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procedure TForm1.Button1Click(Sender: TObject);
var
  LBuffer: TBytes;
  LByteOrderMark: TBytes;
  LFileStream: TFileStream;
  LOffset: Integer;
  LEncoding, DestEncoding: TEncoding;
  ComboIndex: Integer;
  ByteString: RawByteString;
  FileText: string;
  ucs4Text: UCS4String;
  wideStringText: WideString;
  utf8Text: UTF8String;
  L: Cardinal;

begin
  LEncoding:= nil;
  ComboIndex := ComboBox1.ItemIndex;
  LFileStream := TFileStream.Create(Edit1.Text, fmOpenRead);
  try
    // Read the file into buffer.
    SetLength(LBuffer, LFileStream.Size);
    LFileStream.ReadBuffer(Pointer(LBuffer)^, Length(LBuffer));
    // Get data encoding of read data.
    LOffset := TEncoding.GetBufferEncoding(LBuffer, LEncoding);
  finally
    LFileStream.Free;
  end;					

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  // Process various types.
  case ComboIndex of

    // AnsiToUtf8
    0:
    begin
      // Identify the encoding and convert buffer to Ansi.
      DestEncoding := TEncoding.ASCII;
      LBuffer := LEncoding.Convert(LEncoding, DestEncoding, LBuffer,
        LOffset, Length(LBuffer) - LOffset);
      LOffset := TEncoding.GetBufferEncoding(LBuffer, DestEncoding);
      FileText := DestEncoding.GetString(LBuffer, LOffset, Length(LBuffer) - LOffset);
      ByteString := AnsiToUtf8(FileText);
      ShowMessage(UTF8ToString(ByteString));
  hier folgen weitere Case-Verzweigungen