Delphi 2005, can't build Indy 10

Question

I'm receiving an error when building the IdMessageHelper.pas unit in the IndyProtocols90 package. All instances of LoadFromStream and LoadFromFile are claiming there is an issue with the signature:

[Error] IdMessageHelper.pas(78): E2250 There is no overloaded version of 'LoadFromStream' that can be called with these arguments

procedure Internal_TIdMessageHelper_LoadFromStream(AMsg: TIdMessage; AStream: TStream;
  const AHeadersOnly: Boolean; const AUsesDotTransparency: Boolean);
var
  LMsgClient: TIdMessageClient;
begin
  if AUsesDotTransparency then begin
    AMsg.LoadFromStream(AStream, AHeadersOnly);
  end else
  begin
    // clear message properties, headers before loading
    AMsg.Clear;
    LMsgClient := TIdMessageClient.Create;
    try
      Internal_TIdMessageClientHelper_ProcessMessage(LMsgClient, AMsg, AStream, AHeadersOnly, False);
    finally
      LMsgClient.Free;
    end;
  end;
end;

I see that IdMessageHelper is new to this version, but the method that's being called ( IdMessage.LoadFromStream for example), the arguments for it haven't changed from the last few versions - at least not for the ones that I have the source.

procedure TIdMessage.LoadFromStream(AStream: TStream; const AHeadersOnly: Boolean = False);
var
  LMsgClient: TIdMessageClient;
begin
  // clear message properties, headers before loading
  Clear;
  LMsgClient := TIdMessageClient.Create;
  try
    LMsgClient.ProcessMessage(Self, AStream, AHeadersOnly);
  finally
    LMsgClient.Free;
  end;
end;

I'm pretty sure I removed all previous versions and packages since this was a clean install of D2005.

Answer 1

The IdMessageHelper unit introduces new LoadFrom...() and SaveTo...() methods for the TIdMessage component, to add an AUsesDotTransparency parameter when loading/saving emails.

In Delphi 2005 and later, it does this by defining a class helper (which is a feature introduced in Delphi 2005) to add new methods to the TIdMessage component without having to modify the IdMessage.pas unit itself. This allows Indy to let people use familiar IdMessage1.LoadFrom...() and IdMessage1.SaveTo...() syntax when using the new functionality ¹ .

Things were done this way so as not to cause interface breaking changes in the IdMessage unit itself. I blogged about this new addition at the time the IdMessageHelper.pas unit was first added to Indy:

New TIdMessage helper

In your case, the error message is complaining about Line 78:

AMsg.LoadFromStream(AStream, AHeadersOnly);

That line is the new 3-parameter TIdMessageHelper.LoadFromStream() method attempting to call the pre-existing 2-parameter TIdMessage.LoadFromStream() method when AUsesDotTransparency is True:

procedure TIdMessage.LoadFromStream(AStream: TStream; const AHeadersOnly: Boolean = False);

I have tested this new class helper in later Delphi versions and it works fine for me. You should not be getting a compiler error, as there should not be any ambiguity.

However, I have not tested the class helper in Delphi 2005 specifically (as I do not have that version installed), so it is possible that the compiler error could be indicating that class helpers (being a new language feature at the time) were still a little buggy, and were fixed later on.

If you can't find the cause of the ambiguity, you can work around the issue by modifying IdMessageHelper.pas to undefine HAS_CLASS_HELPER for Delphi 2005 ¹ , and then recompile Indy again.

¹ : In older versions of Delphi where class helpers are not available, IdMessageHelper.pas also defines several standalone TIdMessageHelper_LoadFrom...() and TIdMessageHelper_SaveTo...() functions, so people can still utilize the new AUsesDotTransparency functionality, just with a less-desirable calling syntax.

---

EDIT : it turns out that class helpers were very buggy in Delphi 2005, and were not officially supported until Delphi 2006 :

Class helpers have now been formally introduced in the Win32 compiler [in Delphi 2006]. In Delphi 2005 class helpers were not formally available, and although you could use them they were actually quite buggy . It was quite easy to get internal compiler errors while using them, nothing you could complain to with Borland about as this feature was not officially supported .

So, I have now disabled the TIdMessageHelper helper class in Delphi 2005, and have updated the above mentioned blog article accordingly.