15 Essential C# Interview Questions *

The output will be:

location is null
1/1/0001 12:00:00 AM

Although both variables are uninitialized, String is a reference type and DateTime is a value type. As a value type, an unitialized DateTime variable is set to a default value of midnight of 1/1/1 (yup, that’s the year 1 A.D.), not null.

There are of course many ways to do this, but two of the most straightforward would be either:

static long TotalAllEvenNumbers(int[] intArray) {
  return intArray.Where(i => i % 2 == 0).Sum(i => (long)i);
}

or:

static long TotalAllEvenNumbers(int[] intArray) {
  return (from i in intArray where i % 2 == 0 select (long)i).Sum();
}

Here are the key things to look for in the answer:

1. Does the candidate take advantage of the C# language constructs which make a one-liner solution possible (i.e., rather than employing a more lengthy solution which contains a loop, conditional statement, and accumulator)?
2. Does the candidate consider the possibility of overflow. For example, an implementation such as return intArray.Where(i => i % 2 == 0).Sum() (regardless of the return type of the function) might be an “obvious” one-line solution, but the probability of overflow here is high. While the approach used in the answers above of converting to long doesn’t eliminate the possibility, it makes it a highly unlikely that an overflow exception will occur. Note that, if the candidate asks about the expected size of the array and the magnitude of its members, he or she is obviously considering this overflow issue, which is part of what we’re looking to ascertain.

One might think that, since a DateTime variable can never be null (it is automatically initialized to Jan 1, 0001), the compiler would complain when a DateTime variable is compared to null. However, due to type coercion, the compiler does allow it, which can potentially lead to headfakes and pull-out-your-hair bugs.

Specifically, the == operator will cast its operands to different allowable types in order to get a common type on both sides, which it can then compare. That is why something like this will give you the result you expect (as opposed to failing or behaving unexpectedly because the operands are of different types):

double x = 5.0;
int y = 5;
Console.WriteLine(x == y);  // outputs true

However, this can sometimes result in unexpected behavior, as is the case with the comparison of a DateTime variable and null. In such a case, both the DateTime variable and the null literal can be cast to Nullable<DateTime>. Therefore it is legal to compare the two values, even though the result will always be false.

The preferred answer would be of the form:

circle.Calculate(r => 2 * Math.PI * r);

Since we don’t have access to the private radius field of the object, we tell the object itself to calculate the circumference, by passing it the calculation function inline.

A lot of C# programmers shy away from (or don’t understand) function-valued parameters. While in this case the example is a little contrived, the purpose is to see if the applicant understands how to formulate a call to Calculate which matches the method’s definition.

Alternatively, a valid (though less elegant) solution would be to retrieve the radius value itself from the object and then perform the calculation with the result:

var radius = circle.Calculate(r => r);
var circumference = 2 * Math.PI * radius;

Either way works. The main thing we’re looking for here is to see that the candidate is familiar with, and understands how to invoke, the Calculate method.

The answer to the first part of the question (i.e., the version of the code with await Task.Delay(5);) is that the program will just output a blank line (not “Hello world!”). This is because result will still be uninitialized when Console.WriteLine is called.

Most procedural and object-oriented programmers expect a function to execute from beginning to end, or to a return statement, before returning to the calling function. This is not the case with C# async functions. They only execute up until the first await statement, then return to the caller. The function called by await (in this case Task.Delay) is executed asynchronously, and the line after the await statement isn’t signaled to execute until Task.Delay completes (in 5 milliseconds). However, within that time, control has already returned to the caller, which executes the Console.WriteLine statement on a string that hasn’t yet been initialized.

Calling await Task.Delay(5) lets the current thread continue what it is doing, and if it’s done (pending any awaits), returns it to the thread pool. This is the primary benefit of the async/await mechanism. It allows the CLR to service more requests with less threads in the thread pool.

Asynchronous programming has become a lot more common, with the prevalence of devices which perform over-the-network service requests or database requests for many activities. C# has some excellent programming constructs which greatly ease the task of programming asynchronous methods, and a programmer who is aware of them will produce better programs.

