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[最佳答案] I think jpbochi missed that you want your groupings to be by pairs of values (Title+SourceId then Title+Index). Here's a LINQ query (mostly) solution: var selectedFew = from doc in docs group doc by new { doc.Title, doc.SourceId } into g from docIndex in g.Select((d, i) => new { Doc = d, Index = i }) group docIndex by new { docIndex.Doc.Title, docIndex.Index } into g select g.Aggregate((a,b) => (a.Doc.SourceId <= b.Doc.SourceId) ? a : b); First we group by Title+SourceId (I use an anon