Leaky Abstractions? Try Asp.Net!
First here's a code example:
Single line 'TextBox':<asp:TextBox ID='dd1' runat='server' TextMode="SingleLine" MaxLength="10"> </asp:TextBox>
^^^^This 'MaxLength' is respected!
Multi line 'TextBox':<asp:TextBox ID='dd1' runat='server' TextMode="MultiLine" MaxLength="10"> </asp:TextBox>
^^^^This 'MaxLength' gets ignored!
Now here it is in pictures:
The single-line TextBox respects the maximum length property. The multi line textbox does not. We see other differences too: these two 'TextBoxen' differ from each other in far more than the number of rows.
The cause here is a classic 'Leaky Abstraction' - and asp.net is riddled with such leaks.
The 'abstraction' of "TextBox" is used to emit either one of three underlying html elements -- "TextArea", "Input type=text" or "input type=password" -- depending on the 'TextMode' property. The three controls are wildly different animals.
Edge cases are frequently missed, for example:
- Once maxlength is reached, ignore input, but don't ignore: Arrow keys, Delete key and Backspace. Others?.
- Pasting of text should truncate down to maxlength.
That's pretty nasty stuff to have to perform -- and worse, you're given no warning that you need to perform it.
The API tells you that there is a maxlength property you can use... but the API, in this case, is lying.
I'm not really concerned with this problem technically. And I think the asp.net abstraction is so extreme that it's admirable. I'd like to know how to patch this kind of leak.
Here's what I'm wondering:
Is there a way the text box class could have been implemented so that you get a compile error for setting the maxlength property when the textmode is multiline?
That would be enough to make it a safe abstraction in my books. It's more extreme than the static typing afforded by most static type systems.
I expect that such a constraint could be achieved with a language/tool like Spec#.
An alternative would be to throw an exception when setting the maxlength property if the textbox type is multi-line, and vice versa. But that seems like quite a faulty approach.
I'm just not happy about this at all. I think I will need to write a very stern letter to Scott Guthrie.Next → ← Previous