>
>if you hover over the var declaration in the linq statement, it will tell you type. Then simply move it to the top with that type.

Ok, this seems to compile

EnumerableRowCollection<DataRow> query; 
                  if (checkType && checkCategory)
                      query = from c in dtMax4SaleLimits.AsEnumerable() 
                                 where (2==c.Field<Byte>("type") && lsn_cat == c.Field<String>("lsn_cat"))
                                 || (3==c.Field<Byte>("type") && lessontype == c.Field<Int32>("lessontype"))
                              select c;