c# group by doesn't work

i have some problems with my c# code everywhere in the Examples they do it like me but somehow i gonna get some errors

Compiler says at g.Datum he doesn' t know Datum

and at "return query" he says - cannot convert, there is a explicit convert

     var query = (from p in dataContext.Untersuchungen
                  orderby p.Datum
                  group p by p.Datum into g 
                  let number = (from n in dataContext.Untersuchungen
                                where n.Datum == g.Dat开发者_StackOverflowum
                                select n).Count()
                  select new StatsistikObjekt() { Date1 = g.Datum, number1 = number });
       return query;

hope you can help me =)

The type of the range variable g is the group, which indeed doesn't have a Datum value.

You can fix that bit easily, given your grouping (which uses Datum as the key)- and make your query simpler too by just counting the size of the group:

var query = (from p in dataContext.Untersuchungen
             orderby p.Datum
             group p by p.Datum into g
             select new StatsistikObjekt() { Date1 = g.Key,
                                             number1 = g.Count() });

As for the return value - we can't really help you on that one, as we don't know the return type you're trying to return.

Try

g.Key instead of g.Datum