Top 20 .NET Core Inteview Questions 2026

Introduction

As we navigate the software landscape of 2026, .NET 10 has solidified its position as the premier framework for cross-platform, cloud-native development. For senior developers and architects, the interview process has shifted away from basic syntax toward deep architectural patterns, performance optimization, and the integration of AI-driven workflows. This guide covers the top 20 interview questions designed to challenge even the most seasoned .NET professionals.

1. Explain the evolution of Native AOT in .NET 10 and its impact on cold-start performance.

Native AOT (Ahead-of-Time) compilation has matured significantly by 2026. Unlike the JIT (Just-In-Time) compiler, Native AOT compiles your code directly into machine code at publish time. This eliminates the need for the JIT compiler at runtime, resulting in faster startup times and lower memory footprints, which is critical for serverless environments and containerized microservices.

// Example of a minimal API optimized for Native AOT
// In 2026, we ensure all libraries used are "trim-ready"
using System.Text.Json.Serialization;

var builder = WebApplication.CreateSlimBuilder(args);

// Source generators are mandatory for JSON serialization in AOT
builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.TypeInfoResolver = AppJsonContext.Default;
});

var app = builder.Build();
app.MapGet("/", () => "AOT Optimized Response");
app.Run();

[JsonSourceGenerationOptions(WriteIndented = true)]
[JsonSerializable(typeof(string))]
internal partial class AppJsonContext : JsonSerializerContext { }

2. How does the "Modular Monolith" architecture compare to Microservices in the 2026 .NET ecosystem?

In 2026, many organizations have pivoted back from microservices to Modular Monoliths to reduce operational complexity. In .NET, this is achieved through strictly decoupled projects or folders, using internal libraries and shared kernels, and communicating via in-memory event buses or MediatR, while retaining the ability to split into microservices if scaling requirements demand it.

3. Describe the role of Semantic Kernel in modern .NET AI integration.

Semantic Kernel is now the standard SDK for integrating Large Language Models (LLMs) into .NET applications. It allows developers to orchestrate AI "plugins," manage memory (vector databases), and create "planners" that can automatically execute sequences of C# functions based on natural language prompts.

// Integrating an AI plugin with Semantic Kernel
using Microsoft.SemanticKernel;

var kernel = Kernel.CreateBuilder()
    .AddAzureOpenAIChatCompletion("model-id", "endpoint", "key")
    .Build();

// Defining a native function for the AI to call
kernel.ImportPluginFromFunctions("HelperPlugin", new[]
{
    kernel.CreateFunctionFromMethod((string input) => 
        $"Processed: {input}", "ProcessData")
});

// The AI can now invoke 'ProcessData' based on intent

4. What are "Primary Constructors" in C# and how have they evolved for classes?

Introduced in earlier versions and refined by 2026, primary constructors allow you to define constructor parameters directly in the class header. This reduces boilerplate and simplifies dependency injection for services.

// Dependency Injection using Primary Constructors
public class OrderService(IOrderRepository repository, ILogger<OrderService> logger)
{
    public async Task ProcessOrder(Guid id)
    {
        logger.LogInformation("Processing order {Id}", id);
        var order = await repository.GetByIdAsync(id);
        // ... business logic
    }
}

5. Explain the concept of "Zero-Copy" parsing with Span<T> and Utf8JsonReader.

Performance-critical applications in 2026 avoid heap allocations by utilizing Span<T> and ReadOnlySpan<T>. When parsing JSON or large text streams, Utf8JsonReader works directly with memory buffers without creating intermediate string objects, drastically reducing GC pressure.

6. How does .NET 10 handle Distributed Tracing and OpenTelemetry by default?

Observability is baked into the framework. .NET 10 provides first-class support for OpenTelemetry via the System.Diagnostics.DiagnosticSource and Activity classes. Middleware automatically captures traces and exports them to collectors like Jaeger or Honeycomb without requiring extensive manual instrumentation.

7. What is the difference between "Required" members and "Init-only" properties?

The required keyword ensures that a property must be set during object initialization, providing a compile-time safety net. init properties allow setting a value only during initialization, making the object immutable thereafter. In 2026, these are used together to create robust Data Transfer Objects (DTOs).

public class UserDto
{
    public required Guid Id { get; init; }
    public required string Email { get; init; }
    public string? DisplayName { get; init; }
}

// This will throw a compile error if Email is missing
var user = new UserDto { Id = Guid.NewGuid(), Email = "dev@example.com" };

8. Discuss the impact of Green IT and Carbon-Aware SDKs in .NET.

Sustainability has become a key metric. Modern .NET libraries now offer carbon-aware scheduling, where non-critical background tasks (via IHostedService) are delayed until the local energy grid has a higher percentage of renewable energy, often integrated through specialized cloud-native APIs.

9. How do "Extension Types" (C# 14/15 feature) differ from Extension Methods?

Extension Types allow developers to add new members—including properties and static methods—to existing types without using the traditional static class syntax. This provides a cleaner, more object-oriented way to extend third-party libraries.

