How to remove non-duplicates from a list in C#

Question

I want to do the opposite thing as here

I have a list and I know how to remove the duplicates. But I want to have an option where the user can select which duplicate to keep. Some query quere I can have a list that will only show the duplicates. Something like:

Lets say my list is:

"tom" "bob" "Frank" "bob" "Lacey" "Frank"

I know that if I use the distinct method I will get:

"tom" "bob" "Frank" "Lacey"

I don't know what method to I have to use to get:

"bob" "bob" "frank" "frank"

or to get

"bob" "frank"

cause those are the ones that repeat.

In other words I want to do the same thing that iTunes does when you click on the option that says show duplicate songs. — Tono Nam
– Tono Nam, Commented May 25, 2011 at 17:08
@Joe: your answer match perfectly question asked. Why don't you post it as an answer to allow the OP to approve it? — Sylver
– Sylver, Commented May 25, 2011 at 17:11
@Sylverdrag I do not know LINQ well enough to provide an adequate answer — Joe Phillips
– Joe Phillips, Commented May 25, 2011 at 17:13

BrokenGlass · Accepted Answer · 2011-05-25 17:09:46Z

24

You can use GroupBy to filter out the items that only occur once, then flatten the remaining items back into a list:

var resultList = list.GroupBy(x => x)
                     .Where(g => g.Count() > 1)
                     .SelectMany(g => g)
                     .ToList();

answered May 25, 2011 at 17:09

BrokenGlass

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3 Comments

BrokenGlass Over a year ago

Also, slightly more efficient for the general case of an IEnumerable when having a large number of duplicates you could use .Where( g=> g.Skip(1).Any()) instead of the above Where condition.

Sandeep Over a year ago

Why do we need SelectMany after Where condition? I am new to Linq please help me out in understanding this.

BrokenGlass Over a year ago

@Sandeep: SelectMany "flattens" a list of lists to a single lists that consists of all the elements contained in the original lists

Tono Nam · Accepted Answer · 2011-05-25 17:23:21Z

1

I needed to compare them by a specific property theretofore I just modified your query BrokenGlass to

var resultList = itemsThatNeedToBeAdded.GroupBy(x => x.property1)
                     .Where(g => g.Count() > 1 )
                     .SelectMany(g => g)
                     .ToList();

answered May 25, 2011 at 17:23

Tono Nam

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