combined datarows with the same values

Question

User2108892867 posted

Hello everyone, I have this datatable that I built from a gridview:

DataTable dt = new DataTable();
            dt.Columns.Add("productId", typeof(string));
            dt.Columns.Add("unitId", typeof(string));
            dt.Columns.Add("price", typeof(string));
            dt.Columns.Add("quantity", typeof(string));

            foreach (GridViewRow r in GridView1.Rows)
            {
                TextBox txtQuantity = r.FindControl("txtQuantity") as TextBox;
                TextBox txtPrice= r.FindControl("txtPrice") as TextBox;
                DropDownList ddlUnit = r.FindControl("ddlUnit") as DropDownList;
                string productTypeId= GridView1.DataKeys[r.RowIndex].Value.ToString();
                    DataRow row;
                    row = dt.NewRow();
                    row["productId"] = productTypeId;
                    row["unitId"] = ddlUnit.SelectedValue;
                    row["price"] = txtPrice.Text;
                    row["quantity"] = txtQuantity.Text;
                    dt.Rows.Add(row);
            }

After I have this datatable, I would like to loop through each row in the datatable and combine rows with the same product id, unit id and price by adding to the quality. How could I do that? In SQL, it should be something like this 

Select SUM(quantity) FROM table group by productId, unitId, price

Thanks.