Multiple GroupBy in 1 linq expression

Question

I cannot seem to combine 2 GroupBy statements in 1 linq expression..

For now i'm doing something like this:

double maxInvestment = 0;

foreach (var playerAction in TotalUserActions.GroupBy(p => p.Player))
{
    var MaxInvestmentPerPlayer = playerAction.GroupBy(p => p.RoundId)
                                             .Select(p => p.LastOrDefault())
                                             .Sum(p=> p.BetSize);

    if(MaxInvestmentPerPlayer > maxInvestment)
        maxInvestment = MaxInvestmentPerPlayer;
}

What I would like to do is something like this...

double maxInvestment = TotalUserActions.GroupBy(p => p.Player)
                                       .GroupBy(p => p.RoundId)
                                       .Select(p => p.LastOrDefault())
                                       .Sum(p=> p.BetSize);

But that wont work.. Can someone help me on this?

Thanks!

Answer 1

Looks like this is what you want, the key takeaway being the inner query is wrapped in an outer call to Select():

var maxInvestment = TotalUserActions.GroupBy(p => p.Player)
                                    .Select(g => g.GroupBy(x => x.RoundId)
                                                  .Select(x => x.LastOrDefault())
                                                  .Sum(x => x.BetSize))
                                    .Max();

I do question your use of LastOrDefault() though as, since you have not specified any ordering, you may as well use FirstOrDefault() and save the hassle of skipping to the last element.