ArrayList arr1 = new ArrayList(new int[] { 3, 4, 5, 6, 7 });
ArrayList arr2 = new ArrayList(new int[] { 4, 5, 6, 7, 8 });
ArrayList result = new ArrayList();
foreach (int i in arr2)
{
    if (arr1.Contains(i))
        arr1.Remove(i);
    else
        result.Add(i);
}
result.AddRange(arr1);

//查看结果
foreach (int i in result)
    richTextBox2.Text += i.ToString() + "\n";