Distinct() method is not working

Question

I have a LINQ similar to this:

var list = (from g in myPreBuiltQuery
            where g.Address != null
            orderby g.Schools.SchoolName
            select new ResultFindSummary
            {
               // fill some fields for this class
            }).Take(100).Distinct().ToArray();

But still it is returning extra duplicate results.

A grid of expected result and actual results would also help in this case. — abhi, Feb 12 '14 at 14:38
Have you overridden `Equals` and `GetHashCode`? Apart from that, i would swap the `Take` and `Distinct` if you want up to 100 distinct results. — Tim Schmelter, Feb 12 '14 at 14:40
You state that you have a LINQ query **similar** to the one above. What is your **actual** LINQ query? Are you materializing your results to a C# collection before calling `.Distinct()`? — Jon Senchyna, Feb 12 '14 at 15:31

score 4 · Answer 1 · answered Feb 12 '14 at 14:39

The problem is that you're creating 100 different instances of a custom type (ResultFindSummary), and that instances of a custom type are only considered equal when they reference the same object.

The solution is to override the Equals() and GetHashCode() methods in the ResultFindSummary class.

score 3 · Accepted Answer · answered Feb 12 '14 at 14:40

Distinct uses an IEqualityComparer on items in memory. Now you are making a distinction on an sql query which is probably translated wrongly. (You can inspect that with the SQL profiler).

Two possible solutions => use groupby in combination with a first. If you want to work on the sql directly.

Or use the distinct after you converted your result set to memory (using .ToList()) and implement the IEqualityComparer on the SomeClass.

score 2 · Answer 3 · answered Feb 12 '14 at 14:39

2

Have you implemented the Equals method in your ResultFindSummary class ?

Here is a topic from MSDN on how to Distinct() function works : http://msdn.microsoft.com/en-us/library/bb348436(v=vs.110).aspx

answered Feb 12 '14 at 14:39

gretro

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score 2 · Answer 4 · answered Feb 12 '14 at 14:39

You are trying to do a Distinct on a ResultFindSummary object. To use Distinct on an object you would need to use the Distinct override (http://msdn.microsoft.com/en-us/library/bb338049%28v=vs.110%29.aspx) that provides a class that implements the IEqualityComparer interface...

http://msdn.microsoft.com/en-us/library/ms132151%28v=vs.110%29.aspx

Distinct() method is not working

4 Answers4