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https://issues.apache.org/jira/browse/LUCENENET-612?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16986934#comment-16986934
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Shad Storhaug commented on LUCENENET-612:
-----------------------------------------

The documentation has now been updated to inform end users that the dictionary 
implementation chosen will provide the desired behavior. This issue can be 
closed after the next release after beta00006.

> SERIOUS issues with PerFieldAnalyzerWrapper in 4.8
> --------------------------------------------------
>
>                 Key: LUCENENET-612
>                 URL: https://issues.apache.org/jira/browse/LUCENENET-612
>             Project: Lucene.Net
>          Issue Type: Bug
>          Components: Lucene.Net.Analysis.Common
>    Affects Versions: Lucene.Net 4.8.0
>            Reporter: Shad Storhaug
>            Priority: Major
>   Original Estimate: 16h
>  Remaining Estimate: 16h
>
> This came in on the user mailing list on 15-July-2019 and was originally 
> reported by Bryan Rojo (bryanr...@elliotelectric.com)
>  
> {quote}Not necessarily a bug, but for some people who use 
> PerFieldAnalyzerWrapper like I do this might be worth noting.
> PerFieldAnalyzerWrapper has been "improved" in 4.8 and now uses a 
> PER_FIELD_REUSE_STRATEGY which means that the tokenized fields will be stored 
> in a dictionary, so If you have multiple fields with the same name in your 
> document, then you will only be able to index the very first one that makes 
> it into that dictionary.
> So the problem with this is that you can potentially lose thousands of terms 
> in your index, which could cause your searches to be of very low quality.
> BEWARE.
> {quote}
>  
> There are 2 issues that need to be resolved to address this:
> 1. The documentation for {{PerFieldAnalyzerWrapper}} should be updated to 
> inform users that if they need to use multiple dictionary keys with the same 
> name, they should use {{TreeDictionary<K, V>}}.
> 2. {{TreeDictionary<K, V>}} does not currently implement 
> {{System.Collections.Generic.IDictionary<TKey, TValue>}}, as it was brought 
> over from C5 as-is.
> Another thing of note is that C5 has added support for .NET Standard 1.0 
> since this was brought over.
> However, there still seems to be a few problems that make the C5 types 
> incompatible with Lucene.Net, most notably the lack of support for 
> {{System.Collections.Generic.IDictionary<TKey, TValue>}} in 
> {{TreeDictionary}} and {{System.Collections.Generic.ISet<T>}} in {{TreeSet}} 
> (the latter of which has already been patched in 
> {{Lucene.Net.Support.TreeSet}}).
> I [reported|https://github.com/sestoft/C5/issues/53] the lack of support for 
> {{ISet<T>}} on 6-Nov-2016, but although the maintainers agree this should be 
> done, it still hasn't been. Perhaps a PR to the C5 project is the way to get 
> this done, which would allow us to finally remove these collection copies 
> from Lucene.Net.Support and add a package dependency on C5.
> Another option is to shop around to see if there are any other generic 
> TreeSet/TreeDictionary implementations that have popped up since late 2016 
> that we can check for compatibility.



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