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.NET Real-Time Scenario-Based Interview Questions and Answers

1. How do you handle concurrency in a multi-threaded .NET application?

Answer:
Concurrency issues arise when multiple threads access shared resources simultaneously, leading to race conditions. In .NET, you can handle this by:

  • Locks: Using lock statements ensures that only one thread can access the critical section at a time.

  • Mutex: For multi-process synchronization, Mutex can be used.

  • Thread-Safe Collections: Using collections like ConcurrentQueue ensures safe access in a multi-threaded environment.

2. How would you manage large data retrieval to avoid performance issues in .NET?

Answer:
To efficiently handle large datasets:

  • Paging: Implement pagination by limiting the number of records fetched using Skip() and Take() in LINQ queries or SQL queries with LIMIT and OFFSET.

  • Lazy Loading: In Entity Framework, enable Lazy Loading to only load related data when it’s needed.

  • Asynchronous Operations: Use asynchronous methods (async and await) to avoid blocking the main thread during data retrieval.

3. What steps would you take to improve the performance of a slow-running SQL query in a .NET application?

Answer:
To optimize slow-running SQL queries:

  • Indexing: Ensure proper indexing on columns that are used frequently in WHERE, JOIN, and ORDER BY clauses.

  • Execution Plan: Analyze the query execution plan using SQL tools like SQL Profiler to identify bottlenecks.

  • Caching: Implement caching using tools like Redis or MemoryCache to reduce database hits.

  • EF Core Optimization: If using Entity Framework, ensure that AsNoTracking() is used for read-only queries to improve performance.

4. How would you implement a logging mechanism in a .NET application?

Answer:
For logging, you can use ILogger in ASP.NET Core:

  • ILogger Interface: Integrate the built-in ILogger interface to log messages at different severity levels (e.g., Information, Warning, Error).

  • Third-Party Libraries: Use NLog, Serilog, or log4net for advanced logging features.

  • Centralized Logging: Use centralized logging systems like ELK Stack or Splunk to collect and analyze logs from different application instances.

5. How do you handle dependency injection in a .NET Core application?

Answer:
.NET Core provides built-in Dependency Injection (DI), which you can leverage to improve code testability and maintainability. Here’s how:

  • Service Registration: Register services in the Startup.cs file using services.AddScoped(), services.AddSingleton(), or services.AddTransient().

  • Constructor Injection: Inject the required services into your classes through constructors, avoiding tight coupling.

Example:

public class MyController : Controller
{
private readonly IMyService _myService;

public MyController(IMyService myService)
{
_myService = myService;
}
}

6. How do you implement JWT authentication in a .NET application?

Answer:
To implement JWT authentication:

  • Token Generation: Generate a JWT token on successful login using System.IdentityModel.Tokens.Jwt.

  • Token Validation: Use JwtBearer middleware in Startup.cs to validate tokens and protect API endpoints.

Example of generating JWT:

var claims = new[]
{
new Claim(ClaimTypes.Name, username)
};
var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("secretKey"));
var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
var token = new JwtSecurityToken(
issuer: "your-issuer",
audience: "your-audience",
claims: claims,
expires: DateTime.Now.AddMinutes(30),
signingCredentials: creds);
return new JwtSecurityTokenHandler().WriteToken(token);

7. How would you implement API versioning in a .NET Core application?

Answer:
To implement API versioning:

  • URL Versioning: Version the API by including the version in the URL, e.g., /api/v1/, /api/v2/.

  • Header Versioning: You can also version via headers, such as using Accept: application/vnd.myapi.v1+json.

  • Query Parameter Versioning: You can pass the version as a query string parameter, like /api/products?version=1.

In Startup.cs, configure versioning like so:

services.AddApiVersioning(options =>
{
options.ReportApiVersions = true;
options.AssumeDefaultVersionWhenUnspecified = true;
options.DefaultApiVersion = new ApiVersion(1, 0);
});

8. How do you implement caching in .NET to improve performance?

Answer:
Caching can significantly reduce the load on your server and database. In .NET:

  • MemoryCache: For in-memory caching, use the MemoryCache class.

