C# Signature:

[DllImport("advapi32.dll", SetLastError=true)]
static extern bool LookupAccountName (
    string lpSystemName,
    string lpAccountName,
    [MarshalAs(UnmanagedType.LPArray)] byte[] Sid,
    ref uint cbSid,
    StringBuilder ReferencedDomainName,
    ref uint cchReferencedDomainName,
    out SID_NAME_USE peUse);

VB .NET Signature:

<DllImport("advapi32.dll", SetLastError:=True)> _
Private Shared Function LookupAccountName(lpSystemName As String, lpAccountName As String, <MarshalAs(UnmanagedType.LPArray)> Sid As Byte(), ByRef cbSid As UInteger, ReferencedDomainName As StringBuilder, ByRef cchReferencedDomainName As UInteger, peUse As SID_NAME_USE) As Boolean
End Function

User-Defined Types:

None.

Notes:

From the SDK:

The LookupAccountName function attempts to find a SID for the specified name by first checking a list of well-known SIDs. If the name does not correspond to a well-known SID, the function checks built-in and administratively defined local accounts. Next, the function checks the primary domain. If the name is not found there, trusted domains are checked.

Use fully qualified account names (for example, domain_name\user_name) instead of isolated names (for example, user_name). Fully qualified names are unambiguous and provide better performance when the lookup is performed. This function also supports fully qualified DNS names (for example, example.example.com\user_name) and user principal names (UPN) (for example, someone@example.com).

Windows NT: DNS and UPN names are not supported by this function.

In addition to looking up local accounts, local domain accounts, and explicitly trusted domain accounts, LookupAccountName can look up the name for any account in any domain in the forest.

Windows NT: Forest lookup is not supported.

Tips & Tricks:

Please add some!

C# Sample Code:

   using System;
   using System.Runtime.InteropServices;
   using System.Text;

   namespace test
   {
    class clsLookupAccountName
    {
        const int NO_ERROR = 0;
        const int ERROR_INSUFFICIENT_BUFFER = 122;
        const int ERROR_INVALID_FLAGS = 1004; // On Windows Server 2003 this error is/can be returned, but processing can still continue

        enum SID_NAME_USE
        {
            SidTypeUser = 1,
            SidTypeGroup,
            SidTypeDomain,
            SidTypeAlias,
            SidTypeWellKnownGroup,
            SidTypeDeletedAccount,
            SidTypeInvalid,
            SidTypeUnknown,
            SidTypeComputer
        }

        [DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError = true)]
        static extern bool LookupAccountName (
            string lpSystemName,
            string lpAccountName,
            [MarshalAs(UnmanagedType.LPArray)] byte[] Sid,
            ref uint cbSid,
            StringBuilder ReferencedDomainName,
            ref uint cchReferencedDomainName,
            out SID_NAME_USE peUse);        

        [DllImport("advapi32", CharSet=CharSet.Auto, SetLastError=true)]
        static extern bool ConvertSidToStringSid(
            [MarshalAs(UnmanagedType.LPArray)] byte [] pSID,
            out IntPtr ptrSid);

        [DllImport("kernel32.dll")]
        static extern IntPtr LocalFree(IntPtr hMem);

        [STAThread]
        static void Main(string[] args)
        {
            string accountName = "Administrator";
            byte [] Sid = null;
            uint cbSid = 0;
            StringBuilder referencedDomainName = new StringBuilder();
            uint cchReferencedDomainName = (uint)referencedDomainName.Capacity;
            SID_NAME_USE sidUse;

            int err = NO_ERROR;
            if (!LookupAccountName(null,accountName,Sid,ref cbSid,referencedDomainName,ref cchReferencedDomainName,out sidUse))
            {
                err = Marshal.GetLastWin32Error();
                if (err == ERROR_INSUFFICIENT_BUFFER || err == ERROR_INVALID_FLAGS)
                {
                    Sid = new byte[cbSid];
                    referencedDomainName.EnsureCapacity((int)cchReferencedDomainName);
                    err = NO_ERROR;
                    if (!LookupAccountName(null,accountName,Sid,ref cbSid,referencedDomainName,ref cchReferencedDomainName,out sidUse))
                        err = Marshal.GetLastWin32Error();
                }
            }
            else
            {
                // Consider throwing an exception since no result was found
            }
            if (err == 0)
            {
                IntPtr ptrSid;
                if (!ConvertSidToStringSid(Sid,out ptrSid))
                {
                    err = Marshal.GetLastWin32Error();
                    Console.WriteLine(@"Could not convert sid to string. Error : {0}",err);
                }
                else
                {
                    string sidString = Marshal.PtrToStringAuto(ptrSid);
                    LocalFree(ptrSid);
                    Console.WriteLine(@"Found sid {0} : {1}",sidUse,sidString);
                }
            }
            else
                Console.WriteLine(@"Error : {0}",err);
        }
    }
   }

VB.Net Sample Code:

    Dim ret As Boolean
    Dim ptrSid As IntPtr
    Dim cbSid As Integer
    Dim ptrSidString As IntPtr
    Dim SidString As String
    Dim refDomainName As New StringBuilder
    Dim cbRefDomainName As Integer
    Dim peUse As SID_USE

    'First get the buffer sizes
    ret = LookupAccountName(Nothing, UserName, Nothing, cbSid, Nothing, cbRefDomainName, peUse)

    'Adjust the buffers to the needed size
    ptrSid = Marshal.AllocHGlobal(cbSid)
    refDomainName.EnsureCapacity(cbRefDomainName)

    'Get the data again, now with the changed buffers
    ret = LookupAccountName(Nothing, UserName, ptrSid, cbSid, refDomainName, cbRefDomainName, peUse)

    If ret Then
       'If return doesn't fail (ret=true) then convert the buffer data at ptrSid to the SidString)
        If ConvertSidToStringSid(ptrSid, ptrSidString) Then

           'Since ptrSidString is a memory pointer we need to make it a string
        Sid = Marshal.PtrToStringAuto(ptrSidString)
           msgBox(Sid)
           'Finally free up the memory
        LocalFree(ptrSidString)

        End If
    End If

    Marshal.FreeHGlobal(ptrSid)

Alternative Managed API:

TODO