Use of nested “try/finally” “try/except” statements

问题

I have seen this code posted here on StackOverflow:

with TDownloadURL.Create(nil) do
  try
    URL := 'myurltodownload.com';
    filename := 'locationtosaveto';
    try
      ExecuteTarget(nil);
    except
      result := false;
    end;
    if not FileExists(filename) then
      result := false;
  finally
    free;
  end;

Can't it be simplified to look like:

 Result:= FALSE;               <--------- Compiler complains
 DeleteFile(Dest);
 dl:= TDownloadURL.Create(NIL);
 TRY
   dl.URL:= URL;
   dl.FileName:= Dest;
   dl.ExecuteTarget(NIL);           
   Result:= FileExists(Dest);
 FINALLY
   dl.Free;
 END;

The final Result:= ... will never be executed if something went wrong in 'ExecuteTarget' be cause the program will jump directly to 'finally'. Right? So, the function will return FALSE. Am I doing something wrong?

PS:

I intend to use this code in a thread.
I just put the function in Delphi and the compiler complaints about the first line: "Value assigned never used."

回答1:

The difference is that your second example passes exceptions back to the caller, while the original traps them and returns false. I'd characterise that style of coding as "I don't care why it failed, I only care whether it succeeded". Which can be reasonable in some cases (like trying to download an update).

So your code is very different from the original in that way - you are expecting the caller to handle exceptions that the original code does not.

Also, the compiler complaint is because there's no branch in your code - either if works and result is determined by the second assignment or you have an exception and Result is irrelevant.

Result := FALSE; //   <--------- Compiler complains
DeleteFile(Dest);
dl := TDownloadURL.Create(nil);
try
   dl.URL := URL;
   dl.FileName := Dest;
   dl.ExecuteTarget(nil);
   Result := FileExists(Dest);
finally
   dl.Free;
end;

回答2:

In the original, if ExecuteTarget throws an exception, it will still test of filename exists.

In yours, if ExecuteTarget throws an exception, result is always false.

Also, unless you skipped a line, on the original, if ExecuteTarget succeeds and the file exists, result is never set.

回答3:

The first version just eat the exception and never raise to upper callers, it treat exception as false return. For your simple version, exception will be thrown out.

来源：https://stackoverflow.com/questions/3526749/use-of-nested-try-finally-try-except-statements

标签

Delphi

try-finally

try-except