Question

string sStoreStockFeed = "";
string sSeparator = "";

var distinctStoreIDList = skuStoreStockLevels.Select(x => x.Item1).Distinct();

    foreach (var storeID in distinctStoreIDList)
    {
        foreach (var item in skuStoreStockLevels)
        {
            if (item.Item1 == storeID)
            {
               // add this one to a job for this store
                       sStoreStockFeed += sSeparator + item.Item1.ToString() + "," + item.Item2.ToString() + "," + item.Item3.ToString();
                       sSeparator = "|";
            }
        }
      // some code to process the string before moving on
      sStoreStockFeed = "";
      sSeparator = "";
    }

In the above code snippet skuStoreStockLevels just happens to be a List of type Tuple and Item1 is the StoreID. having got a distinct list it then iterates through the (non-distinct) list to get every applicable item. The inefficiency is that the (big) inner list is iterated throuh repeatedly for each distinct item (StoreID).

Answer 1

UPDATE: pure LINQ solution. This will give you list of strings, created for each group of items.

var query = skuStoreStockLevel.GroupBy(x => x.Item1)
                .Select(g => g.Aggregate(new StringBuilder(),
                                         (sb, x) => sb.AppendFormat("{0}{1},{2},{3}", sSeparator, x.Item1, x.Item2, x.Item3),
                                         (sb) => sb.ToString()));

foreach(var feed in query)
    // some code to process the string before moving on

Also there are other options – ordering of sequence. Equal items will follow one after another.

int storeID = -1;
StringBuilder builder = new StringBuilder();

foreach (var item in skuStoreStockLevel.OrderBy(x => x.Item1))
{
    builder.AppendFormat("{0}{1},{2},{3}", sSeparator, item.Item1, item.Item2, item.Item3);
    if (item.Item1 != storeID)
    {
        // some code to process the string before moving on
        storeID = item.Item1;
    }       
}

Or you can use grouping

StringBuilder builder = new StringBuilder();

foreach (var storeGroup in skuStoreStockLevel.GroupBy(x => x.Item1))
{
    foreach (var item in storeGroup)
          builder.AppendFormat("{0}{1},{2},{3}", sSeparator, item.Item1, item.Item2, item.Item3);        

    // some code to process the string before moving on
}

And, of course, it’s better to use StringBuilder for creating strings.