  • Distributed Caching: For larger-scale applications, use Redis or SQL Server as distributed caches to share cache across multiple instances.

  • Output Caching: In web applications, you can use Response Caching to cache the HTTP response at the server level.

Example:

var cache = new MemoryCache(new MemoryCacheOptions());
cache.Set("key", "value", TimeSpan.FromMinutes(5));

9. How would you handle error handling in a .NET Core Web API?

Answer:
For effective error handling:

  • Use Middleware: Create global error-handling middleware to catch unhandled exceptions and return appropriate HTTP status codes.

  • Custom Error Response: Return a well-structured error message in the response body to help users and developers understand the error.

  • Logging: Log the details of exceptions to track issues and debug them later.

Example:

app.UseExceptionHandler("/error");
app.UseStatusCodePagesWithReExecute("/error/{0}");

10. How would you secure sensitive data like passwords in .NET Core?

Answer:
To secure sensitive data:

  • Hash Passwords: Use bcrypt or PBKDF2 to hash passwords before storing them.

  • Use Encryption: Encrypt sensitive data using algorithms like AES.

  • Environment Variables: Store sensitive keys and passwords in environment variables or a secure configuration store instead of hardcoding them.

Example using ASP.NET Core Identity to hash passwords:

var passwordHasher = new PasswordHasher<ApplicationUser>();
var hashedPassword = passwordHasher.HashPassword(user, plainTextPassword);

11. How would you implement role-based access control (RBAC) in a .NET application?

Answer:
To implement RBAC:

  • Define Roles: Create roles in the database (e.g., Admin, User, Manager).

  • Assign Permissions: Assign permissions to roles, such as read, write, delete.

  • Authorize Requests: Use [Authorize] attribute in controllers and actions to restrict access based on roles.

Example:

[Authorize(Roles = "Admin")]
public IActionResult AdminDashboard()
{
return View();
}

12. How would you implement file uploads in a .NET Core application?

Answer:
To handle file uploads:

  • Front-End: Use an <input type="file"> field to allow users to select files for uploading.

  • Back-End: Use the IFormFile interface in ASP.NET Core to handle the uploaded files.

Example:

[HttpPost]
public async Task<IActionResult> UploadFile(IFormFile file)
{
if (file != null && file.Length > 0)
{
var path = Path.Combine(Directory.GetCurrentDirectory(), "uploads", file.FileName);
using (var stream = new FileStream(path, FileMode.Create))
{
await file.CopyToAsync(stream);
}
return Ok("File uploaded successfully.");
}
return BadRequest("No file uploaded.");
}

13. How do you handle versioning of a database in a .NET application?

Answer:
For database versioning:

  • Entity Framework Migrations: Use EF Core migrations to manage schema changes. When the schema changes, create a migration using dotnet ef migrations add <MigrationName>.

  • Manual Versioning: Maintain a version table in your database, and track schema versions in code.

14. How do you handle background tasks or long-running processes in .NET?

Answer:
To handle background tasks:

  • Hangfire: Use Hangfire to run background tasks in a recurring or delayed manner without blocking the main thread.

  • Azure Functions: For cloud applications, use Azure Functions to run background tasks asynchronously in the cloud.

15. How would you secure a .NET Web API against common security vulnerabilities?

Answer:
To secure your API:

  • Input Validation: Always validate incoming data to prevent SQL Injection and Cross-Site Scripting (XSS).

  • Authentication & Authorization: Use OAuth 2.0, JWT or ASP.NET Core Identity for securing access to your API.

  • CORS: Use Cross-Origin Resource Sharing (CORS) to limit which domains can access your API.

  • Rate Limiting: Protect against Denial of Service (DoS) attacks by limiting the rate of API calls.

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