20 (Senior Developer) C# Interview Questions and Answers (2023)

Greetings, fellow C# wizards! Do you have what it takes to conquer the ultimate challenge? Welcome to the grand finale of our C# Interview Questions and Answers series!

If you've made it this far, you're a seasoned expert ready to showcase your skills. Get ready to tackle the most difficult questions yet and prove your prowess in the world of C#.

Let's dive in, shall we?

Can you describe just-in-time compilation (JIT) in the context of C#?

Answer

Just-In-Time (JIT) Compilation is a technique used by the .NET runtime to provide significant performance improvements by compiling the Intermediate Language (IL) bytecode to native machine code at runtime. Instead of pre-compiling the entire code to native code before executing, the JIT compiler optimizes and compiles only the required methods at runtime, greatly reducing load times and memory usage.

The main benefits of JIT compilation in the context of C# are:

• Faster application startup: Because only the necessary parts of the code are compiled, the application starts up more quickly.
• Better memory usage: Unused IL bytecode is never converted to native code, leading to lower memory usage.
• Platform-specific optimization: Native code is generated specifically for the runtime platform, allowing better optimization and performance.

The process followed by the JIT compiler in the context of C# consists of three stages:

1. Loading the IL bytecodes: The CLR loads the required IL bytecode of the method to be executed.
2. Compiling IL bytecodes to native code: The JIT compiler compiles the IL bytecodes to native machine code.
3. Executing the native code: The generated native code is executed.

What is the difference between a normal class property and a computed class property in C#?

Answer

A normal class property is a simple property that holds a value and includes a getter and/or a setter method. These properties can be used to store and retrieve data for an object. The setter is responsible for setting the property value, and the getter is responsible for returning the property value.

A computed class property, also known as a calculated property, is a property that does not store any data but rather computes its value based on other property values within the class. Computed properties only have a getter method, which returns the calculated result, and do not have a setter method.

Normal property:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

Computed property:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public string FullName
    {
        get
        {
            return $"{FirstName} {LastName}";
        }
    }
}

In this example, FullName is a computed property that returns the concatenated first name and last name.

How can you implement a custom awaitable type in C#?

Answer

To implement a custom awaitable type in C#, you need to follow these steps:

1. Create a class that represents the awaitable type.
2. Implement the INotifyCompletion interface in the class for notifying when the operation is complete.
3. Add a method named GetAwaiter that returns an instance of the class itself.
4. Implement the IsCompleted property in the class as part of the awaitable pattern.
5. Add a method named OnCompleted(Action continuation) to the class which takes an action that will be executed when the operation is complete.
6. Implement the GetResult method, which will return the result of the asynchronous operation.

Here’s an example of a custom awaitable type:

public class CustomAwaitable : INotifyCompletion
{
    private bool _isCompleted;

    public CustomAwaitable GetAwaiter() => this;

    public bool IsCompleted => _isCompleted;

    public void OnCompleted(Action continuation)
    {
        Task.Delay(1000).ContinueWith(t =>
        {
            _isCompleted = true; 
            continuation?.Invoke();
        });
    }

    public int GetResult() => 42;
}

Why would you use the System.Reflection namespace, and how does it relate to C#?

Answer

System.Reflection is a namespace in .NET that provides functionality to obtain type information (metadata) about classes, objects, and assemblies at runtime. It allows developers to inspect and interact with the code in a dynamic manner, providing the ability to:

• Examine type information such as properties, fields, events, and methods.
• Create and manipulate instances of objects.
• Invoke methods and access fields/properties on instances.
• Discover and examine attributes applied to types and members.
• Load and interact with assemblies.

The following example demonstrates using System.Reflection to get information about a class’s type and its methods:

using System;
using System.Reflection;

class Example
{
    static void Main()
    {
        Type myType = typeof(DemoClass);

        MethodInfo[] myMethods = myType.GetMethods(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly);

        Console.WriteLine("The methods in the DemoClass are:");

        foreach (MethodInfo method in myMethods)
        {
            Console.WriteLine(method.Name);
        }
    }
}

class DemoClass
{
    public void Method1() { }
    public void Method2() { }
    public void Method3() { }
}

What are expression trees in C# and how can they be used?

