Convert a username to a SID string in C#/.NET

Question

There's a question about converting from a SID to an account name; there isn't one for the other way around.

How do you convert a username to a SID string, for example, to find out which HKEY_USERS subkey relates to a user of a given name?

Answer 1

The podcast tells me I should ask, and answer, questions when they're not answered on SO already. Here goes.

The easy way, with .NET 2.0 and up, is this:

NTAccount f = new NTAccount("username");
SecurityIdentifier s = (SecurityIdentifier) f.Translate(typeof(SecurityIdentifier));
String sidString = s.ToString();

The hard way, which works when that won't, and works on .NET 1.1 also:

[DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern bool LookupAccountName([In,MarshalAs(UnmanagedType.LPTStr)] string systemName, [In,MarshalAs(UnmanagedType.LPTStr)] string accountName, IntPtr sid, ref int cbSid, StringBuilder referencedDomainName, ref int cbReferencedDomainName, out int use);

[DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
internal static extern bool ConvertSidToStringSid(IntPtr sid, [In,Out,MarshalAs(UnmanagedType.LPTStr)] ref string pStringSid);


/// <summary>The method converts object name (user, group) into SID string.</summary>
/// <param name="name">Object name in form domain\object_name.</param>
/// <returns>SID string.</returns>
public static string GetSid(string name) {
    IntPtr _sid = IntPtr.Zero; //pointer to binary form of SID string.
    int _sidLength = 0;   //size of SID buffer.
    int _domainLength = 0;  //size of domain name buffer.
    int _use;     //type of object.
    StringBuilder _domain = new StringBuilder(); //stringBuilder for domain name.
    int _error = 0;
    string _sidString = "";

    //first call of the function only returns the sizes of buffers (SDI, domain name)
    LookupAccountName(null, name, _sid, ref _sidLength, _domain, ref _domainLength, out _use);
    _error = Marshal.GetLastWin32Error();

    if (_error != 122) //error 122 (The data area passed to a system call is too small) - normal behaviour.
    {
        throw (new Exception(new Win32Exception(_error).Message));
    } else {
        _domain = new StringBuilder(_domainLength); //allocates memory for domain name
        _sid = Marshal.AllocHGlobal(_sidLength); //allocates memory for SID
        bool _rc = LookupAccountName(null, name, _sid, ref _sidLength, _domain, ref _domainLength, out _use);

        if (_rc == false) {
            _error = Marshal.GetLastWin32Error();
            Marshal.FreeHGlobal(_sid);
            throw (new Exception(new Win32Exception(_error).Message));
        } else {
            // converts binary SID into string
            _rc = ConvertSidToStringSid(_sid, ref _sidString);

            if (_rc == false) {
                _error = Marshal.GetLastWin32Error();
                Marshal.FreeHGlobal(_sid);
                throw (new Exception(new Win32Exception(_error).Message));
            } else {
                Marshal.FreeHGlobal(_sid);
                return _sidString;
            }
        }
    }
}