Only numerical input in a TEdit

Problem/Question/Abstract:

Only numerical input in a TEdit

Answer:

If you want to limit the input of a TEdit to numerical strings only, you can discard the "invalid" characters in its OnKeyPress event handler.

Let's assume that you only want to allow positive integer numbers. The code for the OnKeyPress event handler is as follows:

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  // #8 is Backspace
  if not (Key in [#8, '0'..'9']) then
  begin
    ShowMessage('Invalid key');
    // Discard the key
    Key := #0;
  end;
end;

If you also want numbers with a decimal fraction, you must allow a POINT or a COMMA, but only once. For an international version that looks at the correct decimal separator, the code could be as follows:

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if not (Key in [#8, '0'..'9', DecimalSeparator]) then
  begin
    ShowMessage('Invalid key: ' + Key);
    Key := #0;
  end
  else if (Key = DecimalSeparator) and
    (Pos(Key, Edit1.Text) > 0) then
  begin
    ShowMessage('Invalid Key: twice ' + Key);
    Key := #0;
  end;
end;

And here's a full blown version of the event handler, accepting a decimal separator and negative numbers (minus sign: only accepted once, has to be the first character):

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if not (Key in [#8, '0'..'9', '-', DecimalSeparator]) then
  begin
    ShowMessage('Invalid key: ' + Key);
    Key := #0;
  end
  else if ((Key = DecimalSeparator) or (Key = '-')) and
    (Pos(Key, Edit1.Text) > 0) then
  begin
    ShowMessage('Invalid Key: twice ' + Key);
    Key := #0;
  end
  else if (Key = '-') and
    (Edit1.SelStart <> 0) then
  begin
    ShowMessage('Only allowed at beginning of number: ' + Key);
    Key := #0;
  end;
end;

How about giving that same behaviour to several TEdits on the same form, say 10 of them? In the Object Inspector, you change the name of the event handler of Edit1 from Edit1KeyPress to Edit1_10KeyPress or something similar. Delphi automatically changes the name in the code editor, don't worry.

Then, for each next TEdit, you select its OnKeyPress event and you select Edit1_10KeyPress from the listbox next to the event.

Finally, we have to slightly adapt the code. Intead of pointing to Edit1, we must point to "the TEdit that generated the event", in other words: the edit-box where the cursor was when a key was depressed. When you look at the template for the event handler that Delphi made, you see the parameter Sender: that's a pointer to the component that generated the event. But we are not allowed to use it straight away in our code, we must specify that we're dealing with a component of the type TEdit. That's done with the code Sender as TEdit:

procedure TForm1.Edit1_10KeyPress(Sender: TObject; var Key: Char);
begin
  if not (Key in [#8, '0'..'9', '-', DecimalSeparator]) then
  begin
    ShowMessage('Invalid key: ' + Key);
    Key := #0;
  end
  else if ((Key = DecimalSeparator) or (Key = '-')) and
    (Pos(Key, (Sender as TEdit).Text) > 0) then
  begin
    ShowMessage('Invalid Key: twice ' + Key);
    Key := #0;
  end
  else if (Key = '-') and
    ((Sender as TEdit).SelStart <> 0) then
  begin
    ShowMessage('Only allowed at beginning of number: ' + Key);
    Key := #0;
  end;
end;