Separate collection on 2 different NEW collections

Question

I have collection of elements and one additional small collection as filter.

I need to separate it on 2 new collections by some filter. In my case it is first collection that contains some elements and another that doesn't.

There aren't items that doesn't exists out of that 2 new collections.

I did it like :

var collection1= baseCollection.Where(r => filterCollection.Contains(r.Property)).ToList();
var collection2= baseCollection.Where(r => !filterCollection.Contains(r.Property)).ToList();

But is there another, I hope more elegant way, to separate collection? For me it looks like "I repeat myself", use almost the same code 2 times.

score 8 · Accepted Answer · edited Sep 29 '17 at 17:35

You can create a variable for the function - this way you will not "repeat yourself" (wouldn't use in this case, there are better options below, but still an option):

Func<YourClass,bool> filtering = (r) => filterCollection.Contains(r.Property);

var collection1 = baseCollection.Where(r => filtering(r));
var collection2 = baseCollection.Where(r => !filtering(r));

If your type of the collection overrides Equals and GetHashCode you can use Except:

var collection1 = baseCollection.Where(r => filterCollection.Contains(r.Property));
var collection2 = baseCollection.Except(collection1);

Using Except with a given IEqualityComparer (Check also first comment for guidlines):

public class Comparer : IEqualityComparer<YourClass>
{
    public bool Equals(YourClass x, YourClass y)
    {
        // Your implementation
    }

    public int GetHashCode(YourClass obj)
    {
        // Your implementation
    }
}

var collection1 = baseCollection.Where(r => filterCollection.Contains(r.Property));
var collection2 = baseCollection.Except(collection1, new Comparer());

You can also use GroupBy (probably less good performance wise):

var result baseCollection.GroupBy(r => filterCollection.Contains(r.Property))
                         .ToDictionary(key => key.Key, value => value.ToList());

var collection1 = result[true];
var collection2 = result[false];

Otherwise another way will just to use a loop:

List<YourType> collection1 = new List<YourType>();
List<YourType> collection2 = new List<YourType>();
foreach(var item in baseCollection)
{
    if(filterCollection.Contains(item.Property))
    {
        collection1.Add(item);
    }
    else
    {
        collection2.Add(item);
    }
}

@demo - about performance I'm not sure. One must check. Probably the fastest is option 3 - *o(n) for the baseCollection part (not including the contains because all solutions use it)*. — Gilad Green, Aug 26 '16 at 11:18
`Except` is faster than `Where` since it's using a set based approach to filter. Instead of overriding `Equals` + `GetHashCode` you could also use the [overload](https://msdn.microsoft.com/en-us/library/bb336390(v=vs.110).aspx) that takes an `IEqualityComparer`. — Tim Schmelter, Aug 26 '16 at 11:19

Separate collection on 2 different NEW collections

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