Top 10 .NET Core Web API Interview Questions (with Answers) in 2025

1. What is .NET Core and how is it different from .NET Framework?

Answer:
.NET Core is a cross-platform, open-source framework maintained by Microsoft. Unlike the .NET Framework, which is Windows-only, .NET Core runs on Windows, Linux, and macOS. It’s optimized for modern app development like microservices and cloud-based apps.

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2. What is a Web API in .NET Core?

Answer:
A Web API is a framework for building HTTP-based services that can be consumed by a broad range of clients including browsers, mobile apps, and IoT devices. In .NET Core, Web API uses the ASP.NET Core MVC framework, but it returns data instead of views.

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3. How do you create a simple Web API in .NET Core?

Answer:

• Create a new project using dotnet new webapi.

• Define your controller by inheriting from ControllerBase.

• Use routing attributes like [HttpGet], [HttpPost], etc.

• Return data using ActionResult<T>.

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4. What is Dependency Injection in .NET Core?

Answer:
Dependency Injection (DI) is a design pattern used to achieve Inversion of Control. .NET Core has built-in support for DI through the IServiceCollection interface where you can register services with AddScoped(), AddTransient(), or AddSingleton().

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5. What is Middleware in ASP.NET Core?

Answer:
Middleware are components that form the request pipeline. Each middleware can handle, modify, or pass the HTTP request to the next middleware. Examples include authentication, logging, and error handling.

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6. What are Filters in .NET Core Web API?

Answer:
Filters allow you to add custom processing before or after the execution of an action method. Common types include:

• Authorization filters

• Action filters

• Exception filters

• Resource filters

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7. What is the difference between IActionResult and ActionResult<T>?

Answer:

• IActionResult is a general return type that provides flexibility.

• ActionResult<T> combines the features of IActionResult and allows returning a specific model type (e.g., ActionResult<User>).

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8. How does Routing work in ASP.NET Core Web API?

Answer:
Routing in ASP.NET Core is attribute-based by default. Example:

csharp
[Route("api/[controller]")]
[ApiController]
public class ProductsController : ControllerBase {
    [HttpGet("{id}")]
    public IActionResult GetProduct(int id) { ... }
}

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9. What are some best practices for building a .NET Core Web API?

Answer:

• Use DTOs to shape data.

• Implement versioning (e.g., /api/v1/products).

• Use dependency injection wisely.

• Handle exceptions globally.

• Secure your API using JWT or OAuth2.

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10. How do you implement Authentication and Authorization in .NET Core Web API?

Answer:
Use JWT (JSON Web Tokens) for stateless authentication:

• Configure authentication in Startup.cs using AddAuthentication() and AddJwtBearer().

• Protect endpoints with [Authorize].

Example:

csharp
[Authorize]
[HttpGet]
public IActionResult GetSecureData() { ... }