LINQ Filtering a list by looping on another list

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Hello all,

I have a IEnumerable<Principle> say users which contains 100 items. And I have IEnumerable>DataRow> say groupUsers which contains 10 items.

"Principle" contains fields of id, name, surname, emailaddress
"Datarow" contains fields of company, name, surname, email

I want to loop on users and for each item on users.mailaddress equals to groupUsers.Email I want to add the users's item to a IEnumerable<principle>. Can someone help me write the LINQ statement.

What I have came up so far;

C#

IEnumerable<Principle> result = users.ForEach(x => x.Emailaddress.Select(//another loop here for matching emails));

Thanks in advance.

Posted 16-Jul-14 5:17am

wonder-FOOL

Updated 16-Jul-14 5:23am

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Matt T Heffron · Accepted Answer · 2014-07-16T07:37:00

Look at it from the inner test first:
A Principle should be included in the output if the Emailaddress matches any of the Email values in groupUsers:

C#

groupUsers.Any(gu => gu.Email == u.Emailaddress)

Now do that for each element of users and collect the result:

C#

IEnumerable<Principle> result = users.Where(u => groupUsers.Any(gu => gu.Email == u.Emailaddress));

This, of course, is very inefficient. Time grows proportional to the product of the lengths of the two lists. So make the inner test faster:

C#

HashSet<string> groupEmails = new HashSet<string>(groupUsers.Select(gu => gu.Email));
IEnumerable<Principle> result = users.Where(u => groupEmails.Contains(u.Emailaddress));

This now takes time proportional to the sum of the lengths of the two lists.
Creating and populating the groupEmails is a single pass over the groupUsers collection.
Finding the result is a single pass over the users collection.
The advantage is that the groupEmails.Contains(...) is (approximately) constant time, independent of the number of elements in the groupEmails HashSet.

* Gramatically, you probably mean "Principal" not "Principle"... lookup the difference: Principal vs Principle[^]