// Operates with Q memory
void FantasyMethod() {
    var o = new BigObject();

    {
        var temp = new BigObject();
        Populate(temp); // Populates o1 with N megabytes of data

        o = PerformSomeOperationsOn(temp); // Returns a BigObject of size M (M is close to N)

        // Currently, M+N memory is occupied, we have Q-M-N free
    }

    // Let's tell the garbage collector to catch up
    GC.Collect();
    GC.WaitForPendingFinalizers();

    // Currently, M memory is occupied

    DoUsefulStuffWith(o); // This method can only work if at least Q-M-N/2 memory is free
}

using (var b = new DisposableObject())
{
   ...
}