100 Essential C# interview questions

Answer: Early binding and late binding are concepts associated with polymorphism in C#. Polymorphism is a feature of object-oriented programming that enables a language to utilize the same name in different forms. For instance, a method named Add can perform addition operations on integers, doubles, and decimals. Polymorphism can be achieved through two distinct types:

• Compile time, also known as early binding or overloading.
• Runtime, also known as late binding or overriding.

Answer: Following are the key differences between IEnumerable and IQueryable:

• IEnumerable is in the System.Collections namespace while IQueryable is in the System.Linq namespace
• IEnumerable execute a select query on the server side, load data in-memory on a client-side and then filter data whereas IQueryable implements a ‘select query’ on server-side with all filters.
• IEnumerable is appropriate for querying data from in-memory collections like List, Array. On the other hand, IQueryable is appropriate for querying data from out-memory (like remote database, service) collections
• IEnumerable is helpful for LINQ to Object and LINQ to XML queries unlike IQueryable which is useful for LINQ to SQL queries

Answer: Constructor chaining in C# is a mechanism that establishes a relationship between two or more classes through inheritance. In constructor chaining, every constructor of a child class is implicitly linked to a constructor of the parent class using the base keyword. When an instance of the child class is created, the constructor of the parent class is called. Without constructor chaining, inheritance between classes would not be possible.

Answer: The Array.Clone() method creates a shallow copy of an array. It copies only the elements of the array, whether they are reference types or value types. However, it does not copy the objects that the references point to. The references in the new array still point to the same objects as those in the original array. On the other hand, the CopyTo() method of the Array class is a static method that copies a section of an array to another array. It copies all the elements of an array to another one-dimensional array. For example, it can copy the contents of an integer array to various object types.

Answer: Indexers in C# are a feature that allows objects to be treated as arrays. Indexers are commonly referred to as “smart arrays” in C#. They provide a way to access elements of a class using the [] array access operator. By defining an indexer, you can create classes that simulate array-like behavior, where instances of the class can be accessed using index values.

Answer: Object pooling is a design pattern used to manage and reuse expensive objects instead of creating and destroying them repeatedly. It involves maintaining a pool or container of reusable object instances. These objects are obtained from the pool when needed and returned to the pool after their immediate use. Object pooling is often used to improve performance and reduce resource consumption in applications.

Answer: Although both the Equality Operator (==) and Equals() method are used to compare two objects for equality, they function differently. The Equality Operator (==) compares object references. It returns true only if both references point to the same object in memory. The Equals() method, on the other hand, compares the values carried by the objects. For example, when comparing two integers int x=10 and int y=10, x == y would compare their references, while x.Equals(y) would compare their values. The Equals() method is typically overridden in classes to provide custom comparison logic.

Answer: The finally block in C# is used to ensure that certain code is executed regardless of whether an exception occurs or not. It is executed irrespective of whether an exception is thrown or caught in the preceding try-catch blocks. The finally block is commonly used for tasks such as closing connections to a database or releasing file handlers, where the cleanup code should always be executed regardless of exceptions.

Answer: Thread signifies an actual OS-level thread, with its own stack and kernel resources. Thread permits the highest level of control; you can Suspend() or Abort() or Resume() a thread, you can observe its state. You can set thread-level properties such as apartment state, stack size, or culture. ThreadPool is a wrapper around a pool of threads maintained by the CLR. The Task class from the Task Parallel Library offers many benefits. Like the ThreadPool, a task doesn’t create its own OS thread. As an alternative, tasks are performed by a TaskScheduler; the default scheduler simply runs on the ThreadPool. Unlike the ThreadPool, Task also lets you discover when it finishes and (via the generic Task) returns a result.

Answer: Lambda expressions in C# are anonymous functions that can be used to create delegates or expression tree types. They provide a concise syntax for defining small and inline functions. Lambda expressions are particularly useful for writing LINQ query expressions and functional programming-style code. They allow you to define functions as parameters or return values, making code more expressive and readable.

Answer: Operators in C# are symbols that represent predefined operations that can be performed on operands. There are several types of operators in C#:

• Arithmetic Operators: Used for performing mathematical operations.
• Relational Operators: Used for comparing values and determining relationships.
• Logical/Boolean Operators: Used for logical operations and combining conditions.
• Bitwise Operators: Used for performing operations at the bit level.
• Assignment Operators: Used for assigning values to variables.
• Type Information Operators: Used for obtaining type information at runtime.
• Miscellaneous Operators: Including conditional operators, lambda operators, and more.

