C# – Select top N rows AFTER sorting from Dataview in c#

For the first issue - IIRC you can't sort a DataView with a comparer. If you just need to sort numerically a field you must be sure that the column type is numeric and not string. Some code would help to elucidate this.

For the second issue also you can't do that directly in the DataView. If you really need to sort the records based on some processing of data in a column then I'd copy the data in an array and use an IComparer on the array:

DataView dv = new DataView(dt);
ArrayList lst = new ArrayList();
lst.AddRange(dv.Table.Rows);
lst.Sort(new MyComparer());
foreach (DataRow dr in lst)
    Debug.WriteLine(dr[0]);

The comparer is like this:

    class MyComparer : IComparer
    {
        public int Compare(object x, object y)
        {
            DataRow rx = x as DataRow;
            DataRow ry = y as DataRow;
            string datax = (string)rx[colName];
            string datay = (string)ry[colName];
            // Process datax and datay here then compare them (ASC)
            return datax.CompareTo(datay);
        }
    }

This will increase the memory consumption, so you need to think if there is maybe a better way to preprocess the data in the table so that you can sort directly the DataView by a column.

P.S. colName is the name of the column you're interested to sort by. Replace the comment with actual code to extract the sorting info from the column. You can also use this method to extract sorting info from more columns. Just use something like this:

int cmp = colAx.CompareTo(colAy);
if (cmp != 0)
    return cmp;
cmp = colBy.CompareTo(colBx);
return cmp;

This will compare ascending by colA values and then descending by colB values (not that the second compare has y first and then x)

Edit: OK, I interpreted wrongly the term comma separated values. From your example I think that you actually meant numbers with thousand separators (1,000,000 = one million). If you store numbers like this in the database then it must be that you're using a text field and that should be the reason your sorting order is alphanumeric.

Based on this assumption I would propose to change the type of that column to numeric, keep normal numbers inside and format them (with thousand separators) only when you display them. This way the sort should work directly in the DataView and you don't have to copy the data.

I prefer LINQ for stuff like this. Instead, I use System.Linq and write:

var rows = MyDataSet.Tables[0].AsEnumerable()
   .OrderByDescending(r => r["columnName"]) 
   .Take(10);

and then just bind to "rows".