Report from the battlefield #4 - Do not waste my and your time

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The Report from the battlefield series is based on my experience as a reviewer. The idea is simple. In order to evaluate programming skills, a candidate is asked to write a simple project. To do so he/she needs to invest some amount of time (roughly speaking a few hours). Taking this into account I assume that he/she must be interested in finding a new job. Otherwise he/she wouldn't spend his/her private time writing a project which rather isn't extremely exciting. The more I'm surprised why some people doesn't care about the first impression.

Here are some examples showing what I'm talking about:

• A connection string used by the application referred to some server that of course wasn't available to me.
• The database used by the application didn't contain any sample data.
• I had to manually create a database used by the application. There was a script but it didn't work without fixes.
• The application crashed immediately when started.
• ...

It's wasting of time from my perspective. It is true that all these problems can be fixed quickly but they require additional effort from me. You can believe me that it is extremely annoying. Instead of making an actual review someone forces me to fix bugs. What's the worst these bugs could be avoided easily with a little bit more effort.

Please remember, the first impression is important. It'll be appreciated if a reviewer can run your application just by pressing F5 in Visual Studio (or in another IDE). You can test it in a straightforward way. Before submitting a project to a review, copy it to another machine and try to run it there. It should work without any additional actions.

Currently, if a project cannot be run without problems I don't make a review. However, I have a soft heart and I give a candidate one chance to fix them. Do you think that it's a good approach? I have my doubts because an employer probably wouldn't do so.

Tips & Tricks: How to tell VS to modify variables in the runtime for us?

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Today, I'd like to share with you a simple but useful trick. Imagine yourself that you are debugging an application and you find a place with the following very simple code:

            
var flag = ReadConfiguration();
if (flag)
{
   //...
}
else
{
   //...
}

The problem is that the flag variable is set to false but you need to check what would happen if it is set to true. Of course you can easily change the value of this variable in Visual Studio. But what would you do if this kind of code is executed dozens, hundreds... of times and every time the flag variable must be set to true? One solution is to modify a configuration, another might be to change the source code. However, all these things require an additional action. It would be much more better to tell Visual Studio to do it for us. How? In order to achieve desired effect we can utilize breakpoints and custom actions. I'll show how to do it in Visual Studio 2015.

Firstly, put a breakpoint in the line with if.

Right click the breakpoint and from a context menu select Actions... Then in the text box enter {flag = true}. You can even use IntelliSense here. At the end click Close button.

An that's all. Now, if you run the application under debugger control a flag variable will be set to true whenever a line with a breakpoint is executed. What's more this trick also works with other types of variables and you can also execute methods in this way e.g.:

At the end I want to say 2 things. Custom actions are usually used to write diagnostic messages to Output window. This trick works because in order to write a message Visual Studio has to execute some code and this code can have side effects. Besides, you can also use this trick in older versions of Visual Studio. The only difference is that from a context menu you need to select When Hit... option.

My list of online editors

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Online editors (testers, debuggers) are awesome if we want to quickly test some code. They are also very useful to check our solution when we want to post an answer on Stack Overflow. Here is my collection of various online editors that I encountered, though personally I use only some of them.

I'm publishing it because it can be helpfull for others and because I'd like to have this list easily accessible in Internet. Of course this list is not complete and there are many other editors. If you know something interesting let me know and I will add it here.

Online Editor	Language / Technology	Share function	Collaborate function
yUML	UML	Yes
draw.io	Diagrams	Yes (via Google docs)	Yes
moqups	UI moqups	Yes	Yes
ideone	C#, Java, Haskell, C++, Ada and many other	Yes
SQL Fiddle	SQL (MySQL, Oracle, PostgreeSQL, SQLLite, MSSQL)	Yes
regular expressions 101	Regular expressions	Yes
.NET Fiddle	C#, VB.NET, F#	Yes	Yes
C# Pad	C#
D3.js	D3.js Java Script library
CodePen	HTML + CSS + JS	Yes
jsfiddle	HTML + CSS + JS	Yes	Yes
JS Bin	HTML + CSS + JS	Yes	Soon
CSS Deck	HTML + CSS + JS	Yes	Yes
Liveweave	HTML + CSS + JS	Yes	Yes
Plunker	HTML + CSS + JS	Yes	Yes
cpp.sh	C++	Yes

By share function I mean a possibility to create a pernament link to our code. Collaborate functions allows a group of developers to write a code together.

