c# DataTable: More efficient Group By and Sum?

Question

I merge two similar tables, where I have 2 fields (ID and Quantity) and end up with a table where I can have multiple records for the same ID. I want the resulting table to have only 1 record for each ID, while SUMMING the respective Quantities for each ID. After long Google search I came up with this code:

    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();

However, I feel that there is a way to do this more efficiently as the table gets scanned twice in this case. Does anyone have any suggestions?

One more thing, as a bonus question I would like to find a total Quantity across the entire resulting table, idealy in an efficient way (that is WHILE grouping it).

UPDATE: yes I know, DataTable is not a wise data structure, but due to the nature of our project I am forced to work with them. I would much appreciate a suggestion on how to use an intermediary data structure here but in the end I need a DataTable.

Answer 1

//your method
public void YourMethod()
{
     Dictionary<int, int> result = new Dictionary<int, int>();

     int length = 0;

     if(dt1.Rows.Count > dt2.Rows.Count)
        length = dt1.Rows.Count
     else
         length = dt2.Rows.Count

     for(int i=0; i < length - 1; i++)
     {
         AddRowValue(dt1, result, i);
         AddRowValue(dt2, result, i);
     }  

}


public AddRowValue(DataTable tbl, Dictionary<int, int> dic, int index)
{
    if( index > tbl.Rows.Count)
       return;

    DataRow row = tbl.Rows[index];

    int idValue = Convert.ToInt32(row["ID"]);
    int quantityValue = Convert.ToInt32(row["Quantity"]);

    if(dic.Keys.Contains(idValue)
         dic[idValue] = dic[idValue] + quantityValue;
    else
         dic.Add(idValue, quantityValue);
}

You need something like this, you can use dictionary at the end the result will be stored in the dictionary.