Not getting Distinct Items when using Linq to List?

Question

Here is how I am attempting to get a distinct List of items...

    var queryResults = PatientList.Distinct();
    PatientList = queryResults.ToList<SelectListItem>();

For some reason, I am not getting a distinct list here.

possible duplicate of [Distinct not working with LINQ to Objects](http://stackoverflow.com/questions/1365748/distinct-not-working-with-linq-to-objects) — dlev, Aug 31 '11 at 12:53

score 8 · Accepted Answer · edited May 23 '17 at 12:18

8

Use

var queryResults = PatientList.GroupBy(x=>x.Id).Select(x=>x.FirstOrDefault())
    PatientList = queryResults.ToList<SelectListItem>();

You can always try

 PatientList = PatientList.GroupBy(x=>x.Id).Select(x=>x.FirstOrDefault()).ToList<SelectListItem>();

It will give you the distinct results based off whatever you group by

Check out http://blog.jordanterrell.com/post/LINQ-Distinct()-does-not-work-as-expected.aspx

Also another question for reference: Returning a Distinct IQueryable with LINQ?

edited May 23 '17 at 12:18

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answered Aug 31 '11 at 12:55

Gage

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The type argument for method yadda yadda cannot be inferred from the usage. Try specifying the type arguments explicitly. – SoftwareSavant Aug 31 '11 at 13:06
@DmainEvent, Change x.Id to whatever you need to group everything by. Not sure what you mean by your comment, which argument cannot be inferred? – Gage Aug 31 '11 at 13:14
sorry about that. I am working on a different computer right now to right this message than the one that has the code (Little lazy this morning :-) ). But I did change it to the right value... Not sure why it isn't working. – SoftwareSavant Aug 31 '11 at 13:18
@DainEvent, hmm Ok well which line did you get that error on? The groupby or where your converting to a list? The groupby should work correct what are you grouping everything by? What you are doing is similar to http://stackoverflow.com/questions/4472369/returning-a-distinct-iqueryable-with-linq – Gage Aug 31 '11 at 13:41
@Gage... I was doing my usual and being an idiot. I forgot to put () at the end of FirstOrDefault. It works now. Thanks a ton. – SoftwareSavant Aug 31 '11 at 14:30

score 1 · Answer 2 · edited May 23 '17 at 12:02

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Not sure what kind of items your PatientList contains, but I guess you have to implement IEquatable on your custom object.

This have been aswered before here: Distinct not working with LINQ to Objects

edited May 23 '17 at 12:02

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answered Aug 31 '11 at 12:51

Muttok

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If you implement it in the objects being compared, that would be IEquatable, not IEqualityComparer... – Thomas Levesque Aug 31 '11 at 12:53
It is actually a List of SelectListItem's – SoftwareSavant Aug 31 '11 at 12:56

score 1 · Answer 3 · answered Aug 31 '11 at 12:53

Your SelectListItem class needs to override Equals and GetHashCode (and optionally implement IEquatable<SelectListItem>). Otherwise, different instances with the same values will be considered different.

Another option is to implement a IEqualityComparer<SelectListItem> and pass it as the second parameter to Distinct.

Not getting Distinct Items when using Linq to List?

3 Answers3