SqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
string addStr = reader.GetString("emp_firstname");
comboBox1.Items.Add(addStr);
}
string addStr = reader.GetString(emp_firstname);
comboBox1.Items.Add(addStr);
insted of above code i am wrote below code i got expected result
comboBox1.Items.Add(reader["emp_fristname"].ToString());