Dynamic Linq to DataTable: Max of DateTime with GroupBy

Question

I've used the below excellent post for the basis of my dynamic linq queries

http://social.msdn.microsoft.com/Forums/is/linqprojectgeneral/thread/8f6f2352-34f6-46a4-b61f-2953417bfdfc

One thing I am having trouble with is how to perform a min/max on DateTime columns in a dyanmic linq query with groups.

e.g. The below will get the Min on a float field named Amount

var query  = myDT.AsEnumerable().AsQueryable();
var newquery = query.GroupBy("new (get_item(@0).ToString()  AS Column1)", "it", "Column1");
newquery = newquery.Select("new (it.Key.Tier.ToString() as Column1, @0(it) as Amount)",  GetGroupByLambdaExpression(Min, "Amount", GetType(Float)));

This works fine, however if I wanted to do exactly the same on a field of Type DateTime rather than float I cannot figure out how to do it. The underlying problem is that Enumerable type doesn't have a Min method which accepts a DateTime.

Any ideas?

Answer 1

BF&I method is:

• convert the DateTime to a string in format "yyyy-mm-dd hh-mm-ss",
• extract the Min of that, and
• convert back to a DateTime.

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Answer 2

Actually, use OrderBy and then Take 1

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Answer 3

My first thought was to use order by however I'm dealing with groups and possibly multi-level grouping. 

my query in pseudocode is something like

.Select("company as company, division as division, max(lastactivitydate)

.GroupBy("company, division")

Therefore I would need to iterate over every group then orderby and then Take 1