把 TBytes 转换为十六进制字符串

function BytestoHexString(ABytes: TBytes; len: Integer): AnsiString;
begin
  SetLength(Result, len*2);
  BinToHex(@ABytes[0], PAnsiChar(Result), len);
end;

例子：

客户端：

    IdTCPClient1: TIdTCPClient;
    Button1: TButton;
    mmo1: TMemo;

procedure TForm4.Button1Click(Sender: TObject);
var
  b:TBytes;
  Len:Integer;
  str:string;
begin
  IdTCPClient1.Host:= '127.0.0.1';
  IdTCPClient1.Port:= 8090;
  if not IdTCPClient1.Connected then
    IdTCPClient1.Connect;
  if IdTCPClient1.Connected then
  begin
    IdTCPClient1.IOHandler.WriteLn('QUERYDATA');
    mmo1.Lines.Add('向服务器发送：QUERYDATA') ;
    try
      IdTCPClient1.IOHandler.CheckForDataOnSource(250);//等待
      Len:= IdTCPClient1.IOHandler.InputBuffer.Size;
      IdTCPClient1.IOHandler.ReadBytes(b,len,false);
      mmo1.Lines.Add('接收到：' +BytestoHexString(b,length(b)));
    finally
      IdTCPClient1.Disconnect;
    end;
  end;
end;

服务端

    IdTCPServer1: TIdTCPServer;
    Button1: TButton;
    Memo1: TMemo;

procedure TForm4.Button1Click(Sender: TObject);
begin
  if  IdTCPServer1.Active then
     IdTCPServer1.Active:= False;
  IdTCPServer1.Bindings.Clear;
  with IdTCPServer1.Bindings.Add do
  begin
    ip:= '0.0.0.0';
    Port:= 8090;
  end;
  IdTCPServer1.DefaultPort:= 8090;
  IdTCPServer1.Active:= True;

  Memo1.Lines.Add('服务器监听已启动！')
end;

procedure TForm4.IdTCPServer1Execute(AContext: TIdContext);
var
  Cmd:string;
  b:TBytes;
begin
  if not AContext.Connection.IOHandler.Connected then
  begin
    Exit;
  end;
  try
    Cmd:= AContext.Connection.IOHandler.ReadLn;
    Memo1.Lines.Add('接收到数据：'+Cmd);
    if Cmd = 'QUERYDATA' then
    begin
      setlength(b,2);
      b[0]:= 1;
      b[1]:= 2;
      AContext.Connection.IOHandler.Write(b);
      Memo1.Lines.Add('发送:'+BytestoHexString(b,Length(b)));
    end;
  except
     Exit;
  end;
end;