Interview Questions by MK #8

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Title: Sunset seen from the top of Mount Fuji, Source: own resources, Authors: Agnieszka and Michał Komorowscy

This is the first post from Interview Questions by MK series for a long time. The motivation to write it was a short talk with my colleague. His company really want to hire new .NET developers. The situation on the market is difficult for employers so they are also considering juniors without experience. And still they have a problem to find someone. Why?

The requirements are not extremely high. I'd say that they are standard. They don't demand God knows what. The ideal candidate doesn't have to: know all formatting options available in .NET, enumerate classes in System.DirectoryServices namespace, tell about all new features introduced in any .NET version or any other thing that can be checked in the documentation within seconds. However, they want someone with general knowledge. What I mean by that?

• It's good to know how to write a class, properties, derived a class... but it's also good to understand and can explain principals of the object oriented programming. For example could you tell why OOP is better than the procedural programming? Or maybe it isn't? Could you justify why encapsulation is actually a good thing?
• You don't have to know all possible collections available in .NET API but it's worth knowing some of them and their characteristics. Just to mention the list, the dictionary, the stack or the queue.
• You don't have to be very good in algorithms but knowing how to search a binary tree is not the rocket science.
• Writing a code that compiles without errors is only a first step. You should also know how to write a readable and a maintainable code. This knowledge comes from experience but at the beginning you should hear about refactoring, knows that the method with 50 parameters is not the best choice...
• It's not a problem if you have never worked with the continuous integration but you should at least know that there is something like that.
• As a developer you'll probably not work directly with IT infrastructure but knowing what is the load balancing or the computer cluster does not seem very demanding.
• ...

I can continue and continue this enumeration. According to me these are basic things, but still many candidates don't know them. Sometimes even developers with a few or more years of experience.

If you are one of them and you want to have better chances on the job market, I recommend one simple thing i.e. reading books, blogs, web sites... whatever you want. Several minutes (better more) every day, regularly, will make a big difference.

You may also say that you don't care because you'll get a job anyway. Well, it's true at least for now. Nonetheless it'll be only some job.

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.

Interview Questions for Programmers by MK #6

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Question #6
What is the arithmetic overflow and how is it handled in .NET?

Answer #6
It is a situation when the result of an arithmetic operation exceeds (is outside of) the range of a given numeric type. For example the maximum value for byte type in .NET is 255. So in the following example, an operation a+b will cause an overflow:

byte a = 255;
byte b = 20;
byte c = a + b;

The final result depends on the used numeric types:

• For integer types either OverflowException will be thrown or the result will be trimmed/cropped (the default behaviour). It depends on the compiler configuration and usage of checked / unchecked keywords.
• For floating point types OverflowException will never be thrown. Instead the overflow will lead either to the positive or the negative infinity.
• For decimal type OverflowException will be always thrown.

var b = byte.MaxValue;
//The result will be zero because:
//b = 255 = 1111 1111 
//b++ = 256 = 1 0000 0000
//The result has 9 bits so the result will be trimmed to 8 bits what gives 0000 0000
b++; 
         
checked
{
 b = byte.MaxValue;
 //Exception will be thrown 
 b++; 
}

var f = float.MaxValue;
//The result will be float.PositiveInfinity
f *= 2;  

decimal d = decimal.MaxValue;
//Exception will be thrown
d++; 

Interview Questions for Programmers by MK #5

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Question #5
Here you have a very simple implementation of Template method pattern.

public abstract class BaseAlgorithm
{
   protected SomeObject Resource { get; set; }
   //Other resources

   public void Start()
   {
      // Configure
      Resource = new SomeObject();
      //...
      try
      {
         InnerStart();
      }
      finally
      {
         // Clean up
         Resource.Dispose();
         Resource= null;               
         //...
      }
   }

   protected abstract void InnerStart();
}

public class Algorithm1: BaseAlgorithm
{
   protected override void InnerStart()
   {
      //Do something with allocated resources
   }  
}

At some point someone decided to create a new class Algorithm2 derived from BaseAlgorithm. The difference between the new class and the previous one is that Algorithm2 starts an asynchronous operation. A programmer decided to use async/await keywords to handle this scenario. What do you think about this approach? What could possibly go wrong?

public class Algorithm2: BaseAlgorithm
{
   protected async override void InnerStart()
   {
      var task = DoAsyncCalculations();
      await task;

      //Do something with allocated resources
   }

   private Task DoAsyncCalculations()
   {
      //Let's simulate asynchronous operation
      return Task.Factory.StartNew(() => Thread.Sleep(1000));
   }
}

Answer #5
I think that the developer who created Algorithm2 doesn't understand well how async/await keywords work. The main problem is that finally block inside Start method will be executed before DoAsyncCalculations method will end calculations. In other words resources will be disposed in the middle of calculations and this will cause an exception. Sequence of events will be as follows:

• Start method begins.
• SomeObject is created.
• InnerStart method begins.
• InnerStart method starts an asynchronous operation and uses await to suspend its progress.
• This causes that control returns to Start method.
• Start method cleanups resources.
• When the asynchronous operation is finished InnerStart method continues processing. It tries to use resources, that have been already disposed, what leads to an exception.

It is also not recommended to have async void methods (except event handlers). If an async method doesn't return a task it cannot be awaited. It is also easier to handle exceptions if an async method returns a task. For details see also this article.