Interview Questions for Programmers by MK #7

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Question #7
You have the following code that uses Entity Framework to retrieve data from Northwind database. Firstly it finds customers that are from London and then process their orders. All data model classes were generated with the code first from database approach. Unfortunately, this code contains a bug that can lead to performance problems. Identity this problem and propose a fix.

using (var ctx = new NorthwindContext())
{
   var londoners = ctx.Customers.Where(e => e.City == "London");
   foreach (var londoner in londoners)
   {
      foreach (var o in londoner.Orders)
      {
         foreach (var d in o.OrderDetails)
         {
            //....
         }
      }
   }               
}

Answer #7
This code is a classical example of N+1 select problem where too many queries are sent to a database. The first query will be sent to a database in order to find customers from London. Then for each customer another query will be sent to read orders. Finally, for each order another query will be sent to retrieve details of a given order. Instead, all data could be retrieved by sending only one query. To do so we need to tell EF that we want to read customers together with their orders and orders details. It can be achieved with Include method.

using (var ctx = new NorthwindContext())
{
   londoners = ctx.Customers.Include("Orders").Include("Orders.Order_Details").Where(e => e.City == "London");
   ...
}

A quick test will show that originally 53 queries are sent to a database and after a fix only 1.

Report from the battlefield #3 - IEnumerable vs IQueryable

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Sometime ago I was reviewing the data access layer that was based on Entity Framework. I found a code which immediately attracted my attention. The simplified version is shown below.

public IList<Product> GetAll()
{
   return ctx.Products.Select(p => new Product() { ... }).ToList(); 
}
...
var numberOfProducts = GetAll().Count();

GetAll method is pretty simple because it just reads products from a database. The result returned from this method is used to count number of products in the database. Although it is simple it contains a serious bug. The problem is that it uses ToList method to return a list of products. It causes that ALL products must be retrieved from the database in order to return them in the form of a list. In other words there is no deferred execution here.

If we work with a local database and the number of products is small it shouldn't be a problem. However, this kind of code might lead to difficult to analyse performance problems. For example if our application uses a remote database and/or there are thousands of products. The desired behaviour is that products are counted by a database engine. So let's try to make a fix:

public IEnumerable<Product> GetAll()
{
   return ctx.Products.Select(p => new Product() { ... });
}
...
var numberOfProducts = GetAll().Count();

Now it looks much more better, doesn't it? GetAll doesn't use ToList and returns IEnumerable. interface. Unfortunately this solution is far from being perfect. In comparison to the first version, the only difference is the moment when all products are retrieved from the database. This time it will happen when Count method is executed. Why? Before I'll explain let's see the correct solution:

public IQueryable<Product> GetAll()
{
   return ctx.Products.Select(p => new Product() { ... });
}
...
var numberOfProducts = GetAll().Count();

This time I used IQueryable instead of IEnumerable. This small change is crucial. It causes that no products are read from a database. Entity Framework "sees" that we only wanto to count number of products and an appropriate query is sent to a database. In other words LINQ To Entities is used.

The situation is completely different when we work with IEnumerable. In order to understand a difference we have to realise one thing. Count method for IEnumerable is something different than Count method for IQueryable. With IEnumerable we use LINQ To Objects and LINQ To Objects operates on objects in memory, it cannot communicate with a database. It is why all products must be read from a database if we want to count them.

Now someone inquiring can say that for virtual methods it shouldn't matter if we have variables of type IEnumerable or of type IQueryable if these variables points the same objects. After all C# is an object oriented language that supports polymorphism etc. Well, it is all true but only for virtual methods and Count is not a virtual method. It is an extension method and extension methods don't support polymorphism.