C# developer interview questions and answers

1. Can you explain the difference between string and StringBuilder in C#?

A string in C# is an object of the System.String class, representing a sequence of characters. String objects are immutable, meaning they cannot be modified once they are created. StringBuilder, on the other hand, is also a class in C# that represents a mutable string. Developers use StringBuilder whenever they need to modify a string by appending or deleting characters, as this is not possible with the class String.

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

An abstract class is a class that cannot be instantiated and must be inherited. An abstract class provides a base implementation that can be inherited by its derived classes, and it may also have concrete methods. Interfaces, on the other hand, are just a collection of method signatures and properties that a class can implement. Interfaces are commonly used to provide a common set of methods or properties that different classes should implement.

3. Can you explain the "using" statement in C#?

The "using" statement in C# is used to declare a scope at the end of which an object will be automatically disposed of. It can be used with any object that implements the IDisposable interface, which means that it has a Dispose() method defined. For example, if you create an instance of a StreamReader object for reading a file, you can use the "using" statement to make sure that the StreamReader object is disposed of properly after the file has been read.

4. What is boxing and unboxing in C#?

The .NET runtime uses a unified type system, which means that all types in .NET inherit from a common base type called System.Object. Boxing in C# means converting a value type to an object type, which is done by copying the value from the stack to the heap. Unboxing is the opposite process - taking an object type and converting it back to a value type. One common example of boxing and unboxing is when storing value types in collections like List, where objects are required to be stored.

5. What is the difference between a value type and a reference type?

Value types are types that are directly stored in memory, and they contain their own value. Examples of value types include numeric types like int, long, and double, as well as the bool type. Reference types, on the other hand, are types that are stored on the heap and contain a reference to the memory location where the data is stored. Examples of reference types include classes, arrays, and strings. In general, value types are faster to allocate and access because they don't require dereferencing.

6. What is the difference between an event and a delegate in C#?

A delegate in C# is a reference to a method, and it can be used to invoke the method at a later time. An event in C# is a mechanism for signaling that something has occurred, and it is implemented using delegates. An event is essentially a wrapper around a delegate, which provides a way for a class to notify other objects when it has changed state. In other words, while a delegate represents a call to a method, an event represents a state change in an object.

7. What is exception handling in C#?

Exception handling is a mechanism in C# that allows you to deal with runtime errors in a structured and controlled manner. Whenever an error occurs during the execution of a program, an exception is thrown, which means that the normal flow of execution is interrupted. By using try-catch-finally blocks, you can handle exceptions gracefully by catching the exception and taking appropriate action, such as logging an error message or displaying a user-friendly error message.

8. What is LINQ and how is it used in C#?

LINQ, or Language-Integrated Query, is a set of language features and libraries built into C# that allows you to perform data queries directly within your code. LINQ offers a uniform syntax for querying different data sources, such as arrays, collections, and databases. With LINQ, you can write queries that are type-safe and easy to read, and you don't need to know the underlying SQL or other query language syntax.

9. What is the difference between an abstract method and a virtual method in C#?

An abstract method is a method that doesn't have a body, and it is used to define a method signature that must be implemented by a derived class. An abstract method cannot be instantiated, and it must be implemented by a derived class. A virtual method, on the other hand, is a method that can be overridden by a derived class, but it has a default implementation provided by the base class. Virtual methods are used when a base class provides a default behavior that can be overridden by a derived class.

10. How do you manage memory in C#?

C# uses a garbage collector to manage memory automatically, which means that you don't need to worry about deallocating memory manually. The garbage collector keeps track of all objects that are no longer being used by your program and deallocates them automatically. However, you can help the garbage collector do its job more efficiently by following some best practices, such as minimizing the use of unmanaged resources like file handles or network sockets, avoiding circular references between objects, and using the "using" statement to dispose of objects that implement IDisposable.