System.Guid.NewGuid() in linq select

Question

I wanted to generate a unique identifier for the results of a Linq query i did on some date. Initially i thought of using Guid for that but stumbling upon this problem i had to improvise. However I'd like to see if anyone could have a solution using Guid so here we go.

Imagine we have:

class Query
{
    public class Entry
    {
        public string Id { get; set; }
        public int Value { get; set; }
    }

    public static IEnumerable<Entry> GetEntries( IEnumerable<int> list)
    {
        var result = 
            from i in list
            select new Entry
            {
                Id = System.Guid.NewGuid().ToString("N"),
                Value = i
            };
        return result;
    }
}

Now we want Id to be unique for each entry, but we need this value to be the same for each traversal of the IEnumerable we get from GetEntries. This means that we want calling the following code:

List<int> list = new List<int> { 1, 2, 3, 4, 5 };
IEnumerable<Query.Entry> entries = Query.GetEntries(list);
Console.WriteLine("first pass");
foreach (var e in entries) { Console.WriteLine("{0} {1}", e.Value, e.Id); }
Console.WriteLine("second pass");
foreach (var e in entries) { Console.WriteLine("{0} {1}", e.Value, e.Id); }

to give us something like:

first pass

1 47f4a21a037c4ac98a336903ca9df15b
2 f339409bde22487e921e9063e016b717
3 8f41e0da06d84a58a61226a05e12e519
4 013cddf287da46cc919bab224eae9ee0
5 6df157da4e404b3a8309a55de8a95740

second pass

1 47f4a21a037c4ac98a336903ca9df15b
2 f339409bde22487e921e9063e016b717
3 8f41e0da06d84a58a61226a05e12e519
4 013cddf287da46cc919bab224eae9ee0
5 6df157da4e404b3a8309a55de8a95740

However we get:

first pass

1 47f4a21a037c4ac98a336903ca9df15b
2 f339409bde22487e921e9063e016b717
3 8f41e0da06d84a58a61226a05e12e519
4 013cddf287da46cc919bab224eae9ee0
5 6df157da4e404b3a8309a55de8a95740

second pass

1 a9433568e75f4f209c688962ee4da577
2 2d643f4b58b946ba9d02b7ba81064274
3 2ffbcca569fb450b9a8a38872a9fce5f
4 04000e5dfad340c1887ede0119faa16b
5 73a11e06e087408fbe1909f509f08d03

Now taking a second look at my code above I realized where my error was: The assignment of Id to Guid.NewGuid().ToString("N") gets called every time we traverse the collection and thus is different everytime.

So what should i do then? Is there a way i can reassure that i will get with only one copy of the collection everytime? Is there a way that i'm sure that i won't be getting the new instances of the result of the query?

Thank you for your time in advance :)

Answer 1

This is a inherent to all LINQ queries. Being repeatable is coincidental, not guaranteed.

You can solve it with a .ToList() , like:

IEnumerable<Query.Entry> entries = Query.GetEntries(list).ToList();

Or better, move the .ToList() inside GetEntries()

Answer 2

Perhaps you need to produce the list of entries once, and return the same list each time in GetEntries.

Edit:
Ah no, you get each time the different list! Well, then it depends on what you want to get. If you want to get the same Id for each specific Value, maybe in different lists, you need to cache Ids: you should have a Dictionary<int, Guid> where you'll store the already allocated GUIDs. If you want your GUIDs be unique for each source list, you would perhaps need to cache the input the return IEnumerables, and always check if this input list was already returned or not.

Edit:
If you don't want to share the same GUIDs for different runs of GetEntries, you should just "materialize" the query (replacing return result; with return result.ToList();, for example), as it was suggested in the comment to your question.

Otherwise the query will run each time you traverse your list. This is what is called lazy evaluation. The lazy evaluation is usually not a problem, but in your case it leads to recalculating the GUID each query run (i.e., each loop over the result sequence).