布尔中的数据类型不匹配错误

Question

I need to make an authentication on the VotingStatus(Boolean) of the user, once that his voting status is checked in the database he shouldn't be able to vote again.

This code below shows a data type mismatch error. What is wrong with this?

MessageBox.Show("Welcome!");
OleDbCommand comd1 = new OleDbCommand();
comd1.Connection = connection;
comd1.CommandText = "SELECT VoterID FROM tbl_voter where Uname='" + txt_user.Text + "' and Pword='" + txt_pass.Text + "'";
voterid = Convert.ToString(comd1.ExecuteScalar());
Program.VoterID = voterid;

string comd21 = "Select VotingStatus from tbl_voter where VoterID='" + voterid + "'";
OleDbCommand comd2 = new OleDbCommand(comd21, connection);
var vstatus = (String)comd2.ExecuteScalar();
if (vstatus == "true")
{
      MessageBox.Show("You cannot vote again!");
}

Answer 1

You should change the following lines

var vstatus = (String)comd2.ExecuteScalar();
if (vstatus == "true")

as below:

var vstatus = comd2.ExecuteScalar();
if (vstatus !=null && Convert.ToBoolean(vstatus))
{
    MessageBox.Show("You cannot vote again!");
}

Despite it is irrelevant with that you have asked, it should be mentioned that you must avoid using string concatenation in the creation of SQL statements. Not doing so you are open to SQL injections.