c# - Linq query summing dates in range -

trying work out if there's nice way of doing this, preferably in linq rather loops, i've done now.

i want number of days product active between 2 dates.

product has many producthistories, producthistory has values dateactive , dateinactive. dateinactive can null, , be, show history ongoing. histories won't overlap.

i want sum maximum of startdate or producthistory's dateactive, , subtract total sum of minimum of enddate or producthistory's dateinactive (enddate default if dateinactive has no value).

worked example:

get active number of active days between 2011-01-01 (1st jan) , 2011-02-01 (1st feb)

histories:

active 2010-12-25 : inactive 2011-01-05 active 2011-01-15 : inactive 2011-01-20 active 2011-01-25 : inactive null 

this should total 16 days (4 + 5 + 7)

my best @ moment rather ugly:

    dim integer = (from h in histories             select if(h.activedate > startdate, h.activedate, startdate)).sum(function(d) d.ticks) / timespan.ticksperday      dim b integer = (from h in histories             select if(h.inactivedate.hasvalue andalso h.inactivedate < enddate, h.inactivedate, enddate)).sum(function(d) d.value.ticks) / timespan.ticksperday  return b - 

any thoughts? (either c# or vb fine, i'll translate)

something this? (assumes no overlapping histories)

var numdays = product              .producthistories              .where(ph => ph.active >= start && ph.active < end)              .sum(ph => ph.active                           .subtract(end > ph.inactive ? ph.inactive.value : end)                           .duration() // because doing start - end                           .days); 

1. only consider histories start within date-range.
2. for each history, construct timespan delta between active-date , earlier of inactive-date , end-date.
3. sum them up.

to improve this, suggest:

1. write isbetween extension let write lambda ph.active.isbetween(start, end).
2. extract sum lambda out different method, , write more intuitively. there's use of "lifted" operator on datetime? many find quite unintuitive.