Calculate time interval between 2 dates on different rows - linq

Question

I have login and logout events and i need to calculate time between them.
I guess i could group each 2 rows (each two messages) and then do the calculation, but how would you do that?
Example XML i need to query:
<Log>
<Message>
<DateTime>2009-12-02 14:38:41</DateTime>
<Priority>Local3.Info</Priority>
<Source_Host>192.168.0.100</Source_Host>
<MessageText>Dec 2 14:38:41 root: login,ng1,192.168.0.160,janis.veinbergs</MessageText>
</Message>
<Message>
<DateTime>2009-12-02 15:28:19</DateTime>
<Priority>Local3.Info</Priority>
<Source_Host>192.168.0.100</Source_Host>
<MessageText>Dec 2 15:30:33 root: logout,ng1,,janis.veinbergs</MessageText>
</Message>
<Message>
<DateTime>2009-12-02 15:29:11</DateTime>
<Priority>Local3.Info</Priority>
<Source_Host>192.168.0.100</Source_Host>
<MessageText>Dec 2 15:31:25 root: login,ng1,192.168.0.160,janis.veinbergs</MessageText>
</Message>
<Message>
<DateTime>2009-12-02 15:58:22</DateTime>
<Priority>Local3.Info</Priority>
<Source_Host>192.168.0.100</Source_Host>
<MessageText>Dec 2 16:00:37 root: logout,ng1,,janis.veinbergs</MessageText>
</Message>
</Log>
Thankyou.

Answer 1

Given SQL doesn't have any aggregate Diff method, I would suggest joining the table onto itself, and selecting each row that you're after from each side of the join.
Something like:
var diff = from a in db.Events
join b in db.Events on a.SessionId equals b.SessionId
where a.EventType == 'Login' && b.EventType == 'Logout'
select b.EventTime - a.EventTime;
Haven't tried this, but something along those lines should work.
EDIT: updated to suit new provided info.
Try the following. Could perhaps be more concise, but does the job. Have broken down into a couple of queries for easier reading.
var query = from a in (from log in data.Elements()
select new {
date = DateTime.Parse(log.Element("DateTime").Value),
msg = log.Element("MessageText").Value
})
select new {
a.date,
type = a.msg.Contains("login") ? "Login" : "Logout",
user = a.msg.Substring(a.msg.LastIndexOf(',') + 1)
};
var results = from a in query
join b in query on a.user equals b.user
where a.type == "Login" && b.type == "Logout"
&& b.date == (query.OrderBy(o => o.date).Where(d => d.date > a.date).FirstOrDefault().date)
select new {
a.user,
Login = a.date,
Logout = b.date
};