Hire the Best Developer with These 15 .NET Interview Questions

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If you’re lucky enough to attract the attention of hard-to-find .NET developers for a position in your organization, you’ll need to be prepared with the right .NET interview questions to ask them. And you’ll also need to know enough about the technology to properly evaluate their answers.

Problem is, if you aren’t a .NET developer yourself or don’t have a strong background in .NET platforms, how can you be certain that your .NET interview questions will help you identify the best candidate?

To help out, we’ve prepared a “cheat sheet” of 15 essential .NET interview questions, along with sample answers. Keep in mind that these Q&As are intended to serve as a guide, not a bible. As a hiring manager, you may need to tailor your questions depending on the responsibilities of your specific position, and you should likewise allow some leeway on each candidate’s answers to reflect their unique background and experience.

The goal is to identify the types of questions commonly asked, and to gain a working knowledge of appropriate answers you might receive from developers during a .NET interview. Let’s get started:

1. What is OOP, and how does it relate to the .NET framework?

OOP stands for object-oriented programming. A good answer to this question would point out that OOP languages are the only languages supported by .NET Framework — Visual Basic .NET, Visual C#, and Visual C++. As a technique, OOP allows .NET developers to create classes containing methods, properties, fields, events and other logical modules. It also lets developers create modular programs, which they can assemble as software. OOPs have four basic features: encapsulation, abstraction, polymorphism and inheritance.

2. What is encapsulation?

Encapsulation is one of four basic features of OOP and refers to the inclusion within a program object of methods and data needed for the object to function. In a .NET interview, you’d want the candidate to point out that encapsulation helps keep data from unwanted access through binding code and data in an object, which is the basic, single self-contained unit of a system. Another way of understanding encapsulation is to think of it as “hiding” the state of an object as private or protected. Under this principle of “information hiding,” the internal workings of an object stay hidden from the rest of the program. This is useful because it makes it less likely that other objects can modify the state or behavior of the object in question.

3. Explain the concept of inheritance and how it relates to .NET.

In the .NET framework, all classes are inheritable by default. A strong .NET interview candidate should be able to explain what this means: that developers can create new classes reusing, extending and modifying behavior defined in other classes, which expedites development since developers can reuse code. Inheritance also enables developers to write and debug one class only once, and then reuse that same code as the basis for the new classes. Another way that someone might explain this is that inheritance represents the relationship that exists between two different classes with one type deriving functionality from another type. The relationship is then extended with the addition of new events, properties, methods, fields and constants.

4. What is the difference between an abstract class and an interface?

An abstract class is always used as a base class. It provides some abstract/virtual members that the inheriting entities must implement, as well as a partial implementation for a functionality. For extra credit during a .NET interview, your interviewee might mention that this class can also declare fields. Developers cannot create an object from this class.

An interface, on the other hand, can only declare properties, methods and events (no access modifiers). The developer must implement all declared members. In short, an interface designates a contract/behavior that implementing classes should have.

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5. What is the difference between a class and an object, and how do these terms relate to each other?

A class is a comprehensive data type that is the primary building block — or template — of OOP. Class defines attributes and methods of objects and contains an object’s behavior and data.

An object, on the other hand, represents an instance of class. As a basic unit of a system, objects have identity and behavior as well as attributes.

Make sure that your job applicant in a .NET interview can explain how these terms are related to each other as well: the relationship is based on the fact that a class defines the states and properties that are common to a range of objects.

6. What is a delegate in .NET?

A delegate allows the developer to encapsulate a reference to a method inside a delegate object. Examples of this are pointers or callbacks. It is similar to a class used for storing the reference to a method. Developers can also use delegates within a specific class to create custom events. As long as the methods have the same signatures as the delegate, then a delegate can hold the reference for them.

7. Explain the difference between a stack, a queue and a heap.

It’s essential that all contenders in a .NET interview understand and clearly articulate that a stack keeps track of what is executing and contains stored value types to be accessed and processed as Last In First Out, with elements both inserted and deleted from the top end. A queue lists items on a First In First Out basis in terms of both insertion and deletion, with the developer inserting items from the rear end and deleting items from the front end of the queue. A heap contains stored reference types and is responsible for keeping track of more precise objects.

