DataTable dt1 = new DataTable("Table1");
    DataTable dt2 = new DataTable("Table2");

    dt1.Columns.Add("ID", typeof(Int32));
    dt1.Columns.Add("Quantity", typeof(Int32));
    dt1.Rows.Add(new object[] { 1, 1 });
    dt1.Rows.Add(new object[] { 2, 2 });
    dt1.Rows.Add(new object[] { 3, 3 });
    dt1.Rows.Add(new object[] { 4, 4 });

    dt2.Columns.Add("ID", typeof(Int32));
    dt2.Columns.Add("Quantity", typeof(Int32));
    dt2.Rows.Add(new object[] { 1, 100 });
    dt2.Rows.Add(new object[] { 3, 100 });
    dt2.Rows.Add(new object[] { 4, 100 });

    dt1.Merge(dt2);

    var datas = dt1.AsEnumerable()
       .GroupBy(r => new { Col1 = r["ID"] })
       .Select(g => g.First()["Quantity"] = g.Sum(x => int.Parse(x["Quantity"].ToString())))
       .ToList();

    dt1 = dt1.AsEnumerable().GroupBy(r => new { Col1 = r["ID"] })
        .Select(x => x.First()).CopyToDataTable();