With regard to the second part of the question, if await Task.Delay(5); was replaced with Thread.Sleep(5), the program would output Hello world!. An async method without at least one await statement in it operates just like a synchronous method; that is, it will execute from beginning to end, or until it encounters a return statement. Calling Thread.Sleep() simply blocks the currently running thread, so the Thread.Sleep(5) call just adds 5 milliseconds to the execution time of the SaySomething() method.

This program will output the number 10 ten times.

Here’s why: The delegate is added in the for loop and “reference” (or perhaps “pointer” would be a better choice of words) to i is stored, rather than the value itself. Therefore, after we exit the loop, the variable i has been set to 10, so by the time each delegate is invoked, the value passed to all of them is 10.

Yes! It is possible to do so because the array can be of type object that can not only store any datatype but also the object of the class as shown below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication8
{
    class Program
    {
        class Customer
        {
            public int ID { get; set; }
            public string Name { get; set; }
            public override string ToString()
            {
                return this.Name;
            }
        }
        static void Main(string[] args)
        {
            object[] array = new object[3];
            array[0] = 101;
            array[1] = "C#";
            Customer c = new Customer();
            c.ID = 55;
            c.Name = "Manish";
            array[2] = c;
            foreach (object obj in array)
            {
                Console.WriteLine(obj);
            }
            Console.ReadLine();
        }
    }
}

Classes and Structs in C# do have a few things in common, namely:

• Are compound data types
• Can contain methods and events
• Can support interfaces

But there are a number of differences. Here’s a comparison:

Classes:

• Support inheritance
• Are reference (pointer) types
• The reference can be null
• Have memory overhead per new instance

Structs:

• Do not support inheritance
• Are value types
• Are passed by value (like integers)
• Cannot have a null reference (unless Nullable is used)
• Do not have memory overhead per new instance (unless “boxed”)

TestValue : 10

The static constructor of a class is called before any instance of the class is created. The static constructor called here initializes the TestValue variable first.

The this keyword is used to call other constructors, to initialize the class object. The following shows the implementation:

class ClassA
{
  public ClassA() : this(10)
  { }

  public ClassA(int pValue)
  {  }
}

Hello
C

Dependency injection is a way to de-couple tightly linked classes, thereby reducing the direct dependency of classes upon each other. There are different ways by which dependency injection can be achived:

1. Constructor dependency
2. Property dependency
3. Method dependency

using system;
class abc
{
    public static Void Main()
    {
        int ndistance, nresult;
        Console.WriteLine("Enter the distance in kilometers");
        ndistance = convert.ToInt32(Console.ReadLine());
        nresult = ndistance * 1000;
        Console.WriteLine("Distance in meters: " + nresult);
        Console.ReadLine();
    }
}

Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by the value type. Boxing a value type creates an object instance containing the value and stores it on the heap.

Example:

int x = 101;
object o = x;  // boxing value of x into object o

o = 999;
x = (int)o;    // unboxing value of o into integer x

This problem has two parts: Preserving the first occurrence and replacing all the others.

We can solve it using a regular expression and String.Replace():

using System;
using System.Text.RegularExpressions;

class MainClass {
  public static void Main (string[] args) {
    string s = "like for example $  you don't have $  network $  access";
    Console.WriteLine("before: {0}", s);

    GroupCollection halves = Regex.Match(s, @"([^$]*\$)(.*)").Groups;
    string answer = halves[1].Value + halves[2].Value.Replace("$", "");
    
    Console.WriteLine("after: {0}", answer);
    // like for example $  you don't have   network   access
  }
}

Explanation:

• ([^$]*\$)—Group 1 captures any number of non-$ characters, plus a single $ character (escaped with a \)
• (.*)—Group 2 (greedily) captures everything else

With the first occurrence of $ preserved in halves[1].Value, we can simply use String.Replace() on halves[2].Value to eliminate all $ characters found in the remainder of the string, without the need for a second regular expression.