on try..except..

[Paolo]

Hello,

I see that a common action can be used for all possible exception writing code like this (the code is meaninful just to illustrate the concept)

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button2Click(Sender: TObject);
2. var
3.   a, b, c : double;
4. begin
5.   try
6.     a:=1;
7.     b:=0;
8.     c:=a/b;
9.   except
10.     MessageDlg('Something went wrong', mtError, [mbOK], 0);
11.   end;
12. end;
13.  

I see that except section allows specialize the answer like this (Still covering all the possible exception thanks to “else”)

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button3Click(Sender: TObject);
2. var
3.   a, b, c : double;
4. begin
5.   try
6.     a:=1;
7.     b:=0;
8.     c:=a/b;
9.   except
10.     on E:EZeroDivide do
11.       MessageDlg('Division by 0', mtError, [mbOK], 0);
12.     on E:EMathError do
13.       MessageDlg('Computational Error', mtError, [mbOK], 0);
14.     else
15.       MessageDlg('Something unexpected went wrong', mtError, [mbOK], 0);
16.   end;
17. end;  
18.  

Now my question: how implement an exception section that do something in ”all the cases” but then lefts the specialized case do the rest of the job? I would like something like

Code: Pascal  [Select][+][-]

1. var
2.   a, b, c : double;
3. begin
4.   try
5.     a:=1;
6.     b:=StrToFloat(Edit1.text);
7.     c:=a/b;
8.    except
9.     //MessageDlg('Something went wrong', mtError, [mbOK], 0); <-- code needed for all exception cases, it does not compile
10.     on E:EZeroDivide do
11.       MessageDlg('Division by 0', mtError, [mbOK], 0);
12.     on E:EMathError do
13.       MessageDlg('Computational Error', mtError, [mbOK], 0);
14.     on E:EConvertError do
15.       MessageDlg('Conversion Error', mtError, [mbOK], 0);
16.   end;
17. end;
18.  

How can I achieve this behaviour ? are there other ways to achieve this ?

Thank you

[PascalDragon]

Now my question: how implement an exception section that do something in ”all the cases” but then lefts the specialized case do the rest of the job? I would like something like

Code: Pascal  [Select][+][-]

1. var
2.   a, b, c : double;
3. begin
4.   try
5.     a:=1;
6.     b:=StrToFloat(Edit1.text);
7.     c:=a/b;
8.    except
9.     //MessageDlg('Something went wrong', mtError, [mbOK], 0); <-- code needed for all exception cases, it does not compile
10.     on E:EZeroDivide do
11.       MessageDlg('Division by 0', mtError, [mbOK], 0);
12.     on E:EMathError do
13.       MessageDlg('Computational Error', mtError, [mbOK], 0);
14.     on E:EConvertError do
15.       MessageDlg('Conversion Error', mtError, [mbOK], 0);
16.   end;
17. end;
18.  

How can I achieve this behaviour ? are there other ways to achieve this ?

One of the simpler solutions would be to add a nested exception handler block:

Code: Pascal  [Select][+][-]

1. var
2.   a, b, c : double;
3. begin
4.   try
5.     try
6.       a:=1;
7.       b:=StrToFloat(Edit1.text);
8.       c:=a/b;
9.     except
10.       MessageDlg('Something went wrong', mtError, [mbOK], 0); // <-- code needed for all exception cases, it does not compile
11.       raise;
12.     end;
13.   except
14.     on E:EZeroDivide do
15.       MessageDlg('Division by 0', mtError, [mbOK], 0);
16.     on E:EMathError do
17.       MessageDlg('Computational Error', mtError, [mbOK], 0);
18.     on E:EConvertError do
19.       MessageDlg('Conversion Error', mtError, [mbOK], 0);
20.   end;
21. end;
22.  

[Paolo]

that was my fisrt idea, thanks

[Warfley]

Exceptions are always class instances, so you can do this:

Code: Pascal  [Select][+][-]

1. procedure ExceptionTest(toRaise: TObject);
2. begin
3.   try
4.     raise toRaise;
5.   except on E: TObject do
6.   begin
7.     WriteLn('Exception');
8.     if E is EConvertError then
9.       WriteLn('Convert Error: ' + Exception(E).Message)
10.     else if E is Exception then
11.       WriteLn(Exception(E).Message);
12.   end;
13.   end;
14. end;
15.  
16. begin
17.   ExceptionTest(EConvertError.Create('Convert')); // Exception + Convert Error Convert
18.   ExceptionTest(EFormatError.Create('Format'));  // Exception + Format

In most cases the Exception will inherit from the Exception type from SysUtils, so you could even use this rather than TObject
Alternatively you might use AcquireExceptionObject, but then you extract that and have to ReleaseExceptionObject it afterwards

[Thaddy]

I am missing in all answers the quite essential

Code: Pascal  [Select][+][-]

1.  else raise;

[Paolo]

thanks to all for the suggestions