10 C# API Interview Questions and Answers

C# has established itself as a robust and versatile programming language, particularly in the realm of developing APIs. Its strong typing, object-oriented features, and seamless integration with the .NET framework make it a preferred choice for building scalable and maintainable web services. C# APIs are widely used in enterprise environments, offering powerful tools for creating efficient and secure communication channels between different software systems.

This article provides a curated selection of interview questions focused on C# API development. By working through these questions and their detailed answers, you will gain a deeper understanding of key concepts and best practices, enhancing your ability to tackle technical challenges and demonstrate your expertise in C# API development during interviews.

C# API Interview Questions and Answers

1. Describe the different HTTP methods and their typical uses in API operations.

HTTP methods are used to perform operations on resources in RESTful APIs. The most commonly used HTTP methods are:

• GET: Retrieves data from the server, such as fetching user details from a database.
• POST: Sends data to the server to create a new resource, like creating a new user account.
• PUT: Updates an existing resource on the server, replacing the entire resource with the provided data.
• PATCH: Partially updates an existing resource, applying partial modifications.
• DELETE: Deletes a specified resource from the server.
• HEAD: Similar to GET, but only retrieves the headers and status line, useful for checking resource existence.
• OPTIONS: Describes the communication options for the target resource.

2. How would you serialize and deserialize JSON data when making API calls in C#?

Serialization converts an object into a format like JSON for transport or storage, while deserialization does the reverse. In C#, the Newtonsoft.Json library is commonly used for these tasks.

Example:

using Newtonsoft.Json;

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

// Serialization
Person person = new Person { Name = "John", Age = 30 };
string json = JsonConvert.SerializeObject(person);

// Deserialization
Person deserializedPerson = JsonConvert.DeserializeObject<Person>(json);

3. Discuss the various authentication mechanisms that can be used to secure an API.

There are several authentication mechanisms to secure an API in C#:

• Basic Authentication: The client sends the username and password encoded in Base64. It is not very secure unless used over HTTPS.
• Token-Based Authentication: The client sends a token, usually a JSON Web Token (JWT), with each request. The server validates the token and grants access if it is valid.
• OAuth: An open standard for access delegation, allowing third-party services to exchange tokens on behalf of the user. OAuth 2.0 is widely used for securing APIs.
• API Keys: Unique identifiers passed along with API requests. They are simple to implement but should be used with caution as they can be easily shared or exposed.

4. How would you implement versioning in an API to ensure backward compatibility?

To implement versioning in an API, you can use several strategies:

• URL Path Versioning: The version number is included in the URL path.
• Query String Versioning: The version number is specified as a query parameter.
• Header Versioning: The version number is included in the request header.
• Accept Header Versioning: The version is specified in the Accept header using media types.

Example of URL Path Versioning in a C# Web API:

[ApiController]
[Route("api/v1/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult GetProductsV1()
    {
        return Ok(new { Message = "This is version 1" });
    }
}

[ApiController]
[Route("api/v2/[controller]")]
public class ProductsV2Controller : ControllerBase
{
    [HttpGet]
    public IActionResult GetProductsV2()
    {
        return Ok(new { Message = "This is version 2" });
    }
}

5. What is rate limiting and why is it important in API management?

Rate limiting controls the number of requests an API can handle within a specified time frame. It is essential for:

• Preventing Abuse: Limiting the number of requests a single user or client can make.
• Ensuring Fair Usage: Ensuring all users have fair access to resources.
• Maintaining Performance: Controlling the number of requests to handle traffic efficiently.
• Protecting Backend Systems: Ensuring backend systems are not overloaded with too many requests.

6. Describe how you would implement pagination in an API response.

Pagination in an API response allows clients to request data in smaller chunks, improving response times. In a C# API, pagination can be implemented using query parameters like page and pageSize.