8. What is .NET web service?

Web services are reusable components that allow developers to publish an application’s function over the internet to make it accessible and able to directly interact with other applications and objects online. Web services communicate by using standard web protocols and data formats including HTTP, XML and SOAP that allow them to connect across different platforms and programming languages. ASP.NET provides a simple way to develop web services. The .NET framework provides built-in classes for building and consuming web services.

9. What is API, and when should you use .NET Web API?

A strong candidate in your .NET interview will be well versed in the fact that API is a set of building blocks that provides developers with tools and protocols to help them build software apps. They do so by specifying how various components of the software should interact. Different types of operating systems and websites use different types of apps. In .NET, developers can use ASP.NET as a framework when they want to build HTTP services. Web API offers a framework for many types of clients — from smartphones and browsers to tablets — to consume these services. As an open-source platform, Web API is also useful for building REST-ful services on the .NET framework.

10. What is JSON data, and what is one way that .NET developers can work with JSON?

JSON (JavaScript Object Notation) data provides developers with a way to organize and store data in a way that is easy to access and read. JSON is important to developers because it allows them to manipulate JSON feeds from other sites and to load them more quickly and easily than via SML/RSS feeds. Json.NET provides a way for .NET developers to define classes to parse objects and arrays from JSON text. You can also use Json.NET if you need to serialize value types into JSON text. Json.NET runs on .NET 2, .NET3, and .NET4.

11. Explain the difference between managed and unmanaged code.

Applicants in a .NET interview should be able to point out that all Intermediate Language (IL) is managed code. (See Question 15 below.) One difference between managed and unmanaged code is that Visual Basic and C# compiler create managed code in .NET, but the only way to get unmanaged code is to write the application in C or C++. Since the .NET compiler creates managed code, this type of code does not depend on the target machine’s architecture. The Common Language Runtime (CLR) executes it, and not the operating system (OS). In contrast, unmanaged code is dependent on the architecture of the target machine. It is executed directly by the OS and directly compiled to native machine code.

12. Define LINQ.

LINQ stands for Language Integrated Query. This is a Microsoft programming model and methodology that offers developers a way to manipulate data using a succinct yet expressive syntax. It does so by instilling Microsoft .NET-based programming languages with the ability to make formal queries.

13. Read-only variables and constants have many similarities, but what is at least one way that they differ?

Here are two possible answers that would be acceptable in a .NET interview:

  • Read-only variables can support reference-type variables. Constants can hold only value-type variables.
  • Developers evaluate read-only variables at the runtime. They evaluate constants at the compile time.

14. What do the terms “boxing” and “unboxing” mean?

The terms “boxing” and “unboxing” refer to a value type being allocated on the heap rather than the stack. (See Question 7 above.) The boxing process is the implicit conversion of a value type to an object (or reference type). Unboxing, on the other hand, is explicit conversion of the same reference type back to a value type, which is unboxed from the heap and allocated on the stack.

15. What are three of the most common acronyms used in .NET and what do they stand for?

This should be an easy one for .NET interview candidates as long as they understand the basics. The question allows them some flexibility in choosing terms with which they are most familiar. For example, interviewees might tell you that IL, CIL and CLI are three frequently used acronyms in .NET.

  • IL stands for Intermediate Language, which is an object-oriented programming language that is a partially compiled code that .NET developers will then compile to native machine code.
  • CIL stands for Common Intermediate Language — this was formerly called Microsoft Intermediate Language (MSIL). This is another programming language that .NET developers use, and it represents the lowest possible level for a language that humans can still read.
  • CLI stands for Common Language Infrastructure. This is a compiled code library that Microsoft developed as an open specification. Developers use CLI for security, versioning and deployment purposes.

Other acronyms commonly used in .NET include JIT (Just-in-Time compiler), which uses the target machine’s CPU architecture to perform a .NET operation, OOP (object-oriented programming), defined in Question 1 above, CLR (Common Language Runtime), defined in Question 11 above and LINQ (Language Integrated Query), defined above in Question 12.

There you have it — our list of 15 essential .NET interview questions with sample answers to get you started. If you have additional .NET interview questions you’ve asked job candidates, please post them in the comments section.

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This post has been updated. It originally appeared on 12/10/13.