10. Describe the "Vertical Slice Architecture" and why it is replacing N-Tier.

Vertical Slice Architecture organizes code by features rather than technical concerns (UI, Business, Data). Each feature "slice" contains everything from the API endpoint to the database logic, reducing the need to jump across projects and minimizing the "Change Ripple" effect.

11. What is the purpose of the FrozenDictionary<TKey, TValue>?

Introduced for high-performance scenarios, FrozenDictionary is an immutable collection optimized for read-heavy workloads. It performs significantly faster than ReadOnlyDictionary because it builds a highly optimized lookup structure at creation time.

using System.Collections.Frozen;

var source = new Dictionary<string, int> { { "A", 1 }, { "B", 2 } };
// Created once, read many times with O(1) performance
var optimizedConfig = source.ToFrozenDictionary(); 

12. Explain "Interceptors" in C# and their use in source generation.

Interceptors allow source generators to "intercept" calls to specific methods and redirect them to optimized versions. This is used heavily in ASP.NET Core to replace runtime reflection with compile-time generated logic for things like routing and dependency injection.

13. How do you implement Resilience Patterns using the modern Polly v8+ SDK?

Polly has evolved to be more performance-oriented with a functional "Resilience Pipeline" approach. This allows for combining retries, circuit breakers, and rate limiters into a single, cohesive execution policy.

// Defining a resilience pipeline in .NET 10
var pipeline = new ResiliencePipelineBuilder()
    .AddRetry(new RetryStrategyOptions
    {
        MaxRetryAttempts = 3,
        BackoffType = DelayBackoffType.Exponential
    })
    .AddCircuitBreaker(new CircuitBreakerStrategyOptions { ... })
    .Build();

await pipeline.ExecuteAsync(async token => await CallRemoteService(token));

14. What are "Discriminated Unions" in C# and how do they benefit Error Handling?

Expected to be a major feature by 2026, Discriminated Unions allow a method to return one of several different types. This replaces the "Result" pattern or throwing exceptions for flow control, making domain logic more explicit.

// Speculative syntax for 2026
public union Result<T> { T Value; Error Failure; }

public Result<User> GetUser(int id)
{
    var user = _db.Find(id);
    return user != null ? user : new Error("Not Found");
}

// Usage with pattern matching
var result = GetUser(1);
var message = result switch {
    User u => $"Found {u.Name}",
    Error e => $"Error: {e.Message}"
};

15. Explain the shift from Newtonsoft.Json to System.Text.Json for legacy migration.

By 2026, System.Text.Json is the default and only recommendation due to its performance and AOT compatibility. Migration involves handling missing features like polymorphic serialization through JsonPolymorphic attributes and custom JsonConverter implementations.

16. How does the "BFF" (Backend for Frontend) pattern apply to Blazor WebAssembly in 2026?

Blazor applications now predominantly use the BFF pattern to handle authentication and API aggregation. The Blazor WASM client only communicates with its specific ASP.NET Core host, which manages Secure Cookies and OIDC flows, mitigating XSS and token theft risks.

17. What is "Rate Limiting" middleware in ASP.NET Core and how does it prevent DoS?

The built-in Rate Limiting middleware allows developers to define policies (Fixed Window, Sliding Window, Token Bucket) to restrict the number of requests a user or IP can make, ensuring system stability under heavy load.

18. Describe the use of IAsyncEnumerable<T> in Streaming APIs.

For large data sets, IAsyncEnumerable<T> allows the server to stream results to the client as they are read from the database, reducing memory spikes and improving perceived performance.

// Streaming data from a repository to the API response
[HttpGet("stream")]
public async IAsyncEnumerable<DataPoint> GetLargeData()
{
    using var reader = await _db.ExecuteReaderAsync("SELECT * FROM Logs");
    while (await reader.ReadAsync())
    {
        yield return new DataPoint(reader["Value"].ToString());
    }
}

19. What are "Collection Expressions" and how do they improve code readability?

Introduced in C# 12 and ubiquitous by 2026, collection expressions provide a unified syntax [] for creating arrays, lists, and spans, allowing the compiler to choose the most efficient implementation.

// Unified collection syntax
int[] array = [1, 2, 3];
List<string> list = ["A", "B", "C"];
ReadOnlySpan<char> span = ['h', 'e', 'l', 'l', 'o'];

// Spread operator in collections
int[] extra = [0, ..array, 4]; // [0, 1, 2, 3, 4]

20. How does .NET 10 approach "Shadow Database Mirroring" for Zero-Downtime Deployments?

In high-availability systems, .NET 10 integrates with EF Core to support shadow mirroring where migrations are applied to a secondary schema and verified before the production traffic is toggled, ensuring database schema changes never cause application downtime.

Conclusion

The .NET ecosystem in 2026 is defined by extreme performance, AI integration, and a focus on developer productivity through language simplification. Mastering these topics—from Native AOT and Semantic Kernel to advanced C# features—is essential for any senior professional aiming to lead technical teams in the current era. By focusing on architectural patterns and the underlying mechanics of the framework, you will be well-prepared to tackle any challenge presented in a high-level technical interview.