LINQ - Group DataTable by multiple columns determined at runtime
// column source
IEnumerable<string> columns;
DataTable table;

IEnumerable<IGrouping<object, DataRow>> groupings = table
    .AsEnumerable()
    .GroupBy(row => ???);

Func<DataRow, IEnumerable<string>, string> f = (row, cols) =>
    String.Join("|", cols.Select(col => row[col]));

IEnumerable<IGrouping<object, DataRow>> groupings = table
    .AsEnumerable()
    .GroupBy(row => f(row, columns));