Answer

Expression trees in C# are a data structure that represents code – specifically, Expressions – in a tree-like format, where each node is an object that represents a part of the expression. Expression trees enable developers to inspect, manipulate, or interpret code in a structured way at runtime. They allow operations such as modification, compilation, and execution of the expressed code.

Expression trees are mainly used in scenarios such as:

• Building dynamic LINQ queries for data manipulation.
• Dynamic code generation for performance-critical code paths.
• Serialization and deserialization of expression trees.
• Analyzing lambda expressions for parallelism or converting them to another form.

Here’s an example of creating an expression tree for a simple lambda expression:

using System;
using System.Linq.Expressions;

class Program
{
    static void Main()
    {
        Expression<Func<int, int, int>> addExpression = (a, b) => a + b;

        // Access the expression tree structure for examination or manipulation
        BinaryExpression body = (BinaryExpression)addExpression.Body;
        ParameterExpression left = (ParameterExpression)body.Left;
        ParameterExpression right = (ParameterExpression)body.Right;

        Console.WriteLine("Expression: {0} + {1}", left.Name, right.Name);

        // Compile the expression tree to a delegate to execute it
        Func<int, int, int> addFunc = addExpression.Compile();

        int result = addFunc(3, 5);
        Console.WriteLine("Result: {0}", result);
    }
}

What is the real-world use case for the ‘yield’ keyword in C#?

Answer

The yield keyword in C# is used in iterator methods to create a stateful iterator and return a sequence of values on-the-fly, without storing the entire sequence in memory. It generates a custom implementation of the IEnumerator<T> interface based on the code in the iterator method and remembers the current execution state between MoveNext() calls. This lazy evaluation of the iterator improves memory usage and performance, especially for large or infinite sequences.

Real-world use cases for the yield keyword include:

• Implementing custom collections or sequences that need to support foreach iteration.
• Generating an infinite sequence of values or a sequence that should not be stored in memory.
• Processing large data sets or streaming data incrementally without consuming a lot of memory.

Here’s an example demonstrating the use of the yield keyword for generating an infinite sequence of even numbers:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        foreach (int evenNumber in GenerateEvenNumbers())
        {
            if (evenNumber > 50)
            {
                break;
            }

            Console.WriteLine(evenNumber);
        }
    }

    public static IEnumerable<int> GenerateEvenNumbers()
    {
        int number = 0;

        while (true)
        {
            yield return number;
            number += 2;
        }
    }
}

Explain the role of the ‘volatile’ keyword in C#.

Answer

The volatile keyword in C# is applied to fields to indicate that they can be accessed by multiple threads and that the field’s value may change unexpectedly, due to optimizations performed by the .NET runtime or underlying hardware.

When a field is marked as volatile, the compiler and the runtime will not reorder or cache its read and write operations, ensuring that the most recent value is always read and that writes are immediately visible to other threads. This provides a memory barrier, forcing atomic read and write operations and preventing unexpected side effects due to optimizations.

The volatile keyword should be used in scenarios where multiple threads must access a shared field, and proper synchronization is required to maintain the consistency of data.

Here’s an example demonstrating the use of a volatile variable:

using System;
using System.Threading;

class VolatileExample
{
    private static volatile bool _shouldTerminate = false;

    static void Main()
    {
        Thread workerThread = new Thread(Worker);
        workerThread.Start();

        Console.ReadLine();
        _shouldTerminate = true;

        workerThread.Join();
    }

    static void Worker()
    {
        int counter = 0;

        while (!_shouldTerminate)
        {
            counter++;
        }

        Console.WriteLine("Terminated after {0} iterations.", counter);
    }
}

In this example, the _shouldTerminate field is used to signal the worker thread when to terminate. Marking it as volatile ensures that the worker thread will see the updated value immediately.

What are weak references, and when would you use them in C#?

