Distinct in Linq query

Question

How I can make this Distinct work:

   var blockIdMap = (from panelEntry in panelEntries
                          select new {panelEntry.BlockNo, panelEntry.BlockID})
                          .Distinct()
                          .ToDictionary(mc => mc.BlockNo , mc => mc.BlockID);

I need to have only unique entries of BlockNo with it's BlockId because I enter them to Dictionary and BlockNo should be unique. I want just to take the first one.

You could always `GroupBy` `BlockNo` and then select `First` `BlockID`. The only way to change `Distinct` is with an `IEqualityComparer` or to use a class that implements `IEquatable` and overrides `GetHashCode`. You cannot do the former with anonymous types, and they have their own implementation for the latter that is locked. — Adam Houldsworth, Jul 11 '12 at 11:31
@AdamHouldsworth - [The compiler overrides GetHashCode and Equals for anonymous types](http://stackoverflow.com/a/543591/219661) — Jamiec, Jul 11 '12 at 11:34
@Jamiec I know, I amended my comment to state that you cannot change the implementation as my first draft was inaccurate. The default implementation will take all properties into account, not just the `BlockNo`. — Adam Houldsworth, Jul 11 '12 at 11:35

score 6 · Accepted Answer · answered Jul 11 '12 at 11:30

6

var blockIdMap = panelEntries.GroupBy(pe => pe.BlockNo)
            .ToDictionary(k => k.Key, v => v.First())

answered Jul 11 '12 at 11:30

Jamiec

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score 1 · Answer 2 · answered Jul 11 '12 at 11:48

In this case your linq query doesn't work beacuse .Distinct() method equals panelEntry.BlockNo and panelEntry.BlockID and not only panelEntry.BlockNo. So a solution could be use MoreLinq and the method .DistinctBy():

var blockIdMap = (from panelEntry in panelEntries
                 select new {panelEntry.BlockNo, panelEntry.BlockID})
                 .DistinctBy(mc => mc.BlockNo)
                 .ToDictionary(mc => mc.BlockNo , mc => mc.BlockID);

Distinct in Linq query

2 Answers2