Delphi - make the Idhttp run in loop if return page not found error

Question

How i can make the Idhttp run in loop if return 404 page not found the problem is 'GOTO CheckAgain' leads into or out of TRY statement

label
  CheckAgain;
begin  
  CheckAgain:
  try
    idhttp.Get(sURL+WebFile[I], S);
  except
    on E: EIdHTTPProtocolException do
    if AnsiPos('404',E.Message) <> 0 then
    begin
      I := I+1;
      goto CheckAgain;
    end;
  end;
end;

Answer 1

You have three options:

1. use a try/except block in a loop, catching EIdHTTPProtocolException exceptions and checking their ErrorCode property for 404:

   repeat
     try
       IdHttp.Get(sURL+WebFile[I], S);
     except
       on E: EIdHTTPProtocolException do
       begin
         if E.ErrorCode <> 404 then
           raise;
         Inc(I);
         Continue;
       end;
     end;
     Break;
   until False;
   

2. if you are using an up-to-date version of Indy, you can enable the hoNoProtocolErrorException flag in the TIdHTTP.HTTPOptions property, and then your loop can remove the try/except and check the TIdHTTP.ResponseCode property instead:

   repeat
     IdHttp.Get(sURL+WebFile[I], S);
     if IdHttp.ResponseCode <> 404 then
       Break;
     Inc(I);
   until False;
   

3. use the overloaded version of the TIdHTTP.Get() method that has an AIgnoreReplies parameter, then you can tell Get() to not raise an EIdHTTPProtocolException exception on a 404 response:

   repeat
     IdHttp.Get(sURL+WebFile[I], S, [404]);
     if IdHttp.ResponseCode <> 404 then
       Break;
     Inc(I);
   until False;
   

Answer 2

As a general rule, I'd avoid using goto. There are occasions where goto should be used, but they are rare and your problem is not suited.

A while loop would be more commonly used. You'd be able to build in a maximum retry mechanism too. Perhaps like this:

RetryCount := 0;
Succeeded := False;
while RetryCount < MaxRetryCount do
  try
    idhttp.Get(sURL+WebFile[I], S);
    Succeeded := True;
    break; // success, break out of while loop
  except
    on E: EIdHTTPProtocolException do
      if E.ErrorCode = 404 then
        inc(RetryCount)
      else
        raise;

  end;