listnode.cs

public class ListNode
{
    public int val;
    public ListNode next;

    public ListNode(int val = 0, ListNode next = null)
    {
        this.val = val;
        this.next = next;
    }

}

solution.cs

public ListNode AddTwoNumbers(ListNode l1, ListNode l2)
{
    return SumLists(l1, l2, 0);
}

public ListNode SumLists(ListNode l1, ListNode l2, int carry)
{
    if (l1 == null && l2 == null && carry == 0)
    {
        return null;
    }
    var val1 = l1 == null ? 0 : l1.val;
    var val2 = l2 == null ? 0 : l2.val;

    var sum = val1 + val2 + carry;

    carry = sum >= 10 ? 1 : 0;
    if (carry == 1)
    {
        sum -= 10;
    }

    return new ListNode(sum, SumLists(l1?.next, l2?.next, carry));
}

test_helper_test.cs

[Test]
public void ShouldBuildListNodeFromIntArray()
{
    var sut = new AddTwoNumbers_Problem();

    var input = new[] {2, 4, 3};
    var node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {5, 6, 4};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {9, 9, 9};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {0, 0, 0, 1};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {7, 0, 8};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {1};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

    input = new[] {0};
    node = sut.BuildList(input);
    input.ShouldBe(sut.Output(node));

}

test_helpers.cs

public ListNode BuildList(int[] input,int index=0)
{
    return index+1 < input.Length ? new ListNode(input[index], BuildList(input, ++index)) : new ListNode(input[index]);
}

public int[] Output(ListNode n)
{
    var res = new List<int>();
    var node = n;
    while (node != null)
    {
        res.Add(node.val);
        node = node.next;
    }

    return res.ToArray();
}

twonum_tests.cs

[Test]
public void ShouldAddNumbers()
{
    var sut = new AddTwoNumbers_Problem();

    var l1 = sut.BuildList(new[] {2, 4, 3});
    var l2 = sut.BuildList(new[] {5,6,4});
    var res = sut.AddTwoNumbers(l1, l2);
    sut.Output(res).ShouldBe(new[] {7, 0, 8});

    l1 = sut.BuildList(new[] {0});
    l2 = sut.BuildList(new[] {0});
    res = sut.AddTwoNumbers(l1, l2);
    sut.Output(res).ShouldBe(new[] {0});

    l1 = sut.BuildList(new[] {9,9,9});
    l2 = sut.BuildList(new[] {9});
    res = sut.AddTwoNumbers(l1, l2);
    sut.Output(res).ShouldBe(new[] {8,0,0,1});

    l1 = sut.BuildList(new[] {9,9,9,9,9,9,9});
    l2 = sut.BuildList(new[] {9,9,9,9});
    res = sut.AddTwoNumbers(l1, l2);
    sut.Output(res).ShouldBe(new[] {8,9,9,9,0,0,0,1});

}