20 Senior .NET Developer Interview Questions to Be Prepared For

Microsoft Message Queuing, or MSMQ, is technology for asynchronous messaging. Whenever there's need for two or more applications (processes) to send messages to each other without having to immediately know results, MSMQ can be used. MSMQ can communicate between remote machines, even over Internet. It's free and comes with Windows, but is not installed by default.

This mainly addresses the common use case of asynchronous message processing: you have a service Service1 that communicates (send messages) with another part of your software architecture, say Service2.

Main problem: what if Service2 becomes suddenly unavailable? Will messages be lost? If you use MSMQ it won't: Service1 will send messages into a queue, and Service2 will dequeue when it is available.

MSMQ will resolve following common issues:

• temporary unavailability of a service: messages are persisted on the disk and will be dequeued when the service becomes available again, so no messages are lost
• as it's fully asynchronous, it'll help a lot in case of punctual peak load: your Service2 won't die under the heavy load, it'll just dequeue and process messages, one after one

In a query expression, it is sometimes useful to store the result of a sub-expression in order to use it in subsequent clauses. You can do this with the let keyword, which creates a new range variable and initializes it with the result of the expression you supply.

Consider:

var names = new string[] { "Dog", "Cat", "Giraffe", "Monkey", "Tortoise" };
var result =
    from animalName in names
    let nameLength = animalName.Length
    where nameLength > 3
    orderby nameLength
    select animalName; 

Single responsibility is the concept of a Class doing one specific thing (responsibility) and not trying to do more than it should, which is also referred to as High Cohesion.

Classes don't often start out with Low Cohesion, but typically after several releases and different developers adding onto them, suddenly you'll notice that it became a monster or God class as some call it. So the class should be refactored.

• Thread represents an actual OS-level thread, with its own stack and kernel resources. Thread allows the highest degree of control; you can Abort() or Suspend() or Resume() a thread, you can observe its state, and you can set thread-level properties like the stack size, apartment state, or culture. ThreadPool is a wrapper around a pool of threads maintained by the CLR.

• The Task class from the Task Parallel Library offers the best of both worlds. Like the ThreadPool, a task does not create its own OS thread. Instead, tasks are executed by a TaskScheduler; the default scheduler simply runs on the ThreadPool. Unlike the ThreadPool, Task also allows you to find out when it finishes, and (via the generic Task) to return a result.

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)
  {  }
}

A lambda expression is an anonymous function that you can use to create delegates or expression tree types. By using lambda expressions, you can write local functions that can be passed as arguments or returned as the value of function calls. Lambda expressions are particularly helpful for writing LINQ query expressions.

In the following example, the lambda expression x => x * x, which specifies a parameter that's named x and returns the value of x squared, is assigned to a variable of a delegate type:

Func<int, int> square = x => x * x;
Console.WriteLine(square(5));
// Output:
// 25

A const is a compile-time constant whereas readonly allows a value to be calculated at run-time and set in the constructor or field initializer. So, a const is always constant but readonly is read-only once it is assigned.

When to use const or readonly:

• const

  • compile-time constant: absolute constant, value is set during declaration, is in the IL code itself

• readonly

  • run-time constant: can be set in the constructor/init via config file i.e. App.config, but once it initializes it can't be changed

Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled,

• in the case of the const value, it is like a find-replace, the value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I'll update I_CONST_VALUE to 20 in the future. AssemblyB would still have 2 till I recompile it.
• in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA. All clients do not need to be recompiled.

Remember: If you reference a constant from another assembly, its value will be compiled right into the calling assembly. That way when you update the constant in the referenced assembly it won't change in the calling assembly!