Delphi calling shgetfileinfo from a thread fails

Question

    function GetFileIcon(const filename:string): HICON;
var
  shfi: TShFileInfo;
begin
  try
    FillChar(shfi, SizeOf(TShFileInfo), 0);
    ShGetFileInfo(PChar(filename), 0, shfi, SizeOf(TShFileInfo), SHGFI_ICON or SHGFI_LARGEICON);
    Result := shfi.hIcon;
  except
    Result := 0;
  end;
end;

Using delphi xe2, on win 7 64bits, this function will often return 0 when called inside a Tthread, but is always working fine when called from main thread. It looks like a shell initialization problem, because after a while it will work in the Thread as well. I found a similar question in stack overflow (Calling SHGetFileInfo in thread to avoid UI freeze) but it is for c++ language so I did not sort it out.

• Update: It seems ShGetFileInfo is not threadsafe. When there are multiple threads calling it simultaneously, it fails. See David Hefferman's answer below. Also using CoInitializeEx instead of Coinitialize does not help with multiple threads. You have to serilize access using a TCriticalSection.

Answer 1

From the documentation:

You must initialize Component Object Model (COM) with CoInitialize or OleInitialize prior to calling SHGetFileInfo.

In a GUI app, the COM is initialized in the main thread. But from other threads that does not happen automatically. You will need to do it explicitly.

Beyond that you are not handling errors correctly. Remember that Windows API functions do not raise exceptions. So your exception handler is pointless and should be removed. Instead you need to check the return value of your call to SHGetFileInfo, as described in the documentation.

Beyond that your code works, as this program demonstrates:

{$APPTYPE CONSOLE}

uses
  Classes, Windows, ActiveX, ShellAPI;

var
  hThread: THandle;
  ThreadId: Cardinal;

function ThreadFunc(Parameter: Pointer): Integer;
var
  shfi: TSHFileInfo;
begin
  CoInitialize(nil);
  Try
    if ShGetFileInfo('C:\windows\explorer.exe', 0, shfi, SizeOf(shfi), SHGFI_ICON or SHGFI_LARGEICON)=0 then
    begin
      Writeln('ShGetFileInfo Failed');
      Result := 1;
      exit;
    end;
    Writeln(shfi.hIcon);
  Finally
    CoUninitialize;
  End;
  Result := 0;
end;

begin
  hThread := BeginThread(nil, 0, ThreadFunc, nil, 0, ThreadId);
  WaitForSingleObject(hThread, INFINITE);
  CloseHandle(hThread);
  Readln;
end.

I expect that any failure you observe is actually related to the particular file that you are trying to inspect.

---

Update: It seems ShGetFileInfo is not threadsafe. When there are multiple threads calling it simultaneously, it fails. I believe that you will need to serialize the calls to ShGetFileInfo with a lock. For instance, TCriticalSection.

The following program, based on the SSCCE you provided in the comments, demonstrates this:

{$APPTYPE CONSOLE}

uses
  SysUtils,
  Classes,
  SyncObjs,
  Windows,
  ActiveX,
  ShellAPI;

var
  hThreads: TWOHandleArray;
  ThreadId: Cardinal;
  Lock: TCriticalSection;

function ThreadFunc(Parameter: Pointer): Integer;
var
  shfi: TSHFileInfo;
  randomnumber: integer;
  fname: string;
begin
  CoInitialize(nil);
  Try
    fname := 'c:\desktop\file'+IntToStr(Integer(Parameter))+'.exe';

    Lock.Acquire;
    try
      if ShGetFileInfo(pchar(fname), 0, shfi, SizeOf(shfi), SHGFI_ICON or SHGFI_LARGEICON)=0 then
      begin
        Writeln('ShGetFileInfo Failed');
        Result := 1;
        exit;
      end;
      Writeln(shfi.hIcon);
    finally
      Lock.Release;
    end;
  Finally
    CoUninitialize;
  End;
  Result := 0;
end;

var
i: integer;
begin
  Lock := TCriticalSection.Create;
  for i := 0 to 9 do
    hThreads[i] := BeginThread(nil, 0, ThreadFunc, Pointer(i), 0, ThreadId);

  WaitForMultipleObjects(10, @hThreads,true, INFINITE);

  Readln;
end.

Remove the critical section, and the calls to ShGetFileInfo succeed, but return 0 for the icon handle. With the critical section, valid icon handles are returned.