How to know whether an user account exists

Question

1. How do I know if an user account exists on my Windows OS (Vista)? I need this information from a stand alone machine that hasn't joined any domain.

2. I want to know whether an user is a part of a group, for example is a user 'admin' part of 'Administrators' group or not?

Answer 1

I have tried the following code and is working fine for me..

    public bool IsUserMemberOfGroup(string userName, string groupName)
    {
        bool ret = false;

        try
        {
            DirectoryEntry localMachine = new DirectoryEntry("WinNT://" + Environment.MachineName);
            DirectoryEntry userGroup = localMachine.Children.Find(groupName, "group");

            object members = userGroup.Invoke("members", null);
            foreach (object groupMember in (IEnumerable)members)
            {
                DirectoryEntry member = new DirectoryEntry(groupMember);
                if (member.Name.Equals(userName, StringComparison.CurrentCultureIgnoreCase))
                {
                    ret = true;
                   break;
                }
            }
        }
        catch (Exception ex)
        {
            ret = false;
        }
        return ret;
    }