Question

我犯错误请告诉我....
当我点击插入按钮时，它会显示此消息

注册号已经存在尝试另一个。

但我每次都尝试新的注册号码。

private void button1_Click(object sender, RoutedEventArgs e)   
{  
cmd.Connection = cn;
cmd.CommandText = "select count(*) from studentreg where registrationno='" + txtrgno.Text + "'";
cn.Open();
dr = cmd.ExecuteReader();
if (dr.Read())  
{  
MessageBox.Show("Registration No. Already Exists Try Another", "Error", MessageBoxButton.OK,     MessageBoxImage.Error);
txtrgno.Text = "";
txtrgno.Focus();
if ((dr != null))  
{  
dr.Close();
}  
return;  
}  
//cn.Close();  
try  
{  
string gen="";  
if(rdb1_m.IsChecked==true)  
{  
gen="Male";  
}  
else if(rdb2_f.IsChecked==true)  
{  
gen="Female";  
}  
string filename = "";  
FileStream fs;  
byte[] x=null;  
if (string.IsNullOrEmpty(filename) == false)  
{  
fs = new FileStream(filename, FileMode.Open, FileAccess.Read);  
BinaryReader br = new BinaryReader(fs);  
x = br.ReadBytes((int)(fs.Length));  
}  
cmd.Connection = cn;
SqlParameter p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16, p17, p18,     p19, p20, p21, p22, p23, p24, p25, p26, p27, p28, p29, p30, p31, p32, p33;
cmd.CommandText = @"insert into studentreg values(@rgno1, @sname1, @dob1, @bloodgroup1, @gender1, @imark11, @imark12, @photo1, @fname1, @fq1, @fo1, @mname1, @mq1, @mo1, @mton1, @religion1, @nationality1, @state1, @distric1, @mobile1, @email1, @caddr1, @paddr1, @academicy1, @adclass1, @adsec1, @addate1, @lschool1, @lclass1, @pyear1, @estudent1, @eclass1, @esec1)";  
p1 = cmd.Parameters.Add("@rgno1", SqlDbType.VarChar, 100);  
p2 = cmd.Parameters.Add("@sname1", SqlDbType.VarChar, 100);  
p3 = cmd.Parameters.Add("@dob1", SqlDbType.Date);  
p4 = cmd.Parameters.Add("@bloodgroup1", SqlDbType.VarChar, 10);  
p5 = cmd.Parameters.Add("@gender1", SqlDbType.VarChar, 10);  
p6 = cmd.Parameters.Add("@imark11", SqlDbType.VarChar, 100);  
p7 = cmd.Parameters.Add("@imark12", SqlDbType.VarChar, 100);  
p8 = cmd.Parameters.Add("@photo1", SqlDbType.Image);  
p9 = cmd.Parameters.Add("@fname1", SqlDbType.VarChar, 100);  
p10 = cmd.Parameters.Add("@fq1", SqlDbType.VarChar, 100);  
p11 = cmd.Parameters.Add("@fo1", SqlDbType.VarChar, 100);
p12 = cmd.Parameters.Add("@mname1", SqlDbType.VarChar, 100);  
p13 = cmd.Parameters.Add("@mq1", SqlDbType.VarChar, 100);  
p14 = cmd.Parameters.Add("@mo1", SqlDbType.VarChar, 100);  
p15 = cmd.Parameters.Add("@mton1", SqlDbType.VarChar, 50);  
p16 = cmd.Parameters.Add("@religion1", SqlDbType.VarChar, 50);  
p17 = cmd.Parameters.Add("@nationality1", SqlDbType.VarChar, 50);  
p18 = cmd.Parameters.Add("@state1", SqlDbType.VarChar, 100);  
p19 = cmd.Parameters.Add("@distric1", SqlDbType.VarChar, 100);  
p20 = cmd.Parameters.Add("@mobile1", SqlDbType.BigInt);  
p21 = cmd.Parameters.Add("@email1", SqlDbType.VarChar, 100);  
p22 = cmd.Parameters.Add("@caddr1", SqlDbType.VarChar);  
p23 = cmd.Parameters.Add("@paddr1", SqlDbType.VarChar);  
p24 = cmd.Parameters.Add("@academicy1", SqlDbType.VarChar, 50);  
p25 = cmd.Parameters.Add("@adclass1", SqlDbType.VarChar, 15);  
p26 = cmd.Parameters.Add("@adsec1", SqlDbType.VarChar, 15);  
p27 = cmd.Parameters.Add("@addate1", SqlDbType.Date);
p28 = cmd.Parameters.Add("@lschool1", SqlDbType.VarChar, 100);  
p29 = cmd.Parameters.Add("@lclass1", SqlDbType.VarChar, 15);  
p30 = cmd.Parameters.Add("@pyear1", SqlDbType.VarChar, 50);
p31 = cmd.Parameters.Add("@estudent1", SqlDbType.VarChar, 100);  
p32 = cmd.Parameters.Add("@eclass1", SqlDbType.VarChar, 15);  
p33 = cmd.Parameters.Add("@esec1", SqlDbType.VarChar, 15);  
cn.Open();  
p1.Value = txtrgno.Text;  
p2.Value = txt_sname.Text;  
p3.Value = dtp_dob.Text;  
p4.Value = cmbx1_bloodgrp.Text;  
p5.Value = gen;  
p6.Value = txt_idm1.Text;  
p7.Value = txt_idm2.Text;  
p8.Value = x;  
p9.Value = txt_fname.Text;  
p10.Value = txt_fq.Text;  
p11.Value = txt_fo.Text;  
p12.Value = txt_mname.Text;  
p13.Value = txt_mq.Text;  
p14.Value = txt_mo.Text;  
p15.Value = txt_tongue.Text;  
p16.Value = txt_religion.Text;  
p17.Value = txt_nationality.Text;  
p18.Value = txt_state.Text;  
p19.Value = txt_dist.Text;  
p20.Value = txt_mobile.Text;  
p21.Value = txt_email.Text;  
p22.Value = txt_caddr.Text;  
p23.Value = txt_paddr.Text;  
p24.Value = cmbx2_academicyear.Text;  
p25.Value = cmbx3_class.Text;  
p26.Value = cmbx4_section.Text;  
p27.Value = dtp_admissiondate.Text;  
p28.Value = txt_pschool.Text;  
p29.Value = cmbx7_pclass.Text;  
p30.Value = cmbx8_pyear.Text;  
p31.Value = txt_esname.Text;  
p32.Value = cmbx5_eclass.Text;  
p33.Value = cmbx6_esec.Text;  
cmd.ExecuteNonQuery();  
cn.Close();  
MessageBox.Show("Record Successfully Inserted");  
}  
catch(SqlException ex)   
{  
MessageBox.Show(ex.Message);  
}  
cn.Close();   
}

