Can I modify a constant in the RTL class System.Classes.TStream and rebuild it at runtime in Delphi XE6?

Question

I am trying to work around a known ugly performance limitation in System.Classes.pas, which has a 1980s era constant buffer limit ($F000) that looks like this:

Answer 1

You can overcome this limitation using a detour, try this sample which uses the Delphi Detours Library

First define the signature of the method to hook

var
 Trampoline_TStreamCopyFrom : function (Self : TStream;const Source: TStream; Count: Int64): Int64 = nil;

then implement the detour

function Detour_TStreamCopyFrom(Self : TStream;const Source: TStream; Count: Int64): Int64;
const
  MaxBufSize = 1024*1024; //use 1 mb now :)
var
  BufSize, N: Integer;
  Buffer: TBytes;
begin
  if Count <= 0 then
  begin
    Source.Position := 0;
    Count := Source.Size;
  end;
  Result := Count;
  if Count > MaxBufSize then BufSize := MaxBufSize else BufSize := Count;
  SetLength(Buffer, BufSize);
  try
    while Count <> 0 do
    begin
      if Count > BufSize then N := BufSize else N := Count;
      Source.ReadBuffer(Buffer, N);
      Self.WriteBuffer(Buffer, N);
      Dec(Count, N);
    end;
  finally
    SetLength(Buffer, 0);
  end;
end;

Finally replace the original function by the trampoline (you can use this code in the initialization part of some unit)

  Trampoline_TStreamCopyFrom     := InterceptCreate(@TStream.CopyFrom,   @Detour_TStreamCopyFrom);

And to release the hook you can use

 if Assigned(Trampoline_TStreamCopyFrom) then
   InterceptRemove(@Trampoline_TStreamCopyFrom);