List.AddRange with IEnumerable<T> parameter not working?

Question

I have the following scenario where I want to add some items to a List...

List<T> items = new List<T>();
IEnumerable<T> addItems = someCollection.Where(...);
items.AddRange(addItems);

Using this code, no items are added to the list but if I add a .ToList() after then Linq statement then the items are added correctly. I guess this is due to deferred execution but I would have thought that given the List.AddRange function accepts an IEnumerable that it would enumerate the items to be added.

Can someone please clear up why this happens?

Answer 1

I guess this is due to deferred execution but I would have thought that given the List.AddRange function accepts an IEnumerable that it would enumerate the items to be added.

It does. There is a short circuit for ICollection<T> (which you wouldn't hit in this case), which would cause it to use ICollection<T>.CopyTo instead of enumerating the items, but otherwise, it will enumerate the collection.

For a working example, try:

using System;
using System.Linq;
using System.Collections.Generic;

internal class Program
{
    private static List<T> RunQuery<T>(IEnumerable<T> someCollection, Func<T, bool> predicate)
    {
        List<T> items = new List<T>();
        IEnumerable<T> addItems = someCollection.Where(predicate);
        items.AddRange(addItems);
        return items;
    }

    static void Main()
    {
        var values = Enumerable.Range(0, 1000);

        List<int> results = RunQuery(values, i => i >= 500);

        Console.WriteLine(results.Count);
        Console.WriteLine("Press key to exit:");
        Console.ReadKey();
    }
}

This uses your exact code, and will print out 500 (the proper number of items in the List<T>).