Answer

In C#, weak references are references to objects that aren’t strong enough to prevent these objects from being garbage collected. They allow you to maintain a reference to an object as long as the object is alive in memory, but without preventing garbage collection (GC) from reclaiming the object when memory pressure increases. With weak references, you can access the object as long as it’s still in memory, but it will not prevent the GC from collecting the object if required.

Weak references are useful in scenarios where you want to hold a reference to a large object for caching purposes but do not want to prevent the object from being garbage collected if the memory pressure increases. This allows for more efficient memory management, especially when working with large data sets or in-memory caches.

To use a weak reference in C#, you create an instance of the WeakReference or WeakReference<T> class.

Here’s an example demonstrating the usage of a weak reference:

using System;

class Program
{
    static void Main()
    {
        WeakReference<MyLargeObject> weakReference = new WeakReference<MyLargeObject>(new MyLargeObject());

        MyLargeObject largeObject;
        if (weakReference.TryGetTarget(out largeObject))
        {
            // The object is still in memory, so we can use it
            Console.WriteLine("Using the large object.");
        }
        else
        {
            // The object has been garbage collected, so we need to recreate it
            Console.WriteLine("The large object has been garbage collected.");
            largeObject = new MyLargeObject();
        }
    }
}

class MyLargeObject
{
    private byte[] _data = new byte[1000000]; // A large object consuming memory
}

In this example, if the GC decides to reclaim the memory used by the MyLargeObject instance, the weakReference.TryGetTarget call returns false. Otherwise, the largeObject will remain accessible through the weak reference.

Describe how to implement a custom attribute in C#.

Answer

To implement a custom attribute in C#, you follow these steps:

1. Create a class that derives from the System.Attribute class.
2. Add properties, fields, or methods to the class as required to store metadata.
3. Apply the attribute to elements in your code (such as classes, methods, or properties) by using the attribute syntax.

Here’s an example of creating and using a custom attribute in C#:

using System;

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class CustomAttribute : Attribute
{
    private string _description;

    public CustomAttribute(string description)
    {
        _description = description;
    }

    public string Description => _description;
}

[Custom("This is a custom attribute applied to the example class.")]
class ExampleClass
{
    [Custom("This is a custom attribute applied to the example method.")]
    public void ExampleMethod()
    {
        // ...
    }
}

In this example, we created a custom attribute called CustomAttribute with a description property. We applied it to the ExampleClass and its ExampleMethod(). The AttributeUsage attribute is used to limit the targets to which the custom attribute can be applied.

Explain the concept of memory-efficient array handling in C# using ArraySegment.

Answer

The ArraySegment<T> structure in C# provides a memory-efficient way of handling arrays by allowing you to work with a segment of an existing array. This is useful in scenarios where you need to process a portion of a large array and want to avoid memory overhead caused by creating new subarrays.

The ArraySegment<T> structure represents a contiguous range of elements within an array and provides properties, such as Array, Offset, and Count, to access the underlying array and the segment boundaries.

Here’s an example that demonstrates the use of ArraySegment<T> for memory-efficient array processing:

using System;

class Program
{
    static void Main()
    {
        int[] largeArray = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        ArraySegment<int> segment = new ArraySegment<int>(largeArray, 2, 3);

        PrintArraySegment(segment);
    }

    static void PrintArraySegment(ArraySegment<int> segment)
    {
        int[] array = segment.Array;
        for (int i = segment.Offset; i < segment.Offset + segment.Count; i++)
        {
            Console.WriteLine(array[i]);
        }
    }
}

In this example, an ArraySegment<int> object is created to represent a portion of the largeArray. The PrintArraySegment method processes the array segment without creating a new subarray, thus reducing memory overhead.

What is the Roslyn Compiler Platform, and how does it relate to C#?

Answer

The Roslyn Compiler Platform is a set of open-source compilers, code analysis APIs, and code refactoring tools developed by Microsoft for C# and Visual Basic .NET (VB.NET) languages. Roslyn exposes a powerful code analysis and transformation API, allowing developers to create more advanced static code analyzers, code fixers, and refactoring tools.

