[LINQ] GroupBy vs ToLookup

[Wiktor Zychla [C# MVP]]

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

 

[Marc Gravell]

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

 

[Jon Skeet [C# MVP]]

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.

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.

 

[Wiktor Zychla [C# MVP]]

Under the bonnet, I suspect that LINQ-to-object uses ToLookup to perform

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.

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

 

[Jon Skeet [C# MVP]]

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?

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.

 

[wzychla]

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