The perils of giving your data objects methods

A colleague of mine hit a strange bug today. It so happened that we use a bastardized dependency injection method that takes into account the WCF session before returning an implementation of an interface. In a piece of code the injection failed and we couldn't see why for a while. Let me give you a simplified version:

var someManager=Package.Get<IManager>();
var someDTOs=Cache.GetDatabaseObjects().Select(x=>x.Pack());

public class DataObject {
    public string Data {get;set;}
    public DataObjectDTO Pack() {
        var anotherManager=Package.Get<IAnother>();
        return new DataObjectDTO {
           Data=anotherManager.Process(Data)
        };
    }
}

Package.Get will attempt to find a session object and if not it will use another mechanism, but if it finds one, it will only use it if it is not expired or invalid, else throwing an exception. This code failed in the Pack method, when trying to get an instance of IAnother. Please take a few moments to reflect on why (and no, it's not that between calls the session expired).

Show explanation

OK, what is going on here? First Package.Get works, then it doesn't. The method GetDatabaseObjects returns some objects that were previously stored in memory. Explanation: the objects were created by another thread. That means that a method of those objects will be executed on that thread, which contains whatever Session another user used.

It is such an insidious bug that we had to think a few minutes to figure it out. And the solutions are not perfect:

• Load all objects in cache from the service loader, rather than lazy, so that they are all instantiated in a thread with no WCF context
• Put a try/catch block around the code that validates the session and treat invalid sessions as null
• Create a helper class that holds the Pack functionality and keep data objects methodless

Obviously all of these might work, but they all have ugly side effects. Personally, I am inclined to recommend the third solution. First of all it respects the separation of concerns of classes, but second of all the helper class can be used as an extension method class, so the usage would be exactly the same.

Now that is interesting, isn't it? A clear difference between instance methods and extension methods is the execution context, something that I had not been consciously aware of before. obj.Pack() translates to DataObjectsExtensions.Pack(obj) which is run on the calling thread. I am now wondering if the web of extensions methods in ASP.Net Core has something to do with this.

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The expression being assigned to '[something] must be constant when using integers inside strings

I've stumbled upon a very funny exception today. Basically I was creating a constant string from adding some other constant strings to each other. And it worked. The moment I added an integer, though, I got The expression being assigned to 'Program.x2' must be constant. The code that generated this error is simple:

const string x2 = "string" + 2;

Note that

const string x2 = "string" + "2";

is perfectly valid. Got the same result when using VS2010 and VS2015, so it's not a compiler bug, it's intended behavior.

So, what's going on? Well, my code transforms behind the scenes into

const string x2 = "string" + 2.ToString();

which is not constant because of ToString!

The only way to solve it was to declare the numeric constant as string as well.

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Getting random rows from a table in T-SQL: TABLESAMPLE [instructional post, but not a recommended method]

This clause is so obscure that I couldn't even find the Microsoft reference page for it for a few minutes, so no wonder I didn't know about it. Introduced in SQL Server 2005, the TABLESAMPLE clause limits the number of rows returned from a table in the FROM clause to a sample number or PERCENT of rows.

Usage:

TABLESAMPLE (sample_number [ PERCENT | ROWS ] ) [ REPEATABLE (repeat_seed) ]

REPEATABLE is used to set the seed of the random number generator so one can get the same result if running the query again.

It sounds great at the beginning, until you start seeing the limitations:

• it cannot be applied to derived tables, tables from linked servers, and tables derived from table-valued functions, rowset functions, or OPENXML
• the number of rows returned is approximate. 10 ROWS doesn't necessarily return 10 records. In fact, the functionality underneath transforms 10 into a percentage, first
• a join of two tables is likely to return a match for each row in both tables; however, if TABLESAMPLE is specified for either of the two tables, some rows returned from the unsampled table are unlikely to have a matching row in the sampled table.
• it isn't even that random!

Funny enough, even the reference page recommends a different way of getting a random sample of rows from a table:

SELECT * FROM Sales.SalesOrderDetail
WHERE 0.01 >= CAST(CHECKSUM(NEWID(), SalesOrderID) & 0x7fffffff AS float) / CAST (0x7fffffff AS int)

Even if probably not really usable, at least I've learned something new about SQL.

Update:
More about getting random samples from a table here, where it explains why ORDER BY NEWID() is not the way to do it and gives hints of what really happens in the background when we invoke TABLESAMPLE.
Another interesting article on the subject, focused more on the statistical probability, can be found here, where it also shows how TABLESAMPLE's cluster sampling may fail in spectacular ways.

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Am I a good person?

I am often left dumbfounded by the motivations other people are assigning to my actions. Most of the time it is caused by their self-centeredness, their assumption that whatever I do is somehow related more to them than to me. And it made me think: am I a good/bad person, or is it all a matter of perception from others?

I rarely feel like I do something out of the ordinary for other people; instead I do it because that's who I am. I help a colleague because I like to help or I refuse to do so because I feel that what I am doing is more important. Same with friends or romantic relationships. Sometimes I need to make an effort to do something, but it's still my choice, my assessment of the situation and my decision to go a certain way. It's not a value judgment on the person, it's not an asshole move or some out of my way effort to improve their life. What I do IS me.

It's also a weird direction of reasoning, since I am aware of the physical impossibility for "free will" and I subscribe to the school of thought that it is all an illusion. I mean, logic dictates that either the world works top-bottom, with some central power of will trickling down reality or it is merely a manifestation of low level forces and laws of physics that lead inexorably towards the reality we perceive. In other words, if you believe in free will, you have to believe in some sort of god, and I don't. Yet living my life as if I have no free will makes no sense either. I need to play the game if I am to play the game. It's kind of circular.

Getting back to my original question: Isn't good or bad just a label I (and other people) assign to a pattern of behavior that belongs to me? And not before I do things, but always afterwards. Just like the illusion of free will there is the illusion of moral quality that guides my path. While one cannot quantify free will, they can measure the effect my behavior has on their life and goals and determine a value. But then is my "goodness" something like an average? Because then it would be more important the number of people I am affecting, rather than the absolute value of the effect per person. Who cares I help a colleague or pay attention to my wife? In the big sea of people, I am just a small fish that affects a few other small fish. We could all die tomorrow in the belly of a whale, all that goodness pointless.

So here I am, asking essentially a "who am I" question - painfully aware it has no final answer - in a world I think is determined by tiny laws of physics that create the illusion of self and with a quantity of consequence that is irrelevant even if it weren't so. I am torturing myself for no good reason, ain't I?

Yet the essence of the question still intrigues me. Is it necessary that I feel a good drive for my actions to be a good person, or is it a posterior calculation of their effect that determines that? If I work really well and fast for a month and then I do less the next, is it that I did good work in the first month or that I am a lazy bastard in the second? If I pay attention to someone or make a nice gesture, is it something to be lauded, or something to be criticized when I don't do it all the time? Is this a statistical problem or an issue of causality?

And I have to ask this question because if I feel no particular drive to do something and just "am myself", I don't think people should assign all kind of stupid motivations to my actions. And if I need to make this sustained effort to go outside my routine just to gain moral value... well, it just feels like a lot of bother. And I have to ask it because the same reasoning can be applied to other people. Is my father making terrible efforts to take care of just about everybody in his life, making him some sort of saint, or is it just what he does and can't help himself, in which case he's just a regular dude?

Personally I feel that I am just an amalgamation of experiences that led to the way I behave. I am neither good nor evil and my actions define me more than my intentions. While there is some sort of consistency that can be statistically assessed, it is highly dependent on the environment and any inference would go down the drain the moment that environment changes. But then, how can I be a good person? And does it even matter?

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