Error on IEqualityComparer

Question

I have been given this solution in one of my question... seems very interesting.. as i am trying and learning to understand LINQ.. I am having this error while implementing this. I dont know whats going wrong here... seeking expert advice.

public class CategotiesEqualityComparer : IEqualityComparer<ac_Categories>
{
    public bool Equals(ac_Categories x, ac_Categories y)
    {
        return x.ThumbnailAltText.Trim() == y.ThumbnailAltText.Trim();
    }

    public int GetHashCode(ac_Categories obj)
    {
        return obj.ThumbnailAltText.Trim().GetHashCode();
    }
}

var catlist = _db.ac_Categories
.Where(c => c.VisibilityId == 0 && c.ThumbnailAltText != null
    && (!c.ThumbnailAltText.StartsWith("gifts/")
    && !c.ThumbnailAltText.StartsWith("email/")
    && !c.ThumbnailAltText.StartsWith("news/")
    && !c.ThumbnailAltText.StartsWith("promotions/")
    && !c.ThumbnailAltText.StartsWith("the-knowledge/")))
    .Distinct(new CategotiesEqualityComparer()).ToList();

ERROR:LINQ to Entities does not recognize the method 'System.Linq.IQueryable1[WebMgr.ac_Categories] Distinct[ac_Categories](System.Linq.IQueryable1[WebMgr.ac_Categories], System.Collections.Generic.IEqualityComparer`1[WebMgr.ac_Categories])' method, and this method cannot be translated into a store expression

NOTE:

I am using Database first approach and ac_Categories is created using that...

Your question is already answered here: http://stackoverflow.com/a/1011014/222163 — Paul D'Ambra, Oct 17 '13 at 10:31

score 4 · Accepted Answer · edited Feb 14 '16 at 21:44

4

You can't pass an IEqualityComparer comparer into a call to an EF query or likely most IQueryable collections. IQueryable are expression trees, that expression tree gets converted into an underlying query (such as SQL with EF). As such it can't execute your code in SQL.

A normal Distinct() call will work just fine if you want to filter out duplicate identical records as it will perform a SQL distinct. If you it to apply your equality comparer you would have to pull the data into memory then apply your distinct.

edited Feb 14 '16 at 21:44

marc_s

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answered Oct 17 '13 at 10:47

Ben Robinson

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2

yes... i got it .... I have to use AsEnumerable() – patel.milanb Oct 17 '13 at 11:07
2

Yes but be aware that this will only execut the WHERE in SQL, the distinct will be performed in memory. Depending on the size of your dataset a group by might be quicker. – Ben Robinson Oct 17 '13 at 11:09

Error on IEqualityComparer

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