How do you read the user's display (first and last) name on all versions of Windows reliably?

Question

I have found that on Windows 7 64 bit, on a machine with a domain name, GetUserNameEx( 3, .... ) which should get the extended name format DisplayName (==3), into a buffer,

Answer 1

I have a solution that appears to work, which in general means:

1. if the GetUserNameEx(3,...) function from secur32.dll works, use that value.
2. fall back to a combination of GetUserNameEx(2,...) calls and NetUserGetInfo calls imported from netapi32.dll
3. The problem with invoking NetUserGetInfo first, is that it fails on domain names, or at least, the implementation below on NetUserGetInfo works only when reading SAM information from a local machine name, on a non-domain/non-ActiveDirectory user namespace.

Sample code (in Delphi), ported to C# below in Matt's answer:

type
EProcError = class( Exception );
TGetUserNameExWProc = function( FormatType : Integer; Buffer : PWideChar; var BufSize : Integer ) : DWORD;      stdcall;
var
  _GetUserNameExW : TGetUserNameExWProc;
procedure GetProcedureAddress( var P : Pointer; const ModuleName, ProcName : string );
var
  ModuleHandle : HMODULE;
begin
  if not Assigned( P ) then
  begin
    ModuleHandle := GetModuleHandle( pChar( ModuleName ) );
    if ModuleHandle = 0 then
    begin
      ModuleHandle := SafeLoadLibrary( pChar( ModuleName ) );
      if ModuleHandle = 0 then
        raise EProcError.Create( 'Unable to load module' );
    end;
    P := GetProcAddress( ModuleHandle, pChar( ProcName ) );
    if not Assigned( P ) then
      raise EProcError.Create( 'Unable to get proc address' );
  end;
end;
function MyGetUserNameEx( aFormat : Integer ) : string;
var
  sz : Integer;
  sz2 : Integer;
  ret : Integer;
begin
  if not Assigned( _GetUserNameExW ) then
    GetProcedureAddress( Pointer( @_GetUserNameExW ), 'secur32.dll', 'GetUserNameExW' );
  if Assigned( _GetUserNameExW ) then
  begin
    sz := 2000;
    SetLength( Result, sz );
    Result[ 1 ] := Chr( 0 );
    ret := _GetUserNameExW( { 3=NameDisplay } aFormat, PWideChar( Result ), sz );
    if ret <> 0 then
    begin
      sz2 := StrLen( PWideChar( Result ) ); // workaround WinXP API bug
      if sz2 < sz then // WinXP bug.
        sz := sz2;
      SetLength( Result, sz )
    end
    else
    begin
      ret := GetLastError;
      if ret = ERROR_NONE_MAPPED then
        Result := ''
      else
        Result := 'E' + IntToStr( ret );
    end;
  end;
end;
function MyNetUserGetInfo : string;
const
  netapi32 = 'netapi32.dll';
type
  TNetUserGetInfo = function( servername, username : LPCWSTR; level : DWORD; var bufptr : PByte ) : DWORD; stdcall;
  TNetApiBufferFree = function( Buffer : PByte ) : DWORD; stdcall;
  USER_INFO_10 = record
    usri10_name : PWideChar;
    usri10_comment : PWideChar;
    usri10_usr_comment : PWideChar;
    usri10_full_name : PWideChar;
  end;
  P_USER_INFO_10 = ^USER_INFO_10;
var
  _NetUserGetInfo : TNetUserGetInfo;
  _NetApiBufferFree : TNetApiBufferFree;
  ret : DWORD;
  servername : string;
  username : string;
  level : Cardinal;
  info : P_USER_INFO_10;
  pbuf : PByte;
  pwuser : PWideChar;
  n : Integer;
begin
  ret := 0;
  _NetUserGetInfo := nil;
  GetProcedureAddress( Pointer( @_NetUserGetInfo ), netapi32, 'NetUserGetInfo' ); // raises EProcError
  if not Assigned( _NetUserGetInfo ) then
    Result := 'FunctionNotFound'
  else
  begin
    // usernamesize := 200;
    username := MyGetUserNameEx( 2 );
    if username = '' then
    begin
      Result := 'CanNotGetUserName';
      Exit;
    end;
    n := Pos( '\', username );      //' recover SO code formatting
    if n > 0 then
    begin
      servername := '\\' + Copy( username, 1, n - 1 );
      username := Copy( username, n + 1, Length( username ) );
    end;
    level := 10;
    pbuf := nil;
    pwuser := PWideChar( username );
    info := nil;
    if servername = '' then
      ret := _NetUserGetInfo( { servername } nil, pwuser, level, pbuf )
    else
      ret := _NetUserGetInfo( PWideChar( servername ), pwuser, level, pbuf );
    if ret = 0 then
    begin
      info := P_USER_INFO_10( pbuf );
      if Assigned( info ) then
        Result := info.usri10_full_name;
      GetProcedureAddress( Pointer( @_NetApiBufferFree ), netapi32, 'NetApiBufferFree' );
      if Assigned( info ) and Assigned( _NetApiBufferFree ) then
        _NetApiBufferFree( pbuf );
    end
    else
    begin
      if ret = 2221 then
        Result := 'Error_USER ' + username
      else if ret = 1722 then
        Result := 'Error_RPC ' + servername
      else
        Result := 'E' + IntToStr( ret );
    end;
  end;
end;

Edit: Nov 2011; Removed dead link.