Who is responsible for checking and handling errors?

Question

Who is responsible for checking and handling errors?

I don't have any of the expensive component libraries like DevExpress or TMS Components, etc., so I can't look at the source to understand how most components handle error handling.

In particular, I want to know if there should be a limit on the number of errors and warnings that component developers should try to capture? Is there a balance between meaningful error checking and just too simplification for developers using your component?

---

Here is an example of using multiple scenarios:

Please note that this is directly from the source of the components (e.g. for purposes)

procedure TMyComponent.AddFromFile(FileName: string);
begin
  FBitmap.LoadFromFile(FileName);
end;

or

procedure TMyComponent.AddFromFile(FileName: string);
begin
  if FileExists(FileName) then
  begin
    FBitmap.LoadFromFile(FileName);
  end
  else
    raise Exception.Create(FileName + ' does not exist.');
end;

And these last two use the component instance at runtime:

procedure TForm1.FormCreate(Sender: TObject);
begin
  MyComponent1.AddFromFile('D:\Test.bmp');
end;

or

procedure TForm1.FormCreate(Sender: TObject);
begin
  if FileExists('D:\Test.bmp') then
  begin
    MyComponent1.AddFromFile('D:\Test.bmp');
  end
  else
    raise Exception.Create('D:\Test.bmp does not exist.');
end;

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Answer 1

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type
  EMyComponentAddError = class(Exception)
  private
    FFileName: String;
  begin
    constructor CreateWithFileName(const AFileName: string);
    property FileName: string read FFileName;
  end;

constructor EMyComponentAddError.CreateWithFileName(const AFileName: string);
begin
  inherited CreateFmt('Unable to add file: %s', [AFileName]);
  FFileName := AFileName;
end;

procedure TMyComponent.AddFromFile(FileName: string);
begin
  try
    FBitmap.LoadFromFile(FileName);
  except
    Exception.RaiseOuterException(EMyComponentAddError.CreateWithFileName(FileName));
  end;
end;

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:

procedure TForm1.FormCreate(Sender: TObject);
begin
  MyComponent1.AddFromFile('D:\Test.bmp');
end;

, MyComponent1.AddFromFile('D:\Test.bmp'); . , :

Unable to add file: D:\Test.bmp

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procedure TForm1.FormCreate(Sender: TObject);
begin
  try
    MyComponent1.AddFromFile('D:\Test.bmp');
  except
    on E: EMyComponentAddError do
    begin
     ShowMessage('There was a problem adding a file:'+sLineBreak+E.FileName+sLineBreak+sLineBreak+E.InnerException.Message);
     Sysutils.Abort;
    end;
  end;
end;

:

procedure TForm1.FormCreate(Sender: TObject);
begin
  try
    MyComponent1.AddFromFile('D:\Test.bmp');
  except
    on E: EMyComponentAddError do
    begin
      raise Exception.CreateFmt('There was a problem adding a file:'#10'%s'#10#10'%s', [E.FileName, E.InnerException.Message]);
    end;
  end;
end;

:

There was a problem adding a file:
D:\Test.bmp
The file was not found

Answer 2

procedure TMyComponent.AddFromFile(FileName: string);
begin
  FBitmap.LoadFromFile(FileName);
end;

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procedure TForm1.FormCreate(Sender: TObject);
begin
  MyComponent1.AddFromFile('D:\Test.bmp');
end;

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Answer 3

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procedure TMyComponent.AddFromFile(FileName: string);
begin
  FBitmap.LoadFromFile(FileName);
end;

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, . TForm1 Application.MainForm, , , . .

• ,

  procedure TForm1.Form1Create(Sender:TObject);
  begin
    MyComponent.AddFromFile( 'D:\Test.bmp' );
  end;
  

  procedure TForm1.Form1Create(Sender:TObject);
  begin
    try
      MyComponent.AddFromFile( 'D:\Test.bmp' );
    except
      on E: Exception do
        raise Exception.Create( 'Sorry, I cannot run, because of: ' + E.Message );
    end;
  end;
  

• , , ,

  procedure TForm1.Form1Create(Sender:TObject);
  var
    LBitmapFiles : TStringList;
    LBitmapIdx : Integer;
    LBitmapLoaded : Boolean;
    LErrorStore : TStringList;
  begin
    LBitmapFiles := nil;
    LErrorStore := nil;
    try
      LBitmapFiles := TStringList.Create;
      LErrorStore := TStringList.Create;
  
      LBitmapFiles.Add( 'D:\Test.bmp' );
      LBitmapFiles.Add( 'D:\Fallback.bmp' );
  
      LBitmapLoaded := False;
      while not LBitmapLoaded and ( LBitmapIdx < LBitmapFiles.Count ) do
        try
          MyComponent.AddFromFile( LBitmapFiles[LBitmapIdx] );
          LBitmapLoaded := True;
        except
          on E: Exception do
          begin
            LErrorStore.Add( LBitmapFiles[LBitmapIdx] + ': ' + E.Message );
            Inc( LBitmapIdx );
          end;
        end;
  
      if not LBitmapLoaded then
        raise Exception.Create( 'Sorry, I cannot run, because of: ' + LErrorStore.Text );
    finally
      LErrorStore.Free;
      LBitmapFiles.Free;
    end;
  end;
  

  Other fallback options are possible, and it also depends on the application (fi sets the dummy bitmap for the component) to make the application work correctly.

• It doesn't matter if we don’t have an image ... we don’t have an image that cares

  procedure TForm1.Form1Create(Sender:TObject);
  const
    CBitmapFile = 'D:\Test.bmp';
  begin
    // check, if there is a file
    if FileExists( CBitmapFile ) then
      try
        MyComponent.AddFromFile( CBitmapFile );
      except
        on E: Exception do
        begin
          // Maybe log the exception
          SomeLogger.Log( E );
          // Maybe set some extra parameters for the application to know, this has failed
          RunningWithoutBitmap();
        end;
      end
    else
      // Maybe set some extra parameters for the application to know, this has failed
      RunningWithoutBitmap();
  end;