无法将类型为“WhereListIterator`1[System.Object]”的对象转换为类型“System.Collections.Generic.IEnumerable`1[System.Int32]”

Question

I'm trying to write function to get return all integers from object list and I keep getting: 'Unable to cast object of type 'WhereListIterator 1[System.Object]' to type 'System.Collections.Generic.IEnumerable 1[System.Int32]'.'

static void Main(string[] args)
    {
        var list = new List<object>() { 1, 2, "a", "b" };
        Console.WriteLine(GetIntegersFromList(list));
    }

    public static IEnumerable<int> GetIntegersFromList(List<object> listOfItems)
    {
        IEnumerable<int> ints = (IEnumerable<int>) listOfItems.Where(x => x is int);
        return ints.ToList();
    }

I tried casting it, not casting it, adding ToList() everywhere and I always get the Invalid Cast Exception.

The output should be: {1, 2}

Answer 1

Linq's Where() returns a WhereListIterator<T> , with T being the T of the source IEnumerable<T> , in your case still object .

Either Cast<T> :

IEnumerable<int> ints = (IEnumerable<int>)listOfItems.Where(x => x is int).Cast<int>();

Or, shorter, use OfType<T>() :

IEnumerable<int> ints = listOfItems.OfType<int>();

Answer 2

If you're trying to write the integers to the console, you will need to convert the IEnumerable to a string :

var list = new List<object>() { 1, 2, "a", "b" };
Console.WriteLine(string.Join(", ", list.OfType<int>()));

// Output: 1, 2

Or iterate over the IEnumerable :

var list = new List<object>() { 1, 2, "a", "b" };
foreach (int i in list.OfType<int>()) Console.WriteLine(i);

// Output:
// 1
// 2

---

If you must implement GetIntegersFromList then you can just create a simple pass through:

public static IEnumerable<int> GetIntegersFromList(List<object> listOfItems)
    => listOfItems.OfType<int>();

Or if you cannot use LINQ:

public static IEnumerable<int> GetIntegersFromList(List<object> listOfItems)
{
    foreach (var item in listOfItems)
    {
        if (item is int i) yield return i;
    }
}