Roslyn’s relation to C#:

• It provides the default C# compiler which transforms C# code into Microsoft Intermediate Language (MSIL) code.
• It offers a modern C# language service implementation for Visual Studio.
• It lets developers take advantage of the code analysis and manipulation APIs for deeper code insights and generation.
• It supports advanced features of modern C# versions like pattern matching, expression-bodied members, and async-await constructs.

Explain the concept of duck typing and how it can be achieved in C#.

Answer

Duck typing refers to a programming concept in which the type of an object is determined by its behavior (methods and properties) rather than its explicit class or inheritance hierarchy. In other words, duck typing is based on the principle: “If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.”

C# is a statically typed language, which means duck typing is not directly supported by the language. However, you can achieve a form of duck typing in C# using dynamic keyword or reflection.

Using dynamic keyword:

public void MakeDuckSwim(dynamic duck)
{
    duck.Swim();
}

But it’s important to note that using dynamic may bring performance overhead, and you lose some compile-time safety. Errors will occur at runtime if the method or property doesn’t exist on the object.

What is the difference between GetHashCode and Equals methods in C#?

Answer

GetHashCode and Equals methods are members of the System.Object class, the base class for all objects in C#. They are used to compare objects for equality and serve different purposes:

• GetHashCode: This method returns an integer (hash code) representation of the object. It is primarily used by hashing-based collections like Dictionary, [HashSet](https://www.bytehide.com/blog/hashset-csharp/ "How To C# HashSet (Tutorial): From A to Z"), etc., to optimize the object’s lookup and storage. When implementing this method, you should make sure objects considered equal have the same hash code value.
• Equals: This method checks whether two objects are equal in their content. It uses their data members to determine equality. By default, the Equals method will compare object references, but it can be overridden to provide custom comparison logic based on the actual object content (like comparing properties).

When you override the Equals method, you should also override the GetHashCode method to maintain consistency between the two methods, ensuring objects that are considered equal have the same hash code.

Explain the concept of unmanaged resources in C# and how they can be managed.

Answer

Unmanaged resources are those not directly controlled by the .NET runtime, such as file handles, network connections, database connections, and other system resources. Because the .NET runtime’s garbage collector does not manage them, developers must handle such resources explicitly to avoid resource leaks or potential application instability.

To manage unmanaged resources properly, you can:

1. Implement the IDisposable interface in your class which uses unmanaged resources. The IDisposable interface provides a Dispose method for cleaning up unmanaged resources.

public class FileWriter : IDisposable
{
    private FileStream fileStream;

    public FileWriter(string fileName)
    {
        fileStream = new FileStream(fileName, FileMode.Create);
    }

    public void Dispose()
    {
        if (fileStream != null)
        {
            fileStream.Dispose();
            fileStream = null;
        }
    }
}

1. Use the using statement to ensure the Dispose method is called automatically when the object goes out of scope.

using (FileWriter writer = new FileWriter("file.txt"))
{
    // Do some operations
}

In this example, the Dispose method will be called automatically when the using block is exited, ensuring proper cleanup of the unmanaged resources.

Describe parallel programming support in C# and its benefits.

Answer

Parallel programming support in C# is provided by the System.Threading, System.Threading.Tasks, and System.Collections.Concurrent namespaces. These parallel execution features allow developers to write applications that leverage modern multi-core and multi-processor hardware for better performance and responsiveness.

Key parallel programming features in C#:

• Task Parallel Library (TPL): Provides a high-level abstraction for executing tasks concurrently, simplifying the parallelism work like managing threads, synchronization, and exception handling.
• Parallel LINQ (PLINQ): A parallel execution version of standard LINQ, enabling developers to easily parallelize data-intensive query operations efficiently.
• Concurrent Collections: Collections like ConcurrentDictionary, ConcurrentQueue, ConcurrentBag, and ConcurrentStack provide thread-safe data structures to help manage shared state in parallel applications.

Benefits of parallel programming in C#:

• Improved performance by taking advantage of multi-core and multi-processor systems.
• Increased responsiveness by executing time-consuming tasks concurrently without blocking the UI thread.
• Simplified parallel programming through high-level abstractions provided by TPL and PLINQ.
• Better utilization of system resources leading to more efficient, scalable applications.

