排序datagrid列，我有代码，它有点工作

I'm trying to sort the columns of a datagrid A to Z, the code below works apart from I get a column beginning with 'c' in between the 'A' (e.g. a,a,a,a,c,a,a,b,b,b), this happens the first time I run the code. If I then use columnNames.Reverse(); (Z to A) and then re-run columnNames.Sort() (A to Z) it sorts correctly. Why would this be?I'm trying to sort the columns of a datagrid A