How to handle exceptions when creating FileStream - delphi

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

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 4

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 5

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.

Answer 6

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