[Solved] C# Combine Multiple Lists of Classes based on Criteria Efficiently [closed]

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An optimal solution for this would have you traverse the entire list of volumes only once, or O(n). The loops you mentioned were doing some multiple of that. There may be a way to do this with Linq to attain O(n) efficiency, but without more testing I do not know.

However, you can get O(n) with a nested for loop. For my suggestion, you could use a dictionary where the key is the date (in format MM/DD/YYYY) and the value is the Master. 

Dictionary<DateTime, Master> masterVolumes = new Dictionary<DateTime, Master>();
List<Tank> tanks = SQLQueryToGetVolumes();
DateTime dt;
Master m;
foreach(Tank tank in tanks)
{
    foreach(Volume volume in tank.volume)
    {
        //Since dates may have times associated, we strip the time.
        dt = new DateTime(volume.date.Year, volume.date.Month, volume.date.Day);

        //See if the day already exists
        if (masterVolumes.ContainsKey(dt))
        {
            //If so, compare the current best volume
            m = masterVolumes[dt];
            if (m.bestVolume < volume.volume)
            {
                //There is a new master!
                m.bestTankName = tank.name;
                m.id = tank.id;
                m.bestVolume = volume.volume;
            }
        }
        else
        {
            //This date doesn't exist yet, so add it
            m = new Master()
            {
                date = dt,
                bestTankName = tank.name,
                id = tank.id,
                bestVolume = volume.volume
            };
            masterVolumes.Add(dt, m);
        }
    }
}

//At the end of this, the masterVolumes.Values will hold a collection of 
// the highest volume Master for each day.

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solved C# Combine Multiple Lists of Classes based on Criteria Efficiently [closed]