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.NET Forum / Languages / C# / March 2008[LINQ] GroupBy vs ToLookup
could someone enlighten me on what would be the difference between GroupBy and ToLookup? I try hard but am not able to spot any difference between these two. the syntax and behavioral semantics is the same. is there any explanation on why we need them both? Thanks in advance, Wiktor Zychla Well, ToLookup is buffered - i.e. it is expected that you can ask (by key) repeatedly. GroupBy, on the other hand, simply iterates the groups (streaming). Under the bonnet, I suspect that LINQ-to-object uses ToLookup to perform GroupBy, but the difference is greater for other providers, such as a database - indeed, for streaming providers (such as TDS), where both are supported it is likely that the roles are reversed, and ToLookup consumes GroupBy. Marc > Well, ToLookup is buffered - i.e. it is expected that you can ask (by key) > repeatedly. GroupBy, on the other hand, simply iterates the groups [quoted text clipped - 5 lines] > supported it is likely that the roles are reversed, and ToLookup consumes > GroupBy. In LINQ to Objects, GroupBy is definitely buffered - and has to be, because it can't give results to a downstream consumer until it's seen all the upstream results.  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk >> Under the bonnet, I suspect that LINQ-to-object uses ToLookup to perform >> GroupBy, but the difference is greater for other providers, such as a [quoted text clipped - 5 lines] > because it can't give results to a downstream consumer until it's seen > all the upstream results. nice, as it still does not reveil my confussion. could you be so nice and provide an example where using GroupBy vs ToLookup would lead to different results? Wiktor Zychla > >> Under the bonnet, I suspect that LINQ-to-object uses ToLookup to perform > >> GroupBy, but the difference is greater for other providers, such as a [quoted text clipped - 10 lines] > could you be so nice and provide an example where using GroupBy vs ToLookup > would lead to different results? For one thing (which I forgot to mention before), GroupBy uses deferred execution whereas ToLookup uses immediate execution. In other words, suppose you call GroupBy, then Console.WriteLine, then foreach over the results - the Console.WriteLine will happen before the grouping is actually executed. If you call ToLookup, then Console.WriteLine, then foreach over the results - the Console.WriteLine happens *after* all the grouping is executed. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk > For one thing (which I forgot to mention before), GroupBy uses deferred > execution whereas ToLookup uses immediate execution. that should be it - the docs says that conversion operators (ToList, ToArray, ToLookup, ToDictionary) causes queries to be executed immediately, where GroupBy can be, of course, deferred. thanks, Jon. Wiktor Zychla
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