string UserName = "孟岩,王金龙,赵海伟,王楠";
string UserName2 = "王金龙,孟岩,赵海伟";
string[] result1 = UserName.Split(',');
string[] result2 = UserName2.Split(',');

string notIn1="", notIn2 = "";
foreach (var name1 in result1)
{
    bool found = false;
    foreach (var name2 in result2)
    {
        if(name1== name2)
        {
            found = true;
        }
    }
    if(!found)
    {
        notIn1 += name1;
    }
}

foreach (var name2 in result2)
{
    bool found = false;
    foreach (var name1 in result1)
    {
        if (name1 == name2)
        {
            found = true;
        }
    }
    if (!found)
    {
        notIn2 += name2;
    }
}
//notIn1,notIn2 就是不同的

            var UserName = "孟岩,王金龙,赵海伟,王楠";
            var UserName2 = "王金龙,孟岩,赵海伟";
            var a = UserName.Split(',').ToList();
            var b = UserName2.Split(',').ToList();
            var result = new List<string>();
            for (int i = 0; i < b.Count; i++)
            {
                for (int j = 0; j < a.Count; j++)
                {
                    if (b[i] == a[j])
                    {
                        a.Remove(a[j]);
                        break;
                    }
                }
            }
            result = a;