select new
                             {
                                 OrderId = grp.Key.Value,
                                 RetDate = grp.Max(s => s.mDate),
                                 Summa = grp.Sum(s => s.mSumma)
                             };

protected void repMainDataBound(object sender, RepeaterItemEventArgs e)

public class cGiveBackRow
{ 
    public Int32 OrderId;
    public DateTime? RetDate;
    public double?  Summa;
}

class Program
    {
        static void Main(string[] args)
        {
            var a = new { Id = 1, Name = "Bob" };
            TestMethod(a);

            Console.Out.WriteLine("Press enter to exit...");
            Console.In.ReadLine();
        }

        private static void TestMethod(Object x)
        {
            // This is a dummy value, just to get 'a' to be of the right type
            var a = new { Id = 0, Name = "" };
            a = Cast(a, x);
            Console.Out.WriteLine(a.Id + ": " + a.Name);
        }

        private static T Cast<T>(T typeHolder, Object x)
        {
            // typeHolder above is just for compiler magic
            // to infer the type to cast x to
            return (T)x;
        }
    }