delphi - How to check a DLL if a function exists?

Question

I'm working on something which dynamically loads specially formulated DLL's. I need to be able to check the DLL and make sure all the expected functions exist before I consider using this DLL. If it's missing some certain functions, I should not try to load it. I know I could attempt to call one of the functions and see if there's an exception, but I would see errors in debug mode.

How should I go about checking a DLL if a function exists? I'd like to check it before I load it (using LoadLibrary) but I guess it's OK if I have to load it to perform this check too.

UPDATE

I've accepted David's answer below, and thought I'd post my final code to show the whole process. I turned it into a function returning a Bool, whether it succeeded or not, cleaned the code a bit, and added another function at the bottom which uses this one to check each name one by one.

I decided to use this method instead of reading GetProcAddress because it will help me in the future with other things.

type
? PIMAGE_NT_HEADERS = ^IMAGE_NT_HEADERS;
? PIMAGE_EXPORT_DIRECTORY = ^IMAGE_EXPORT_DIRECTORY;

function ImageNtHeader(Base: Pointer): PIMAGE_NT_HEADERS; stdcall; 
  external 'dbghelp.dll';
function ImageRvaToVa(NtHeaders: Pointer; Base: Pointer; Rva: ULONG; 
  LastRvaSection: Pointer): Pointer; stdcall; external 'dbghelp.dll';

function ExportedFunctionNames(const ImageName: string; NamesList: TStrings): Bool;
var
  i: Integer;
  FileHandle: THandle;
  ImageHandle: THandle;
  ImagePointer: Pointer;
  Header: PIMAGE_NT_HEADERS;
  ExportTable: PIMAGE_EXPORT_DIRECTORY;
  NamesPointer: Pointer;
  Names: PAnsiChar;
  NamesDataLeft: Integer;
begin
  Result:= False;
  NamesList.Clear;
  FileHandle:= CreateFile(PChar(ImageName), GENERIC_READ, FILE_SHARE_READ,
    nil, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
  if FileHandle = INVALID_HANDLE_VALUE then Exit;
  try
    ImageHandle:= CreateFileMapping(FileHandle, nil, PAGE_READONLY, 0, 0, nil);
    if ImageHandle = 0 then Exit;
    try
      ImagePointer:= MapViewOfFile(ImageHandle, FILE_MAP_READ, 0, 0, 0);
      if not Assigned(ImagePointer) then Exit;
      try
        Header:= ImageNtHeader(ImagePointer);
        if not Assigned(Header) then Exit;
        if Header.Signature <> $00004550 then Exit; // "PE" as a DWORD.
        ExportTable:= ImageRvaToVa(Header, ImagePointer,
          Header.OptionalHeader.DataDirectory[0].VirtualAddress, nil);
        if not Assigned(ExportTable) then Exit;
        NamesPointer:= ImageRvaToVa(Header, ImagePointer,
          Cardinal(ExportTable.AddressOfNames), nil);
        if not Assigned(NamesPointer) then Exit;
        Names:= ImageRvaToVa(Header, ImagePointer, Cardinal(NamesPointer^), nil);
        if not Assigned(Names) then Exit;
        NamesDataLeft:= Header.OptionalHeader.DataDirectory[0].Size;
        for i:= 0 to ExportTable.NumberOfNames - 1 do begin
          NamesList.Add(Names);
          while (Names^ <> chr(0)) and (NamesDataLeft > 0) do begin
            Inc(Names);
            Dec(NamesDataLeft);
          end;
          Inc(Names);
        end;
        Result:= True;
      finally
        UnmapViewOfFile(ImagePointer);
      end;
    finally
      CloseHandle(ImageHandle);
    end;
  finally
    CloseHandle(FileHandle);
  end;
end;

function IsMyDLL(const Filename: String): Bool;
var
  H: THandle;
  L: TStringList;
  function InList(const Func: String): Bool;
  begin
    Result:= L.IndexOf(Func) >= 0;
  end;
begin
  Result:= False;
  L:= TStringList.Create;
  try
    if ExportedFunctionNames(Filename, L) then begin
      Result:=//Names of functions which need to exist
        InList('GetName') and
        InList('GetDescription') and
        InList('GetVersion') and
        InList('Start') and
        InList('Stop');
    end;
  finally
    L.Free;
  end;
end;

Answer 1

You have to use LoadLibrary, and then use GetProcAddress for each function you want to check existence for. There's really no other reasonable choice (unless there are specific reasons you need to avoid`LoadLibrary). Since your intent seems to be just to check to see if the functions are present and nothing more, LoadLibrary and GetProcAddress are the simplest means to do so; you can do all of the work in very few lines of code, and error checking is extremely simple and straightforward.