Overriding Dictionary Not Found Exception

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Paulustrious - 21 Dec 2005 11:35 GMT

Is it possible to override the behaviour of a dictionary if an object is not
found?

string ss=null;
Dictionary<string, string> dict = new Dictionary<string,
string>();

// method 1

try { ss = dict["x"]; }
catch (KeyNotFoundException) {/* Ignore */}

// method 2

if (dict.ContainsKey("x")) ss = dict["x"];

In Method 1, if the key is not there then we have the considerable overhead
of an Exception.
In Method 2 it looks as though we need two dictionary scans.

So is it possible to change the default behaviour of a dictionary so it
returns a null value if not found rather than throwing the exception?

Signature

Paul

Reply to this Message

Anders Borum - 21 Dec 2005 11:44 GMT

Hello!

The Dictionary class is not sealed, so you could create a new concrete
Dictionary with the new functionality. The indexer is not marked virtual, so
you would have to use the "new" keyword when defining the new indexer (you
should make sure that all apropriate methods are re-implemented to ensure
the null-behaviour you're looking for).

Another approach would be to use the Dictionary.TryGetValue() method instead
and adapt your client code to the default non-null behaviour.

Signature

With regards
Anders Borum / SphereWorks
Microsoft Certified Professional (.NET MCP)

Reply to this Message

Jon Skeet [C# MVP] - 21 Dec 2005 11:47 GMT

<=?Utf-8?B?UGF1bHVzdHJpb3Vz?= <msdn_whoisat_paulcotter.com>> wrote:

> Is it possible to override the behaviour of a dictionary if an object is not
> found?
[quoted text clipped - 18 lines]
> So is it possible to change the default behaviour of a dictionary so it
> returns a null value if not found rather than throwing the exception?

You could use Dictionary.TryGetValue - it looks like that's what you're
after.

From a pragmatic (but not elegant) point of view, are you sure that
catching the exception *actually* incurs an overhead which is
significant in your application? You might be surprised just how cheap
exceptions are. See
http://www.pobox.com/~skeet/csharp/exceptions.html for some discussion
on this. I only mention this because many people have bought into the
mantra of "exceptions are expensive" without really looking into it.

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

Reply to this Message

Ignacio Machin ( .NET/ C# MVP ) - 21 Dec 2005 13:29 GMT

HI,

> I only mention this because many people have bought into the
> mantra of "exceptions are expensive" without really looking into it.

I will read you article, but regarding the cost of an exception IIRC even
MS offer that view , that exception are expensive.

Signature

Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation

Reply to this Message

Jon Skeet [C# MVP] - 21 Dec 2005 15:20 GMT

<"Ignacio Machin \( .NET/ C# MVP \)" <ignacio.machin AT
dot.state.fl.us>> wrote:

> > I only mention this because many people have bought into the
> > mantra of "exceptions are expensive" without really looking into it.
>
> I will read you article, but regarding the cost of an exception IIRC even
> MS offer that view , that exception are expensive.

They're expensive if you're going to end up throwing them thousands and
thousands of times per second - otherwise they're almost always lost in
the noise.

As for whether or not to just follow MS's advice - I tend to trust the
evidence of my own eyes above MSDN. After all, it was only after
badgering them for quite a while that MSDN was changed to admit that
System.Decimal was a floating point type. (It had previously been
inaccurately described as a fixed point type.) MS are just as capable
of making mistakes as the rest of us :)

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

Reply to this Message

Chris Dunaway - 21 Dec 2005 14:47 GMT

Jon wrote:

> http://www.pobox.com/~skeet/csharp/exceptions.html for some discussion
> on this. I only mention this because many people have bought into the
> mantra of "exceptions are expensive" without really looking into it.

Very interesting article. My experience has been that the *first* time
an exception is thrown it seems to take a few seconds (presumably
because it has to jit compile the assembly that contains the exception
type), but after that, if the *same* exception is thrown there is
practically no performance hit.

I wonder if there is a way to "pre-load" the exception types during the
start up of the app or is it even worth doing it.

Reply to this Message

Jon Skeet [C# MVP] - 21 Dec 2005 15:18 GMT

> > http://www.pobox.com/~skeet/csharp/exceptions.html for some discussion
> > on this. I only mention this because many people have bought into the
[quoted text clipped - 8 lines]
> I wonder if there is a way to "pre-load" the exception types during the
> start up of the app or is it even worth doing it.

Have you experienced that when running a release build not in the
debugger? Do you have a complete program which demonstrates that? I've
only seen that happen in the debugger, which is a very different
situation.

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

Reply to this Message

rossum - 28 Dec 2005 20:27 GMT

>Jon wrote:

[snip]

>I wonder if there is a way to "pre-load" the exception types during the
>start up of the app or is it even worth doing it.

Presumably you could explicitly throw it yourself:

try {
throw new myPreLoadException();
}
catch (myPreLoadException) { /* Do nothing */ }

Whether it is worth doing is another question.

rossum

--

The ultimate truth is that there is no ultimate truth

Reply to this Message

Jesse McGrew - 21 Dec 2005 11:47 GMT

You can use TryGetValue instead.

string ss=null;
Dictionary<string, string> dict = new Dictionary<string,
string>();

if (dict.TryGetValue("x", out ss) == true) {
/* the key was found, and ss now contains the value */
} else {
/* the key wasn't found, and ss is null */
}

Jesse

Reply to this Message

Paulustrious - 21 Dec 2005 12:26 GMT

To All Above... Thanks - next time I will read ALL the methods before
posting. (As an excuse - it IS the last one in the method list :)

To Anders Borum.. TryGetValue is what I need, but I'll give yours a go as a
learning exercise.

Signature

Paul

Reply to this Message

Overriding Dictionary Not Found Exception

Free Magazines