C# lambda get distinct list of value conditionally

Question

I have a list of users as given below:

List<User> users = new List<User>();

users.Add(new User(){ UserId = "11", City = "London" });
users.Add(new User(){ UserId = "12", City = "London" });
users.Add(new User(){ UserId = "12", City = "London" });
users.Add(new User(){ UserId = "11", City = "Newyork" });
users.Add(new User(){ UserId = "14", City = "Virginia" });

Here, I want to get distinct UserIDs those have different City by C# lambda expression

So, in above case I should get a List<string> which will only contains UserId = 11 item because UserId is same but city is different for both the item.

Could you please let me know how would I do this by C# lambda code.

Thanks in advance.

@SynerCoder I tried by GroupBy lambda clause, but not succeed. — nunu, Aug 29 '12 at 11:02
Here is similar question: http://stackoverflow.com/questions/1183403/how-to-get-distinct-instance-from-a-list-by-lambda-or-linq — JleruOHeP, Aug 29 '12 at 11:03

score 8 · Accepted Answer · answered Aug 29 '12 at 11:05

8

Something like:

var result = users.GroupBy(u => u.UserId)
                  .Where(g => g.Select(u => u.City).Distinct().Count() > 1)
                  .Select(g => g.Key)
                  .ToList();

should do it.

It takes the {UserId,City} pairs and converts into groups of those pairs indexed by UserId; and then looks for cases where there is more than one city in the group. Finally taking the key from the groups for the result.

answered Aug 29 '12 at 11:05

Richard

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Hi Richard... Now I want to get reverse scenarios... I want to select only those UserIds whose have single or multiple SIMILAR cities... Could you please tell me how would I do it?? Thanks in advance! – nunu Aug 29 '12 at 12:02
1

@nunu For the single city change the `> 1` condition to `== 1`. For *similar* you'll need to replace the `Distinct` with something that works with whatever you mean by similar (this might be a custom [`IEqualityComparer`](http://msdn.microsoft.com/en-us/library/bb338049.aspx) passed to `Enumerable.Distinct`'s overload). Without a specific definition of similar it is too ambiguous to be more specific. – Richard Aug 29 '12 at 14:12
Richard do you have an example of how to pass a lambda/anonymous function to Enumerable.Distinct? I.e. like [this](http://stackoverflow.com/a/10476426/575530) but without the separate method. – dumbledad Dec 10 '12 at 17:21
1

@dumbledad Unfortunatelly you cannot. [`Enumerable.Distinct`](http://msdn.microsoft.com/en-gb/library/bb338049.aspx) takes a [`IEqualityComparer`](http://msdn.microsoft.com/en-gb/library/ms132151.aspx) rather than a delegate. And there is no way to create an implementation of an interface with a delegate (whether another function or lambda): you have to have a type. It should be possible to create a helper type that implements `IEqualityComparer` based on a key extraction delegate without too much difficulty (but too long to write here with the limited formatting options available). – Richard Dec 11 '12 at 10:48

score 1 · Answer 2 · answered Aug 29 '12 at 11:06

1

from u in users group u.City by u.UserId into grp //group on the id
where grp.Distinct().Count() > 1 // we only want those with more than one distinct city
select grp.Key //we want the key

answered Aug 29 '12 at 11:06

Jon Hanna

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Your answer is absolutely perfect !! Just the different is Lambda and LINQ :) I upvoted too.. – nunu Aug 29 '12 at 11:24

score 0 · Answer 3 · edited Aug 29 '12 at 14:36

0

var list = users.GroupBy(Obj=>new {Obj.UserID,Obj.City}).Distinct();

edited Aug 29 '12 at 14:36

Druid

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answered Aug 29 '12 at 11:43

Sanjay Gupta

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C# lambda get distinct list of value conditionally

3 Answers3

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