Show the error message and prevent duplicated vote

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C#

protected void LinkButton_Click(Object sender, EventArgs e)
        {

            String MyConnection2 = "Server=localhost;database=ovs;Uid=root;password=; Convert Zero Datetime=True";
            DateTime time = DateTime.Now;              // Use current time
            string format = "yyyy-MM-dd HH:mm:ss";
            string UserName4 = HttpContext.Current.User.Identity.Name;
            GridViewRow grdrow = (GridViewRow)((LinkButton)sender).NamingContainer;
            Label lblStudentId = (Label)grdrow.Cells[0].FindControl("lblID");
          Label lblvID = (Label)GridView2.Rows[0].FindControl("lblvID");
           string label1val = lblvID.Text;

            string studentId = lblStudentId.Text;
            //    string id =;
            //      string testing = this.Encrypt(id.Trim());
            String query = "insert into voting (CandidateStudentID,voterStudentID,DateTime)values ('" + lblStudentId.Text + "','" + Session["UserName"].ToString() + "','" + time.ToString(format) + "')";
           
            if (Session["UserName"].ToString().Equals(lblvID.Text))
            {

                Label1.Text = "You voted beofre";

            }
            else
            {
                MySqlConnection MyConn2 = new MySqlConnection(MyConnection2);
                MySqlCommand MyCommand2 = new MySqlCommand(query, MyConn2);
                MySqlDataReader MyReader2;
                MyConn2.Open();
                MyReader2 = MyCommand2.ExecuteReader();

                Label2.Text = "Thank you for You Vote";


            }


        }


<pre><asp:GridView ID="GridView2" runat="server"  AutoGenerateColumns="False" Font-Size="Medium">
              <Columns>
              <asp:TemplateField HeaderText="Student ID">
      <ItemTemplate>
         <asp:Label ID="lblvID" runat="server"   Width="150px"  Text='<%#Eval("voterStudentID") %>'/>
     </ItemTemplate>
 </asp:TemplateField>

              </Columns>
           </asp:GridView>

What I have tried:

The link button belongs to gridview 1.I'm getting the index out of range error in the

Label lblvID = (Label)GridView2.Rows[0].FindControl("lblvID");.

Posted 18-Oct-20 18:31pm

ycwong99

Updated 19-Oct-20 2:53am

v7

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Patrice T · Answer 1 · 2020-10-18T19:18:00

Quote:
Show the error message and prevent duplicated vote

2 approaches:
- Do a select for the student id, if the result is a record, the student already voted. This can get unlucky if student try to vote simultaneously on 2 computers.
- Design table to have student id unique or primary key, this way, the server will reject insert. To know how the insert did, check its result.

C#

String query = "insert into voting (CandidateStudentID,voterStudentID,DateTime)values ('" + lblStudentId.Text + "','" + Session["UserName"].ToString() + "','" + time.ToString(format) + "')";

Not a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]
SQL Injection Attacks by Example[^]
PHP: SQL Injection - Manual[^]
How can I explain SQL injection without technical jargon? - Information Security Stack Exchange[^]

[Update]

Quote:
I can't select the studentID, because the studentID will show the particular student vote for the particular candidate.

Yes you can. You just have to not query all fields of table, just query 1 field, the student id.