[Solved]-Linq, double left join and double count-LINQ,C#

Wouldn't something like this work?

from main in Main
select new {
    Title = main.Title,
    Details = main.Details.Count(),
    Messages = main.Details.Sum( d => d.Messages.Count())
}

If you have foreign key constraints, LINQ should generate the child-relationships automatically, so you could access main.Details for each Detail associated with main, and main.Messages for each Message associated with main.