Example:

[HttpGet]
public IActionResult GetItems(int page = 1, int pageSize = 10)
{
    var items = _context.Items
                        .Skip((page - 1) * pageSize)
                        .Take(pageSize)
                        .ToList();

    var totalItems = _context.Items.Count();
    var totalPages = (int)Math.Ceiling(totalItems / (double)pageSize);

    var response = new
    {
        Page = page,
        PageSize = pageSize,
        TotalItems = totalItems,
        TotalPages = totalPages,
        Items = items
    };

    return Ok(response);
}

7. Explain how Swagger/OpenAPI can be used for documenting APIs.

Swagger/OpenAPI creates interactive API documentation, allowing developers to explore and test API endpoints. In a C# project, Swagger can be integrated using the Swashbuckle library.

Example:

// Install Swashbuckle via NuGet Package Manager
// Install-Package Swashbuckle.AspNetCore

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.DependencyInjection;

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
        services.AddSwaggerGen();
    }

    public void Configure(IApplicationBuilder app)
    {
        app.UseSwagger();
        app.UseSwaggerUI(c =>
        {
            c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
        });

        app.UseRouting();
        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

8. How do you test APIs in C# to ensure they function correctly?

Testing APIs in C# involves unit testing, integration testing, and using external tools for manual testing.

• Unit Testing: Testing individual components of the API in isolation using frameworks like xUnit or NUnit.
• Integration Testing: Ensuring different parts of the application work together as expected.
• Manual Testing Tools: Tools like Postman and Swagger are used for manual testing of APIs.

Example of a unit test using xUnit and Moq:

using Xunit;
using Moq;
using MyApi.Controllers;
using MyApi.Services;
using Microsoft.AspNetCore.Mvc;

public class MyApiControllerTests
{
    [Fact]
    public void Get_ReturnsOkResult_WithListOfItems()
    {
        var mockService = new Mock<IMyService>();
        mockService.Setup(service => service.GetItems()).Returns(new List<string> { "Item1", "Item2" });
        var controller = new MyApiController(mockService.Object);

        var result = controller.Get();

        var okResult = Assert.IsType<OkObjectResult>(result);
        var items = Assert.IsType<List<string>>(okResult.Value);
        Assert.Equal(2, items.Count);
    }
}

9. What are some security best practices for developing APIs?

When developing APIs in C#, follow security best practices to protect sensitive data and ensure the integrity of the application:

• Use HTTPS: Encrypt data transmitted between the client and the server.
• Authentication and Authorization: Implement strong authentication mechanisms and use role-based access control (RBAC).
• Input Validation: Validate all incoming data to prevent injection attacks.
• Rate Limiting: Implement rate limiting to prevent abuse and denial-of-service (DoS) attacks.
• Logging and Monitoring: Enable logging and monitoring to detect and respond to suspicious activities.
• Data Encryption: Encrypt sensitive data both at rest and in transit.
• API Versioning: Use versioning to manage changes to the API.
• Security Headers: Implement security headers to protect against common web vulnerabilities.
• Regular Security Audits: Conduct regular security audits and code reviews.

10. How do you optimize the performance of an API?

Optimizing the performance of an API in C# involves several strategies:

• Efficient Data Access: Use efficient data access patterns and technologies such as Entity Framework with proper indexing and query optimization.
• Caching: Implement caching mechanisms to store frequently accessed data, reducing the need to repeatedly fetch data from the database.
• Asynchronous Programming: Use asynchronous programming to handle I/O-bound operations without blocking the main thread.

public async Task<IActionResult> GetDataAsync()
{
    var data = await _dataService.GetDataAsync();
    return Ok(data);
}

• Minimizing Payload Size: Reduce the size of the data being sent over the network by using techniques such as compression and selecting only necessary fields.
• Load Balancing: Distribute incoming requests across multiple servers to ensure no single server becomes a bottleneck.
• Monitoring and Profiling: Continuously monitor the API’s performance and use profiling tools to identify and address bottlenecks.