Linq Lambda group by get max

Question

I like to get the Max Sample Num for where the Id is 9

    var samplecount = dbContext.ChemDetails
                      .GroupBy(a => a.Id == 9)
                      .Select(a => a.Max(w => w.Sample_Num))
                      .FirstOrDefault();

What I am getting from above is the max, not from where Id == 9.

Answer 1

You just need a Where clause, then take the max:

var maxNumber = dbContext.Where(a => a.Id == 9).Max(a => a.Sample_Num);

If you wanted to get the max value for each Id , that's when you'd use GroupBy .

Dictionary<int,int> MaxLookup = dbContext.ChemDetails
    .GroupBy(a => a.Id)
    .ToDictionary(g => g.Key, g => g.Max(item => item.Sample_Num));
int maxForId9 = 0;
if (MaxLookup.ContainsKey(9)) maxForId9 = MaxLookup[9];