To fix a problem BaseAlgorithm must be aware of asynchronous nature of calculations. For example InnerStart method can return a task which will be awaited inside try block. However, it also means that synchronous version of InnerStart method in Algorithm1 will have to be changed. It may not be acceptable. Generally, providing asynchronous wrappers for synchronous methods is debatable and should be carefully considered.

In this case, I'll consider to have separated implementations of Template method pattern for synchronous and asynchronous algorithms.

Interview Questions for Programmers by MK #4

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Question #4
You have to implement a very fast, time critical, network communication between nodes of a distributed system. You decided to use good old sockets. However, you haven't decided yet whether to use TCP or UDP protocol. Which one would you choose and why?

Answer #4
If a speed is the only one important factor, I'd choose UDP. UDP is faster than TCP because it has a smaller overhead. In comparison to TCP it is a connection-less, unreliable protocol that doesn't provide features like retransmission, acknowledgment or ordering of messages.

However, it also means that usage of UDP might be more difficult and will require additional coding in some cases. For example, if we have to assure that sent messages have been delivered. In this case, I'd certainly use TCP.

Finally, there is one more thing in favor of UDP. It provides broadcasting and multicasting. So, if it is required I'd also use UDP instead of TCP.

Interview Questions for Programmers by MK #3

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Question #3
You found the following code and were asked to refactor it if needed:

var sb = new StringBuilder();
sb.AppendLine("<root>")
sb.AppendLine(String.Format("   <node id="{0}"/>", 1));
sb.AppendLine(String.Format("   <node id="{0}"/>", 2));
sb.AppendLine(String.Format("   <node id="{0}"/>", 3));
//Many, many lines of code
sb.AppendLine("</root>");

What would you do and why?

Answer #3
It is not the best idea to create XML documents using string concatenation because it is error prone. Besides created documents are not validated in any way. In .NET we have a few possibilities to refactor this code.

I recommend to use XmlWriter in this case because we want to create a new document and we do not want to edit an existing one. However, if we also want to modify existing XML documents, the good choice will be XDocument or XmlDocument class.

In the case of small XML documents (when performance is not critical) it might be a good idea to use XDocument or XmlDocument even if we don't want to edit existing documents. Especially XDocument can be simpler in use than XmlWriter.

Comments #3
I remember that when I wanted to create an XML document in C# for the first time I did it by using string concatenation. The code worked ok and I was surprised that it didn't pass a review :)

Interview Questions for Programmers by MK #2

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I prepared this question a long time ago in order to check knowledge of basic good practices.

Question #2
You have the following method which is responsible for updating a client in a database:

public void UpdateClient(
   int id,
   string name,
   string secondname,
   string surname,
   string salutation,
   int age,
   string sex,
   string country,
   string city,
   string province,
   string address,
   string postalCode,
   string phoneNumber,
   string faxNumber,
   string country2,
   string city2,
   string province2,
   string address2,
   string postalCode2,
   string phoneNumber2,
   string faxNumber2,
   ...)
{
   //...
}

Do you think that this code requires any refactoring? If yes, give your proposition. A database access technology doesn't matter in this question.

Answer #2
The basic problem with this method is that it has so many parameters. It is an error prone, difficult to maintain and use approach. I suggest to change the signature of this method in the following way:

public void UpdateClient(Client client)
{
   //...
}

Where Client is a class that models clients. It can look in the following way:

public class Client
{
   public int Id { get; set; }
   public string Name { get; set; }
   public string Secondname { get; set; }
   public string Surname { get; set; }
   public string Salutation { get; set; }
   public int Age { get; set; }
   public string Sex { get; set; }
   public Address MainAddress{ get; set; }
   public Address AdditionalAdddress { get; set; }
   /* More properties */
}

Address class contains details (country, city..) of addresses.

Comments #2
You may also write much more e.g.:

• It may be good to introduce enums for properties like 'Sex' which can take values only from the strictly limited range.
• UpdateClient method should inform a caller about the result of an update operation e.g. by returning a code.

However, the most important thing is to say that UpdateClient method shouldn't have so many parameters. Personally, if I see a code as above I immediately want to reduce the number of parameters. This question seemed and still seems to be very easy, however not all candidates were able to answer it. Maybe it should be more accurate. For example, I should have stressed that a candidate should focus ONLY on available code. What do you think?

Interview Questions for Programmers by MK #1

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Do you know series of posts titled Interview Question of the Week on a SQL Authority blog? If not or if you don't know this blog at all you have to catch up. I learned a lot of from this series so I decided to start publishing something similar but to focus more on .NET and programming.

This is a first post from series which I called Interview Questions for Programmers by MK and in which I'm going to publish questions that I'd ask if I were a recruiter. Of course they are somehow based on my experience as a participant of many interviews.

Question #1
What is a meaning of using statement in the code below? What would you do if using keyword did not exist?

using(var file = File.OpenWrite(path))
{
   //...
}

Answer #1
In this example using statement is used to properly release resources (to call Dispose method) that are owned by an object of a class that implements IDisposable interface. It is a syntactic sugar and could be replaced by using try/finally block in the following way:

var file = File.OpenWrite(path);
try
{
   //...
}
finally
{
   if(file != null)
      file.Dispose();
}