Question

 private void btnCheck_Click(object sender, EventArgs e)
        {
            string costring = connection();
            string MyQuery = "select expense from dbo.KmtAccounts where date between '"+ txtFromDate +"' and '" +txtToDate +"' and    registernumber='" + txtRegNo.Text + "'";
            SqlConnection conn = new SqlConnection(costring);
            SqlCommand cmd = new SqlCommand(MyQuery, conn);
            conn.Open();
            txtResult.Text = Convert.ToString(cmd.ExecuteScalar());
            conn.Close();
        }

Answer 1

There are several point to improve within your code.

1. It seems that txtFromDate and txtToDate are controls?! I think you’d like to access txtFromDate.Text
2. You should really make use of SqlParameters to prevent SQL injection (good example at dotnetperls.com)
3. You may check if your UI values are correct with some kind of validation or even better, choose a control that only allows valid values (DateTimePicker, Calendar, …)