Why can't I do foreach (var Item in DataTable.Rows)?

Rows effectively returns IEnumerable (DataRowCollection), so the compiler can only pick object as the type for var. Use Rows.Cast<DataRow> if you want to use var.

Cast is defined on Enumerable, so you have to include System.Linq.

Brian is absolutely right about the reason for this, but there's a simpler way of avoiding it: use DataTableExtensions.AsEnumerable():

foreach (var row in DataTable.AsEnumerable())
{
    ...
}