Delphi XE2 TIdUDPClient ReceiveString重载不起作用

Question

I am using Embarcadero RAD Studio XE2 Update 4 and the Indy package shipped with it.

My intention is to find a server in LAN with broadcast from a TIdUDPClient that waits for a response from the server to get its IP. Receiving the data works fine if I use the TIdUDPClient method ReceiveString with no arguments.

But when I try to use the overloaded version found in the Indy 10 Documentation version 10.5.8.3 coming with RAD Studio, it does not compile and shows 'E2250: There is no overloaded version of 'ReceiveString' that can be called with these arguments' . Here is my code:

unit Client;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, IdBaseComponent, IdComponent, IdUDPBase,
  IdUDPClient, Vcl.StdCtrls, IdGlobal;

type
  TFormLC = class(TForm)
    UDPClient: TIdUDPClient;
    LServer: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  FormLC: TFormLC;

implementation

{$R *.dfm}

function findServer:string;
var ans, ip : string;
    port: TIdPort;
begin
  with FormLC.UDPClient do begin
    Active := True;
    BroadcastEnabled:=True;
    Broadcast('ServerRequest', 1234);
    ans := ReceiveString(ip, port);
    Active := False;
  end;
  if SameText(ans, 'ServerAccept') then
    result := ip
  else
    result := '';
end;


procedure TFormLC.Button1Click(Sender: TObject);
var ans:string;
begin
  LServer.Caption := findServer;
end;

end.

I noticed that the online documentation of Indy differs from the documentation that comes with the IDE and tried it as described there, without succes.

Any help would be great!

Answer 1

Your issue is caused by the with statement, you are passing the port property of the TIdUDPClient instead of the local variable port to the ReceiveString method.

function findServer:string;
var ans, ip : string;
    port: TIdPort;
begin
  with FormLC.UDPClient do begin
    ....
    ans := ReceiveString(ip, port);//here you are passing the port property 
    Active := False;
  end;
  .... 
end;

As workaround rename you port local variable like so :

 function findServer:string;
var ans, ip : string;
    vport: TIdPort;
begin
  with FormLC.UDPClient do begin
    .... 
    ans := ReceiveString(ip, vport);//now will work
    Active := False;
  end;
end;

or even better don't use the with statement.

Answer 2

TIdUDPClient has 2 overloads for ReceiveString() :

function ReceiveString(const AMSec: Integer = IdTimeoutDefault; AByteEncoding: TIdTextEncoding = nil{$IFDEF STRING_IS_ANSI}; ADestEncoding: TIdTextEncoding = nil{$ENDIF}): string; overload;

function ReceiveString(var VPeerIP: string; var VPeerPort: TIdPort; const AMSec: Integer = IdTimeoutDefault; AByteEncoding: TIdTextEncoding = nil{$IFDEF STRING_IS_ANSI}; ADestEncoding: TIdTextEncoding = nil{$ENDIF}): string; overload;

When you call ReceiveString() without parameters, you are calling the first overload. When trying to call the second overload, your code fails to compile because your with statement is passing the TIdUDPClient.Port property to the second parameter, instead of your local port variable. The compile will not allow you to pass a property to a var parameter.

You need to remove the with statement and/or rename your port variable to resolve the conflict.