How do you perform compile-time code generation in C#, and what are the benefits?

Answer

Compile-time code generation in C# can be achieved using Source Generators, which are introduced in C# 9.0 and .NET 5. Source Generators are components that run during compilation and can inspect, analyze, and generate additional C# source code to be compiled alongside the original code.

A Source Generator is a separate assembly containing one or more classes implementing the ISourceGenerator interface. Visual Studio and the .NET build system will discover Source Generators with the appropriate project references and will run them during the compilation process.

Benefits of compile-time code generation:

• Improved performance: Code generation at compile-time reduces runtime overhead.
• Code reduction: Automatically generating repetitive or boilerplate code reduces the code a developer needs to write and maintain.
• Security: Generated code is verified and secured before runtime, preventing security vulnerabilities arising from runtime code generation.
• Extensibility: Source Generators enable the creation of advanced libraries and frameworks, which can further enhance and optimize code generation and manipulation.

What is stackalloc in C#, and when should it be used?

Answer

In C#, stackalloc is a keyword that allows you to allocate a block of memory on the stack.

int* block = stackalloc int[100];

Normally, when we work with arrays in C#, they are created on the heap. This introduces a level of overhead, as memory must be allocated and then garbage collected when the object is no longer in use. When dealing with large arrays or high performance code, the impact of this can sometimes be significant.

The stackalloc keyword bypasses this by creating the array directly on the stack. This has 3 major implications:

1. Performance: Generally, allocating memory on the stack is faster than allocating it on the heap. There's no need to worry about garbage collection, as the memory will be automatically reclaimed when the method exits. This makes it highly efficient for small arrays.

2. Memory Limit: The stack is a far more limited resource compared to the heap. The exact size depends on settings and other factors, but it's typically in the region of 1MB. This makes stackalloc unsuitable for larger arrays.

3. Lifespan: Memory allocated on the heap can exist as long as your application does, whereas memory allocated on the stack only exists until the end of the current method. Any attempt to work with stack-allocated memory outside of this will lead to issues.

The typical use cases for stackalloc are high performance scenarios that involve relatively small arrays, such as graphical or mathematical operations. Be aware that improper use can easily lead to stack overflows, causing your application to crash.

Consider using stackalloc if the following cases are true:

• You have a small array (typically few hundred elements max).
• The array is local to your method and doesn't need to be returned or passed elsewhere.
• The overhead of garbage collection has a significant impact on performance in your use case.

Here is an example of using stackalloc:

unsafe 
{
    int* fib = stackalloc int[100];

    fib[0] = fib[1] = 1;

    for(int i=2; i<100; ++i)
    {
        fib[i] = fib[i - 1] + fib[i - 2];
    }

    Console.WriteLine(fib[60]); //Prints fibonacci number at position 60
}

Please note that stackalloc uses unsafe code, so you must compile with the /unsafe switch.

Explain the Tuple class and its use case in C#.

Answer

The [Tuple](https://www.bytehide.com/blog/tuple-csharp/ "Tuples in C#: Full Guide") class in C# represents a fixed-size, immutable collection of heterogeneously-typed elements. It’s part of the System namespace and provides a simple way to group objects without defining custom domain-specific classes. Tuples are useful in scenarios where you need to return multiple values from a method or store or pass around data without creating a specific data structure.

Example of using Tuple:

public Tuple<int, string> GetPersonInfo()
{
    int age = 30;
    string name = "John";
    return Tuple.Create(age, name);
}

var personInfo = GetPersonInfo();
Console.WriteLine($"Name: {personInfo.Item2}, Age: {personInfo.Item1}");

In C# 7.0, tuples were improved with the introduction of ValueTuple. ValueTuples are struct-based (instead of class-based) tuples that allow named elements and other enhancements:

public (int Age, string Name) GetPersonInfo()
{
    int age = 30;
    string name = "John";
    return (age, name);
}

var personInfo = GetPersonInfo();
Console.WriteLine($"Name: {personInfo.Name}, Age: {personInfo.Age}");

What are local functions in C# 7.0, and how can they be used?

