using(SqlConnection conn = new SqlConnection("YOUR CONNECTION STRING")
{
  using(SqlCommand cmd = new SQlCommand())
  {
    cmd.Connection = conn;
    cmd.CommandText = "SELECT COUNT([DeptNo]) AS [cnt] FROM [Dept] WHERE [DeptNo=@deptno";
    cmd.Parameters.AddWithValue("@deptno",txtNo.Text);
    if(conn.State==ConnectionState.Closed)
    {
      conn.Open();
    }
    bool isExist = false;
    using(SqlDataReader reader = cmd.ExecuteReader())
    {
      reader.Read();
      isExist = reader.GetInt32(0)>0;
    }
    if(!isExist)
    {
      cmd.CommandText = "INSERT INTO [Dept] ([DeptNo]) VALUES(@deptno)";
      int row_count = cmd.ExecuteNonQuery();
    }
    else{....}
  }
}