Question

Here is my code in c#, it is saying “No data exists for the row/column.”, how can I improve this code to make it work properly?

db = new OleDbConnection();
db.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data source=" + fileName;
db.Open();

string sql = "SELECT * FROM GroupNameNS WHERE GroupName = '" + Groupnametxt.Text.Trim() + "'";

cmd = new OleDbCommand(sql, db);
rdr = cmd.ExecuteReader();

if (Groupnametxt.Text.Trim() == (string)rdr["GroupName"])
{
  MessageBox.Show("Group Name taken, please try another name", "Error in Name", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
  membernumber1.ReadOnly = true;
}
else
{
  sql = "INSERT INTO GroupNameNS VALUES ('" + Groupnametxt.Text.Trim() + "')";
  membernumber1.ReadOnly = false;
  cmd = new OleDbCommand(sql, db);
  rdr = cmd.ExecuteReader();
}

Answer 1

You need to call .Read() on your Reader object in order to increment through the rows of your result set.

rdr.Read();

//Note this will return false if there is no more record(s) to be read, so it is usually used inside and if statement or while loop

You could change your if (Groupnametxt.Text.Trim() == (string)rdr["GroupName"]) to if(rdr.Read()) because if there is at least 1 record in your table that has this GroupName value, it will return true.

            if (rdr.Read())
            {
                MessageBox.Show("Group Name taken, please try another name", "Error in Name", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                membernumber1.ReadOnly = true;
                rdr.Close();
            }
            else
            {
                rdr.Close();
                sql = "INSERT INTO GroupNameNS VALUES ('" + Groupnametxt.Text.Trim() + "')";
                membernumber1.ReadOnly = false;
                cmd = new OleDbCommand(sql, db);
                cmd.ExecuteNonQuery();
            }

I modified a couple other things too: don’t forget to close your Reader object after using it. And use ExecuteNonQuery() for an INSERT