Linq 中的 Take(x) 是否在获取 x 个对象时停止枚举？

Question

For example, if I have this code:

    public static void Main(string[] args)
    {
        List<int> list = new List<int>() { 2, 3, 2, 9, 10, 2, 5 };

        var out = list.Where(x => x == 2).Take(2).ToList();
    }

Is the number of iterations 3 (as the second two is in index 2) or 7 (total number of elements)?

Thanks

Answer 1

Yes, stops.

You can see this clearly by rewriting the code as follows:

var result = list.Where(x =>
    {
        Console.WriteLine("Where: " + x);
        return x == 2;
    })
    .Take(2).ToList();

Answer 2

list will be iterated by the Where function, returning only matching items.
Where will be iterated by Take , which stops after 2 results.
Take is fully iterated by ToList

So the end result is that the iteration of list is stopped by Take at the second item of 2.

Answer 3

You can easily check it yourself. Let's test the hypothesis that 9 is reached (ie at least 4 items has been iterated):

var result = list
  .Where(x => x == 2)        // your query under test
  .Take(2)
  .Select(item => item != 9  // business as usual for the first 3 items
     ? item                  // throw exception on the 4th
     : throw new Exception("Strange execution: 9 (4th item) has been scanned"))
  .ToList();                 // materialization executes the query

Run it and you'll see that 4th item ( 9 ) has not been taken: no exception has been thrown.

Answer 4

I think the most convincing (and simple) answer is to look at the source code of TakeIterator that runs when Take is called:

static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> source, int count) 
{
    if (count > 0) {
        foreach (TSource element in source) {
            yield return element;
            if (--count == 0) break; // Yep, it stops after "count" iterations
        }
    }
}

Answer 5

If you write some test code with your own IEnumerable and IEnumerator, it will be easy to see what happens.

class MyCollection : IEnumerable<int>
{
    public List<int> Data {get; set;} = new List<int>() { 2, 3, 2, 9, 10, 2, 5 };

    public IEnumerator<int> GetEnumerator()
    {
         return new MyEnumerator()
         {
             Data = this.Data,
         };
    }
}

And the enumerator:

class MyEnumerator : IEnumerator<int>
{
    private int index = -1;
    public List<int> Data {get; set;}

    public void Reset()
    {
        this.index = -1;
    }

    public bool MoveNext()
    {
        ++this.index;
        return this.index < this.Data.Count;
    }

    public int Current
    {
        get
        {
            int returnValue = this.Data[this.index];
            Debug.WriteLine("[{0}] {1}", this.index, returnValue);
            return returnValue;
        }
    }
}

Test code:

void Main()
{
    MyCollection collection = new MyCollection();
    var out = collection.Where(x => x == 2).Take(2).ToList();
}