Optimize LINQ to Objects query

Question

I have around 200K records in a list and I'm looping through them and forming another collection. This works fine on my local 64 bit Win 7 but when I move it to a Windows Server 2008 R2, it takes a lot of time. There is difference of about an hour almost!

I tried looking at Compiled Queries and am still figuring it out.
For various reasons, we cant do a database join and retrieve the child values

Here is the code:

//listOfDetails is another collection
List<SomeDetails> myDetails = null;
foreach (CustomerDetails myItem in customerDetails)
{

    var myList = from ss in listOfDetails
                 where ss.CustomerNumber == myItem.CustomerNum
                 && ss.ID == myItem.ID
                 select ss;
     myDetails = (List<SomeDetails>)(myList.ToList());
     myItem.SomeDetails = myDetails;
}

Answer 1

I would do this differently:

var lookup = listOfDetails.ToLookup(x => new { x.CustomerNumber, x.ID });
foreach(var item in customerDetails)
{
    var key = new { CustomerNumber = item.CustomerNum, item.ID };
    item.SomeDetails = lookup[key].ToList();
}

The big benefit of this code is that it only has to loop through the listOfDetails once to build the lookup - which is nothing more than a hash map. After that we just get the values using the key, which is very fast as that is what hash maps are built for.