I think this is the culprit:
cmd = new SqlCommand(query, con);
DataTable dt = Select(query);
cmd.ExecuteNonQuery();
ddtype.DataSource = dt;
I don't know what that code is supposed to do, but it looks like you want to create an SqlDataReader for that, as explained here and all over the web if you search for "SqlCommand DropDownList DataSource":
cmd = new SqlCommand(query, con);
ddtype.DataSource = cmd.ExecuteReader();
Or you can create a DataTable as explained here:
cmd = new SqlCommand(query, con);
SqlDataAdapter listQueryAdapter = new SqlDataAdapter(cmd);
DataTable listTable = new DataTable();
listQueryAdapter.Fill(listTable);
ddtype.DataSource = listTable;