Code Puzzler: How quickly can you figure out why this acting "weird"?

Marc Clifton

And no, this isn't a programming question in the lounge. I started asking this question in the QA forum because it had me totally stumped. As I was filling out the "What I've tried" I had a major :doh: :doh: :doh: :doh: moment. To prove I'm not asking for help on the lounge, I posted a solution on [my blog](https://marcclifton.wordpress.com/2018/05/31/the-fix/). You'll need to use the password "fizbin" as the blog post is specifically for this lounge post and eventually I'll delete the post. It is a fun one though. Here's the code:

public class ModelDataContext : DataContext
{
    public static ModelDataContext Context;

    public ModelDataContext(DbConnection conn) : base(conn)
    {
        Context = this;
    }
}

class Program
{
    static ModelDataContext mdc = new ModelDataContext(new SqlConnection("\[some string\]"));

    static void CreateNewContext(DataContext context, out SqlConnection conn, out DataContext newContext)
    {
        conn = new SqlConnection(context.Connection.ConnectionString);
        newContext = (DataContext)Activator.CreateInstance(context.GetType(), new object\[\] { conn });
        Console.WriteLine(context == newContext);
    }

    static void Main(string\[\] args)
    {
        SqlConnection conn2;
        DataContext newdc;
        CreateNewContext(ModelDataContext.Context, out conn2, out newdc);

        Console.WriteLine(ModelDataContext.Context == newdc);
    }
}

and the result is: False True Why is the second equality True when the first is False??? And for the bonus prize, what's a fix?

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Lost User

No interest to solve the puzzle. For me it goes in the same direction like "Code Puzzler vs. Dumbing down code so it can be maintained by junior devs". Do we really Need to write code where we Need first to solve a puzzle to get what the code is doing :doh: :confused:

It does not solve my Problem, but it answers my question

Marc Clifton

0x01AA wrote:

Do we really Need to write code where we Need first to solve a puzzle to get what the code is doing

Well, this is different. It's a stupid bug on my part. But it's like language. If you can speak at more than a 3rd grade level, you can express your thoughts better. ;)

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Lost User

Quote:

If you can speak at more than a 3rd grade level, you can express your thoughts better

No idea what this means, sounds only I'm undeveloped :sigh: Btw. I'm aware, I'm most probably not able to solve the puzzle

It does not solve my Problem, but it answers my question

Richard Deeming

Roughly 90 seconds, mostly working out which Console.WriteLine call was which. :) Spoilers ahead - select the block to view:

Take a copy of the static field; Create a new instance, thus overwriting the static field; Compare the new instance to the copy of the old value of the static field - result = false; Compare the new instance to the current value of the static field - result = true;

Damnit Chris, we need a <div class="spoiler">!

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

OriginalGriff

You probably could, with a bit of thinking about it - it's obvious when you see it, but it's a stinker to spot if you didn't write the code (and probably even harder if you did if you are anything like me: I tend to see what I meant to write, rather than what I did :-O )

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Lost User

Black in black until I select all the text in the black :-D

It does not solve my Problem, but it answers my question

PIEBALDconsult

Is it a problem? Does it need to be fixed? :confused: I wouldn't presume to say that you didn't write exactly what you intended. Setting a static on each instantiation is unusual though.

Jon McKee

The problem is that you're assigning a static variable inside the instance constructor to that instance . So every time you create an object you're changing the Context property. The reason the tests are different is because you pass the Context as a parameter so even after the update the parameter still has the old Context. A fix would be to not do that because it's bad design ;P I'd make Context an instance property and implement your own ==, !=, Equals, etc unless you want referential equality.

Richard Deeming

Just read the blog post, and I don't think your fix will work. :)

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Jon McKee

It will work in that specific example because he also changed the CreateInstance call to not use conn anymore. But yes, if the parameterized constructor is called it'll still have the same issue :thumbsup:

Richard Deeming

Well, it will "work", in the sense that it won't overwrite the static instance. But it won't work, in the sense that the returned instance will be using the default connection string for the context, not the connection string from the instance passed in. :)

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Marc Clifton

Richard Deeming wrote:

select the block to view:

Now THAT is snazzy!

Latest Article - Building a Prototype Web-Based Diagramming Tool with SVG and Javascript Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny Artificial intelligence is the only remedy for natural stupidity. - CDP1802

Marc Clifton

Richard Deeming wrote:

But it won't work, in the sense that the returned instance will be using the default connection string for the context, not the connection string from the instance passed in.

Hmm. But the parameterless constructor uses the connection string of the (hopefully) already initialized context: public ModelDataContext() : base(Context.Connection) However, it's a moot point anyways as I refactored the whole mess into something much less weird. It'll be an interesting question though to pose to a couple of the devs at work, though I'll reduce the complexity of it omitting the DataContext base class - it's rather superfluous to the example.

Latest Article - Building a Prototype Web-Based Diagramming Tool with SVG and Javascript Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny Artificial intelligence is the only remedy for natural stupidity. - CDP1802

Richard Deeming

OK, so it's not quite as bad as I though. :) You could still break it though:

var context1 = new ModelDataContext(new SqlConnection("server=A;database=B"));
var context2 = new ModelDataContext(new SqlConnection("server=C;database=D"));

CreateNewContext(context1, out var newConn, out var newContext);

// As expected:
Console.WriteLine(newConn.ConnectionString == context1.Connection.ConnectionString); // True

// Not as expected:
Console.WriteLine(newContext.Connection.ConnectionString == context1.Connection.ConnectionString); // False
Console.WriteLine(newContext.Connection.ConnectionString == newConn.ConnectionString); // False
Console.WriteLine(newContext.Connection == newConn); // False

// Also not as expected:
Console.WriteLine(newContext.Connection.ConnectionString == context2.Connection.ConnectionString); // True

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Mike Hankey

I also see what I meant to write as opposed to what I actually wrote. I've been tired and looked at two spellings of same word and get compiler errors and don't spot the error for some time.

Everyone has a photographic memory; some just don't have film. Steven Wright

OriginalGriff

Annoying, isn't it? :-D

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Mike Hankey

Yes very frustrating. As I get older it seems to be more prevalent.

Everyone has a photographic memory; some just don't have film. Steven Wright

Marc Clifton

Richard Deeming wrote:

You could still break it though:

Yeah, but it's never used that way - two different connection strings. But ideally, it shouldn't be possible to use it that way. Sigh. The irony of this is that it's code I wrote a while back that my DataContext extension methods rely on, and looking at this now, it's some serious code smell. Fortunately, fixing it affects only a couple web servers that are in operation, but I feel embarrassed. But I blame .NET's DataContext. :laugh: It does way to much with regards to the state of the Table object. I get what they're trying to do, but there must be a better way that doesn't end up throwing exceptions like "this object was created in a different data context."

Latest Article - Building a Prototype Web-Based Diagramming Tool with SVG and Javascript Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny Artificial intelligence is the only remedy for natural stupidity. - CDP1802

soulesurfer

The reason for the results are pretty obvious, when a new instance is created in "CreateNewContext", the parameter "context" is still referring to the original context, not the new one ==> false; When the main program does the compare afterwards, the comparison is done to the static that holds the new context ==> true. Fixing this is not obvious because you didn't specify what the correct behavior should be. Do you wish it to be "True" always, or "False" always :) Simply update the context parameter in the method to get True always, but I would personally recommend to re-write this whole thing, it is full of code smells. Perhaps some practice in TDD would also be helpful. Cheers, Anthony