.NET / C# Interview Questions with Answers.

OCP (Open/Closed Principle): The Open/Closed Principle states that software entities should be open for extension but closed for modification. This principle allows for the behavior of a system to be extended without modifying the existing code, thus reducing the likelihood of introducing new bugs.

LISKOV Substitution Principle (LSP): The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of their subclasses without altering the correctness of the program. A common example of LSP violation is the "squaring rectangle" problem, where the subclass's behavior deviates from the expected behavior of the superclass.

Fixing LISKOV Problem: To fix the LISKOV problem, one should design the class hierarchy so that subclasses adhere to the behavioral contracts defined by their superclasses. This often involves ensuring that subclasses override methods in ways that do not alter the postconditions in a way that violates the expectations set by the superclass.

Interface Segregation Principle (ISP): The Interface Segregation Principle dictates that no client should be forced to depend on methods it does not use. This principle leads to smaller, more focused interfaces that are easier to implement and client applications are less coupled to the interfaces.

Connection between LSP and ISP: The LISKOV Substitution Principle is a manifestation of the Interface Segregation Principle. If classes adhere to ISP, they naturally satisfy LSP because they only implement interfaces that pertain to them, thus avoiding the pitfalls that lead to LSP violations.

Dependency Inversion Principle (DIP): The Dependency Inversion Principle states two key things: 1) High-level modules should not depend on low-level modules; both should depend on abstractions. 2) Abstractions should not depend on details; details should depend on abstractions. This principle aims to reduce the coupling between modules and makes the code more maintainable and scalable.

Higher-level module and lower-level module: A higher-level module is one that performs complex operations and relies on services or utilities provided by lower-level modules. Lower-level modules are typically more granular and provide fine-grained functionality that can be used by higher-level modules without the need to understand the inner workings of the low-level modules.

Benefit of Dependency Inversion with an example: By inverting the dependency, we decouple our code so that it is not dependent on concrete implementations. For example, consider a class ReportGenerator that generates reports and depends on IDataSource to get data. Instead of ReportGenerator directly depending on a specific data source implementation (e.g., SqlDataSource), it depends on the IDataSource abstraction. This way, if we need to use a different data source in the future (like FileDataSource), we can simply provide an implementation of IDataSource, without changing the ReportGenerator code.

SOLID Principles: SOLID is an acronym for five design principles that help developers write more maintainable, scalable, and understandable code. The five principles are: Single Responsibility Principle (SRP), Open/Closed Principle (OCP), Liskov Substitution Principle (LSP), Interface Segregation Principle (ISP), and Dependency Inversion Principle (DIP).

Stock Example for SOLID: A common stock example to illustrate SOLID principles is a system that handles different shapes, such as Circle, Rectangle, and Square. Each shape class would implement an interface IShape with methods like CalculateArea(). This satisfies SRP (each class has only one reason to change), LSP (if we have a Square class that implements IShape, it should behave as expected for a square), ISP (the IShape interface doesn't require any methods that aren't relevant to all shapes), and DIP (high-level classes like ShapeRenderer that render the shapes would depend on IShape rather than concrete shape types).

Generations in GC: The .NET garbage collector has three generations of object storage: Gen0, Gen1, and Gen2. Most objects are allocated in Gen0. When these objects are no longer referenced, they are collected in a process called a "generational collection." If an object survives this cycle, it is moved to the next generation (Gen1 or Gen2), where it may undergo additional collections. This generational approach is efficient and scales well because objects that survive many generations are likely to be long-lived and thus worth the extra overhead of collecting them separately from transient objects.

Dispose Pattern: The IDisposable interface defines a method called Dispose(). Classes that hold unmanaged resources or have special cleanup requirements should implement this interface. Client code should use the using statement to ensure that Dispose() is called even if an exception occurs.

Finalize vs Dispose: There are differences between Finalize and Dispose. Finalize is a part of the garbage collection process and is called by the runtime when an object is no longer reachable. Dispose, on the other hand, should be explicitly called by the developer to release resources in a timely manner. It's generally better to use Dispose over Finalize for managed resources as it provides more predictability and control.

Weak References: A weak reference holds a reference to an object but does not prevent the object from being garbage-collected if there are no other strong references to it. This is useful when you need to hold onto an object for a short period of time or want to minimize its impact on the memory footprint of the application.

Detecting Memory Leaks: Tools like Visual Studio's Diagnostics Tools, memory profilers, and code analysis can help detect memory leaks by identifying objects that are no longer needed but are still referenced in the application. Analyzing object allocations over time and understanding the memory usage patterns can also help pinpoint leaks.

Memory Leak in .NET: While .NET has a robust garbage collector, memory leaks are still possible, especially when dealing with unmanaged resources or when there are strong references to objects that should be collected. It's important to use patterns like IDisposable correctly and to regularly profile the application for potential memory issues.

Using Weak References: You might use weak references when implementing caching mechanisms where you want to minimize the impact on performance and memory usage. For example, if you cache frequently accessed data, using weak references for the cached objects allows the garbage collector to reclaim their memory once they are no longer needed. This can be done using WeakReference or WeakEventTable classes in .NET.