Answer

Local functions, introduced in C# 7.0, are functions defined within the scope of another method. They enable you to declare a helper function inside the method that needs it, keeping the helper function private and avoiding clutter in the class level namespace.

Local functions offer advantages like:

• Better organization and encapsulation of functionality.
• Access to the containing method’s variables, allowing them to share state easily.
• Limited accessibility, as local functions are not visible outside their containing method.

Example of a local function:

public int CalculateSum(int[] numbers)
{
    // Local function
    int Add(int a, int b)
    {
        return a + b;
    }

    int sum = 0;
    foreach (int number in numbers)
    {
        sum = Add(sum, number);
}
    return sum;
}

In this example, the Add local function is only visible within the CalculateSum method and can access the local variables (e.g., sum) of the containing method.

Explain the concept of marshaling in .NET and its application in C#.

Answer

Marshaling is the process of converting the types, objects, and data structures of one runtime environment to another, especially in the context of communication between managed (.NET) code and unmanaged (native) code. Marshaling is widely used in .NET when you want to interact with components developed in other programming languages, operating systems, or hardware and software platforms (e.g., COM, Win32 API, or other third-party libraries).

In C#, marshaling is facilitated by the System.Runtime.InteropServices namespace, providing required attributes, methods, and structures to:

• Define the layout of structures and unions in memory.
• Map managed types to unmanaged types.
• Specify calling conventions for unmanaged functions.
• Allocate and deallocate unmanaged memory.

Example of using marshaling in C#:

using System;
using System.Runtime.InteropServices;

class Program
{
    // Import Win32 MessageBox function
    [DllImport("user32.dll", CharSet = CharSet.Unicode)]  
    public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);

    static void Main()
    {
        MessageBox(IntPtr.Zero, "Hello, World!", "C# Message Box", 0);
    }
}

In this example, we import the MessageBox function from the user32.dll native library using the DllImport attribute. This enables the managed C# code to call the unmanaged Win32 MessageBox function. The marshaling process will handle the conversion of string values and other necessary data types when calling the unmanaged function.

Congratulations on completing the most challenging level of our C# Interview Questions and Answers series! You've faced the toughest questions and emerged victorious. But don't let your journey end here, my coding connoisseur

Stay connected and follow our content for more exciting adventures in the realm of C#. Remember, the learning never stops. Keep coding, keep growing, and keep pushing your limits.

What's next on your coding conquest? The choice is yours!

Comments

[Kelly Brown]

Your description of duck typing is really good, but dynamic is not an example of duck typing. Look to enumerables or awaitables as appropriate examples: the language only cares that certain methods/properties are present with specific names, and the code will not compile if it is not structured correctly.

[stvndall]

While the wikipedia answer of the jit is correct, the benifits listed are almost entirely not correct, and simply not how it works in practice.

The startup times are slower and aot compiling.

The memory usage is typically the same, or more than what it would be without the jit

The platform specific optimisations, while they can be optimised quickly on the fly, most compilers these days would target a chipset or platform and put the same or better optimisations in place.

Some tangible benifits of the jit would be

Runtime code generation for unknown objects and flows coming through

Reflection based activities are only achievable through the jit

Compile once, run anywhere there is an installed compatible runtime.

Run dynamic generated code on the fly.

Note the jit is very powerful, but if disrespected can also open security gaps to attackers

[kirsten Jacks]

If you are looking to hire a .NET developer, these two insightful interview questions for senior .NET developers can be helpful:

1. Discuss your approach to implementing secure coding practices in a .NET development project. How would you stay updated on the latest security vulnerabilities and best practices in the .NET ecosystem?

2. Explain how you would design a highly scalable and performant microservices architecture for a large e-commerce application using ASP.NET Core. Focus on aspects like fault tolerance, load balancing, and API design principles.

[Davide Bellone]

May I say that this is probably one of the best articles about C# that I've read in a while?

Well done!