Answer: A hashtable is a collection of pairs, generally “keys” and “values.” It offers efficient lookup and retrieval of values on the basis of their associated keys. Keys are unique In a hashtable, and each key is connected with a value. Hashtables use a hashing algorithm to create an index for each key, allowing fast access to the corresponding value. They are frequently used for effective data retrieval and caching.

Answer: An immutable string in C# is a string object that cannot be changed after it is created. While the reference to an immutable string can be reassigned, the actual content of the string remains constant. In general, immutable strings are used for values that should not be modified once assigned. However, it is important to use them judiciously, as attempting to change an immutable string can result in errors.

Answer: In C#, a destructor is a special method that is automatically called when an object is being destroyed or garbage collected. The destructor is defined using the ~ symbol followed by the class name. Its purpose is to release resources, such as closing files or freeing memory before the object is removed from memory. However, it is important to note that C# provides automatic memory management through the garbage collector, so explicit use of destructors is rarely necessary.

Answer: In C#, three types of comments can be used to document code:

• Single-line comments: These comments start with // and are used to document a single line of code.
• Multiline comments: These comments start with /* and end with */. They can span multiple lines and are often used for long explanations or commenting-out sections of code.
• XML comments: These comments start with /// and are used to generate XML documentation for classes, methods, properties, etc. They provide a way to document code elements and generate documentation files.

Answer: A virtual method in C# is a method that can be overridden in derived classes. It allows a derived class to provide its own implementation of the method while preserving the same method signature. Virtual methods are defined in a base class using the virtual keyword and can be overridden in derived classes using the override keyword. On the other hand, an abstract method is a method that is declared in an abstract class or interface but does not provide an implementation. It serves as a contract that derived classes must implement. Abstract methods are defined using the abstract keyword and must be implemented in derived classes using the override keyword. In summary, a virtual method has a default implementation in the base class but can be overridden in derived classes, while an abstract method does not have an implementation and must be overridden in derived classes.

Answer: The static keyword in C# is used to declare members (variables, methods, properties, etc.) that belong to the type itself rather than to a specific instance of the type. Static members are associated with the type itself rather than with individual objects created from the type. They can be accessed directly using the type name without the need to create an instance of the type. Static members are shared among all instances of a class and can be useful for storing data or defining utility methods that are not specific to a particular object. They are typically used for providing global access points or constants that are consistent across all instances of a class.

Answer: Dependency injection is a design pattern in which the dependencies of a class are provided externally rather than being created internally by the class itself. It helps to decouple classes and promotes modular, reusable, and testable code. There are three common ways to achieve dependency injection in C#:

• Constructor injection: Dependencies are passed to a class through its constructor. This ensures that the class has all the required dependencies when it is instantiated.
• Property injection: Dependencies are set through public properties of a class. This allows for more flexibility, as dependencies can be changed or updated after the class is instantiated.
• Method injection: Dependencies are provided as parameters to specific methods when they are called. This allows for fine-grained control over when and how dependencies are used.

By using dependency injection, classes become more flexible, maintainable, and easier to test, as their dependencies can be easily mocked or substituted with alternate implementations.

Answer: Delegates in C# have the following characteristics:

• Delegates are type-safe function pointers that hold references to methods.
• Delegates can be used to create and invoke methods dynamically at runtime.
• Delegates can encapsulate both static and instance methods.
• Delegates have a signature that defines the method parameters and return type they can reference.
• Delegates support multicasting, allowing multiple methods to be invoked through a single delegate.
• Delegates are immutable, meaning that once a delegate is created, it cannot be changed to reference a different method.
• Delegates are commonly used in event handling, callback mechanisms, and implementing the observer pattern.

Answer: In C#, var and dynamic are used to declare variables with implicit or dynamic typing. Both var and dynamic provide flexibility in typing, they differ in their behavior. var is statically typed and resolved at compile-time, while dynamic is dynamically typed and resolved at runtime. var keyword: When a variable is declared using the var keyword, the compiler infers its type based on the assigned value. The actual type of the variable is determined at compile-time and cannot be changed later.  Using var can make code more concise and flexible, especially when working with complex types or anonymous types. dynamic keyword: When a variable is declared using the dynamic keyword, its type is resolved at runtime instead of compile-time. This allows for late binding, dynamic method invocation, and duck typing. Operations on dynamic variables are resolved at runtime, and type checking is deferred until runtime. The dynamic keyword is useful when interacting with dynamic languages, COM objects, or when working with data of unknown or changing types.