How to handle exceptions when creating FileStream

Question

I have a function like this, that I would like to refactor

   function Myfunction(sUrl, sFile: String) : Boolean;
    var
      GetData : TFileStream;
    begin
      Result := False;
      //if the line below fails, I get an unhandled exception
      GetData := TFileStream.Create(sFile, fmOpenWrite or fmCreate);
      try        
        try
          IdHTTP.Get(sUrl, GetData);
          Result := (IdHTTP.ResponseCode = 200);
        except
          on E: Exception do begin
            MessageBox(0, PChar(E.message), 'Niðurhala skrá', MB_ICONERROR or MB_OK);
          end;
        end;
      finally
        GetData.Free;
      end;
    end;

    Procedure SomeOtherCode;
     Begin
        //How can I best defend against the unhandled exception above
        //unless the call to the function is packed in a try .. except block
        //the code jumps skips the if statement an goes to next 
        //exception block on the stack
        if MyFunction('http://domain.com/file.html', 'c:\folder\file.html') then
            ShowMessage('Got the file')
         else
            ShowMessage('Error !');
        End
     end;

Question:

Please refer to the comment within the procedure SomeOtherCode above.

Best Regards

Answer 1

Just wrap the code where you want to trap exceptions in a try..except block:

function MyFunction(...): Boolean;
var
  Stream: TFileStream;
begin
  Result := False;
  try
    Stream := TFileStream.Create(...);
    try
      // more code
      Result := ...
    finally
      Stream.Free;
    end;
  except
    // handle exception
  end
end;

Answer 2

The whole point about exception handling is two-fold:

• finally is for resource cleanup; you see this often in business logic
• except is for reacting on specific exception (and getting rid of state logic through function results and intermediate variables); you hardly see it in business logic

In your case:

Myfunction should not return a Boolean, not contain an except block, and not perform a MessageBox but just let the exceptions propagate.
SomeOtherCode should contain the except block and tell the user what went wrong.

Example:

procedure Myfunction(sUrl, sFile: String);
var
  GetData: TFileStream;
begin
  Result := False;
  //if the line below fails, I get an unhandled exception
  GetData := TFileStream.Create(sFile, fmOpenWrite or fmCreate);
  try        
    IdHTTP.Get(sUrl, GetData);
    if (IdHTTP.ResponseCode <> 200) <> then
      raise Exception.CreateFmt('Download of %s failed, return code %d', [sURl, IdHTTP.ResponseCode]);
  finally
    GetData.Free;
  end;
end;

procedure SomeOtherCode:
begin
  try
    MyFunction('http://domain.com/file.html', 'c:\folder\file.html');
  except
    on E: Exception do begin
      MessageBox(0, PChar(E.message), 'Niðurhala skrá', MB_ICONERROR or MB_OK);
    end;
  end;
end;

Now the code is much cleaner:

• no more UI in your business logic
• one place where your except is being handled
• all failures are handled equally (cannot create file, download failure)

Good luck with this.

--jeroen

Answer 3

One solution which is quite popular is to avoid 'success' or 'failure' return values completely. Instead of a function, use a procedure and handle failures using exceptions instead:

procedure Download(sUrl, sFile: String);

and then

try
  Download ('http://domain.com/file.html', 'c:\folder\file.html');
  ShowMessage('Got the file')
except
  on E:Exxx do 
  begin
    // handle exception
    ShowMessage('Error !');
  end
end;

This has also the effect that nobody can invoke the function and silently ignore the return value.

Answer 4

If you want your function to show messages to the user and return false on any failure, code it as follows:

function Myfunction(sUrl, sFile: String) : Boolean;
var
  GetData : TFileStream;
begin
  Result := False;
  try
    //if the line below fails, I get an unhandled exception
    GetData := TFileStream.Create(sFile, fmOpenWrite or fmCreate);
    try        
      try
        IdHTTP.Get(sUrl, GetData);
        Result := (IdHTTP.ResponseCode = 200);
      except
        on E: Exception do begin
          MessageBox(0, PChar(E.message), 'Niðurhala skrá', MB_ICONERROR or MB_OK);
        end;
      end;
    finally
      GetData.Free;
    end;
  except
    // you can handle specific exceptions (like file creation errors) or any exception here
  end;
end;

Warning IMHO this design is mixing business logic (such as get a resource/file from the Internet and save it to a file) and user interface logic (such as showing messages to the user in case of errors).

In general, is a better approach to separate business from UI logic, because your code is reusable.

For example you might want to re-factor as this:

function DownloadToAFile(const sUrl, sFile: string): boolean;
var
  GetData : TFileStream;
begin
  GetData := TFileStream.Create(sFile, fmOpenWrite or fmCreate);
  try        
    IdHTTP.Get(sUrl, GetData);
    Result := (IdHTTP.ResponseCode = 200);
  finally
    GetData.Free;
  end;
end;

function UIDownloadToAFile(const sUrl, sFile: string): boolean;
begin
  try
    Result := DownloadToAFile(sURL, sFile);
  except
    on E: EIDException do //IndyError
      MessageBox(0, PChar(E.message), 'Internet Error', MB_ICONERROR or MB_OK);
    on E: EFileCreateError do //just can't remember the extact class name for this error
      MessageBox(0, PChar(E.message), 'File create Error', MB_ICONERROR or MB_OK);
  end;
end;

procedure SomeOtherCode:
begin
  if UIDownloadToAFile('http://domain.com/file.html', 'c:\folder\file.html') then
    ShowMessage('Got the file')
   else
     ShowMessage('Error !');
end;

Tomorrow, if you're writing a service, or a DataSnap module, you're free to use the DownloadToAFile or maybe to write a new ServiceDownloadToAFile wich in turns writes errors to a log or windows events, or maybe send a email notifying the HostAdmin about it.

Answer 5

You should use only one try and get in this your all function code.

Answer 6

For some reason most people misuse except-finally combination. Correct sequence is

try 
  // allocate resource here
  try 
  finally
    // free resource here
  end;
except
  // handle exception here
end;

This lets you catch exceptions in constructor and destructor.