_percentage >= 91 && secondterm_percentage <= 100)
{
grade = "A1";
}
else if ((secondterm_percentage >= 81) && (secondterm_percentage <= 90))
{
grade = "A2";
}
else if (secondterm_percentage >= 71 && secondterm_percentage <= 80)
{
grade = "B1";
}
else if (secondterm_percentage >= 61 && secondterm_percentage <= 70)
{
grade = "B2";
}
else if (secondterm_percentage >= 51 && secondterm_percentage <= 60)
{
grade = "C1";
}
else if (secondterm_percentage >= 41 && secondterm_percentage <= 50)
{
grade = "C2";
}
else if (secondterm_percentage >= 33 && secondterm_percentage <= 40)
{
grade = "D";
}
else if (secondterm_percentage >= 21 && secondterm_percentage <= 32)
{
grade = "E1";
}
else if (secondterm_percentage >= 0 && secondterm_percentage <= 20)
{
grade = "E2";
}
}
dr.Close();
cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "CreategradeDetails";
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@st_id", st_id);
cmd.Parameters.AddWithValue("@roll_no", roll_no);
cmd.Parameters.AddWithValue("@admission_no", admission_no);
cmd.Parameters.AddWithValue("@sub_id", sub_id);
cmd.Parameters.AddWithValue("@FA1", DBNull.Value);
cmd.Parameters.AddWithValue("@FA2", DBNull.Value);
cmd.Parameters.AddWithValue("@SA1", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_FirstTerm", DBNull.Value);
cmd.Parameters.AddWithValue("@FA3", DBNull.Value);
cmd.Parameters.AddWithValue("@FA4", DBNull.Value);
cmd.Parameters.AddWithValue("@SA2", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_SecondTerm", grade);
cmd.Parameters.AddWithValue("@Total_FA", DBNull.Value);
cmd.Parameters.AddWithValue("@Total_SA", DBNull.Value);
cmd.Parameters.AddWithValue("@GrandTotal", DBNull.Value);
result = cmd.ExecuteNonQuery();
con.Close();
}
/* end calculate Total_SecondTerm grade;*/
上面代码中的错误
comm.Parameters.AddWithValue("@sub_id",sub_id);
comm.Parameters.AddWithValue("@admission_no", admission_no);
if(dr.Read())
{
total_STmaxmarks = Convert.ToInt32(dr["Total_SecondTerm"].ToString());
答案 0 :(得分:2)
问题在于:
dr.Close(); //Here
cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandText = "CreategradeDetails";
您已在读者阅读之前关闭了连接。只有在连接打开时,阅读器才会读取。
答案 1 :(得分:1)
您正在关闭此行的读者:
dr.Close();
然后四行,你这样做:
if (dr.Read())
错误信息非常清楚:关闭后无法使用阅读器。
请注意,这是您正在使用的dr
的第二个实例,而不是第一个 - 它是您在if块中创建的实例,其中包含注释:< / p>
/*end calculate SA2 grade */
/* start calculate Total_SecondTerm grade;*/
我的建议是,你要稍微清理你的代码,以减少导致问题的混乱,并使其更难找到。此外,您应该将the using
statement与实现一次性模式的类一起使用,例如System.Data.SqlClient.SqlConnection
,System.Data.SqlClient.SqlCommand
和System.Data.SqlClient.SqlDataReader
。例如,这是编写与数据读取器交互的代码的简洁方法:
using (var conn = new System.Data.SqlClient.SqlConnection(usingConnectionString))
{
using (var command = new System.Data.SqlClient.SqlCommand(usingQuery, conn))
{
using (var reader = command.ExecuteReader())
{
// do your work here...
}
}
}
请注意,我不会致电Close
,因为一旦代码存在using
范围,就会自动完成。