Сравнение двух файлов

1-й вариант

function Are2FilesEqual(const File1, File2: TFileName): Boolean;
var
  ms1, ms2: TMemoryStream;
begin
  Result := False;
  ms1 := TMemoryStream.Create;
  try
    ms1.LoadFromFile(File1);
    ms2 := TMemoryStream.Create;
    try
      ms2.LoadFromFile(File2);
      if ms1.Size = ms2.Size then
        Result := CompareMem(ms1.Memory, ms2.memory, ms1.Size);
    finally
      ms2.Free;
    end;
  finally
    ms1.Free;
  end
end;

Пример использования:

procedure TForm1.Button1Click(Sender: TObject);
begin
  if Opendialog1.Execute then
    if Opendialog2.Execute then
      if Are2FilesEqual(Opendialog1.FileName,
                        Opendialog2.FileName) then
        ShowMessage('Files are equal.');
end;

2-й вариант:

function FilesAreEqual(const File1, File2: TFileName): Boolean;
const
  BlockSize = 65536;
var
  fs1, fs2: TFileStream;
  L1, L2: Integer;
  B1, B2: array[1..BlockSize] of Byte;
begin
  Result := False;
  fs1 := TFileStream.Create(File1, fmOpenRead or fmShareDenyWrite);
  try
    fs2 := TFileStream.Create(File2, fmOpenRead or fmShareDenyWrite);
    try
      if fs1.Size = fs2.Size then
      begin
        while fs1.Position < fs1.Size do
        begin
          L1 := fs1.Read(B1[1], BlockSize);
          L2 := fs2.Read(B2[1], BlockSize);
          if L1 <> L2 then
          begin
            Exit;
          end;
          if not CompareMem(@B1[1], @B2[1], L1) then Exit;
        end;
        Result := True;
      end;
    finally
      fs2.Free;
    end;
  finally
    fs1.Free;
  end;
end;