检查列表是否包含一个属性相同的对象的优雅方法，并且仅在另一个属性的日期更晚时才替换

Question

I have a class as follows :

Object1{
   int id;
   DateTime time;
}

I have a list of Object1 . I want to cycle through another list of Object1 , search for an Object1 with the same ID and replace it in the first list if the time value is later than the time value in the list. If the item is not in the first list, then add it.

I'm sure there is an elegant way to do this, perhaps using linq? :

List<Object1> listOfNewestItems = new List<Object1>();
List<Object1> listToCycleThrough = MethodToReturnList();
foreach(Object1 object in listToCycleThrough){
   if(listOfNewestItems.Contains(//object1 with same id as object))
   {
      //check date, replace if time property is > existing time property 
   } else {
      listOfNewestItems.Add(object)
}

Obviously this is very messy (and that's without even doing the check of properties which is messier again...), is there a cleaner way to do this?

Answer 1

var finalList = list1.Concat(list2)
                     .GroupBy(x => x.id)
                     .Select(x => x.OrderByDescending(y=>y.time).First())
                     .ToList();

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here is the full code to test

public class Object1
{
    public int id;
    public DateTime time;
}

List<Object1> list1 = new List<Object1>() 
{
    new Object1(){id=1,time=new DateTime(1991,1,1)},
    new Object1(){id=2,time=new DateTime(1992,1,1)}
};

List<Object1> list2 = new List<Object1>() 
{
    new Object1(){id=1,time=new DateTime(2001,1,1)},
    new Object1(){id=3,time=new DateTime(1993,1,1)}
};

and OUTPUT:

1 01.01.2001 
2 01.01.1992 
3 01.01.1993 

Answer 2

This is how to check:

foreach(var object in listToCycleThrough)
{
    var currentObject = listOfNewestItems
                              .SingleOrDefault(obj => obj.Id == object.Id);

    if(currentObject != null)
    {
        if (currentObject.Time < object.Time) 
                  currentObject.Time = object.Time
    } 

    else 
        listOfNewestItems.Add(object)
}

But if you have large data, would be suggested to use Dictionary in newest list, time to look up will be O(1) instead of O(n)

Answer 3

You can use LINQ. Enumerable.Except to get the set difference(the newest), and join to find the newer objects.

var listOfNewestIDs = listOfNewestItems.Select(o => o.id);
var listToCycleIDs = listToCycleThrough.Select(o => o.id);
var newestIDs = listOfNewestIDs.Except(listToCycleIDs);
var newestObjects = from obj in listOfNewestItems
                    join objID in newestIDs on obj.id equals objID
                    select obj;
var updateObjects = from newObj in listOfNewestItems
                    join oldObj in listToCycleThrough on newObj.id equals oldObj.id
                    where newObj.time > oldObj.time
                    select new { oldObj, newObj };

foreach (var updObject in updateObjects)
    updObject.oldObj.time = updObject.newObj.time;
listToCycleThrough.AddRange(newestObjects);

Note that you need to add using System.Linq; .

Here's a demo: http://ideone.com/2ASli

Answer 4

I'd create a Dictionary to lookup the index for an Id and use that

var newItems = new List<Object1> { ...
IList<Object1> itemsToUpdate = ... 

var lookup = itemsToUpdate.
        Select((i, o) => new { Key = o.id, Value = i }).
        ToDictionary(i => i.Key, i => i.Value);

foreach (var newItem in newitems)
{
    if (lookup.ContainsKey(newitem.ID))
    {
        var i = lookup[newItem.Id];
        if (newItem.time > itemsToUpdate[i].time)
        {
            itemsToUpdate[i] = newItem;
        }
    }
    else
    {
        itemsToUpdate.Add(newItem)
    }
}

That way, you wouldn't need to reenumerate the list for each new item, you'd benefit for the hash lookup performance.

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This should work however many times an Id is repeated in the list of new items.