Answer 1

您使用的cmd.ExecuteReader();会为SELECT语句返回一行，因为它使用COUNT(*)。现在，这可能会返回0个或多个。

您需要调用ExecuteScalar，然后检查返回的值。

类似的东西：

int count = Convert.ToInt32(cmd.ExecuteScalar());

if(count > 0)
{
   // Already exists
}

但更重要的是，您还应该对SELECT查询进行参数化。另外，请务必将SqlConnection和SqlCommand对象括在using statement.中。阅读SQL injection，因为您的第一个查询基于字符串连接，并且很容易进行SQL注入。

您的代码的第一部分应该是：

using (SqlConnection cn = new SqlConnection("ConnectionString"))
{
    using (
        SqlCommand cmd =
            new SqlCommand("select count(*) from studentreg where registrationno = @registrationNumber", cn))
    {
        cmd.Parameters.Add(new SqlParameter("@registrationNumber", SqlDbType.VarChar) {Value = txtrgno.Text});
        //or
        //cmd.Parameters.AddWithValue("@registrationNumber", txtregno.Text);
        cn.Open();

        int count = Convert.ToInt32(cmd.ExecuteScalar);

        if (count > 0)
        {
            // Already exists
            MessageBox.Show("Registration No. Already Exists Try Another", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
            txtrgno.Text = "";
            txtrgno.Focus();
        }
    }
}

如何在C＃中使用datareader？

1 个答案: