Interview Question and Answers¶

Angular¶

Can you explain how the Angular change detection mechanism works and how you can optimize it?¶

Answer: Angular uses a unidirectional data flow and a mechanism called change detection to update the view whenever the model changes. By default, Angular runs change detection for all components on every browser event. To optimize change detection, we can use techniques such as OnPush change detection, manual change detection, or NgZone to limit the scope of change detection.

Code sample: Here's an example of using OnPush change detection to optimize the change detection mechanism:

import { ChangeDetectionStrategy, Component } from '@angular/core';

@Component({
  selector: 'app-my-component',
  template: '<p>{{ message }}</p>',
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class MyComponent {
  message = 'Hello World';
}

What is Lazy Loading in Angular and how do you implement it?¶

Answer: Lazy loading is a technique in Angular that allows us to load parts of our application lazily, on demand. This can improve the application's startup performance by reducing the initial bundle size and only loading the components that are needed. We can implement lazy loading in Angular by using the loadChildren property in the routing configuration.

Code sample: Here's an example of implementing lazy loading in Angular:

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';

const routes: Routes = [
  {
    path: 'customers',
    loadChildren: () => import('./customers/customers.module').then(m => m.CustomersModule)
  },
  {
    path: 'orders',
    loadChildren: () => import('./orders/orders.module').then(m => m.OrdersModule)
  }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

How do you handle errors in Angular?¶

Answer: To handle errors in Angular, we can use the try-catch statement to wrap the code that might throw an error and catch any errors that are thrown. We can also use the error operator in an RxJS catchError function to handle HTTP errors.

Code sample: Here's an example of handling HTTP errors in Angular using the catchError function:

import { HttpClient } from '@angular/common/http';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

@Component({
  selector: 'app-error-handling',
  template: `
    <p>{{ errorMessage }}</p>
    <button (click)="makeRequest()">Make Request</button>
  `
})
export class ErrorHandlingComponent {
  errorMessage = '';

  constructor(private http: HttpClient) {}

  makeRequest(): void {
    this.http.get('https://api.example.com/data')
      .pipe(
        catchError(error => {
          this.errorMessage = error.message;
          return throwError(error);
        })
      )
      .subscribe();
  }
}

Can you explain how the Angular dependency injection system works and how to use it?¶

Answer: The Angular dependency injection system allows us to inject services into components and other parts of the application. This makes it easy to reuse services and promotes a loosely-coupled architecture. To use the dependency injection system, we can declare services as providers in the component or module, and then use the constructor of the component to inject the service.

Code sample: Here's an example of injecting a service into a component in Angular:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class DataService {
  constructor(private http: HttpClient) {}

  getData(): Observable<any> {
    return this.http.get('https://api.example.com/data');
  }
}

@Component({
  selector: 'app-data',
  template: `
    <p>{{ data }}</p>
    <button (click)="getData()">Get Data</button>
  `
})
export class DataComponent {
  data = '';

  constructor(private dataService: DataService) {}

  getData(): void {
    this.dataService.getData().subscribe(data => {
      this.data = data;
    });
  }
}

Can you explain how to use the Angular Animations API?¶

Answer: The Angular Animations API allows us to create reusable animations in Angular. To use the Animations API, we first need to import the BrowserAnimationsModule in our application module. Then, we can use the @Component decorator to specify the animations for a component. The animations can be triggered using the trigger method, and the animation sequence is defined using the state, transition, and style methods.

Code sample: Here's an example of using the Angular Animations API to animate a component:

import { BrowserModule } from '@angular/platform-browser';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
import { Component, animate, state, style, transition, trigger } from '@angular/core';

@Component({
  selector: 'app-animations',
  animations: [
    trigger('openClose', [
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.5,
        backgroundColor: 'green'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  template: `
    <div [@openClose]="openCloseState" (click)="toggle()">
      {{ openCloseState }}
    </div>
  `
})
export class AnimationsComponent {
  openCloseState = 'open';

  toggle(): void {
    this.openCloseState = this.openCloseState === 'open' ? 'closed' : 'open';
  }
}

@NgModule({
  declarations: [
    AnimationsComponent
  ],
  imports: [
    BrowserModule,
    BrowserAnimationsModule
  ],
  providers: [],
  bootstrap: [AnimationsComponent]
})
export class AppModule { }

Can you explain how to use the Angular Router and navigate between components?¶

Answer: The Angular Router allows us to navigate between components in our application. To use the Router, we first need to import the RouterModule and Routes in our application module. Then, we can define a set of routes using the Routes interface and configure them using the RouterModule.forRoot method. Finally, we can use the routerLink directive to navigate to different routes and the router-outlet component to render the component for the current route.

Code sample: Here's an example of using the Angular Router to navigate between components:

import { RouterModule, Routes } from '@angular/router';
import { NgModule } from '@angular/core';
import { Component } from '@angular/core';

@Component({
  selector: 'app-home',
  template: `
    <h1>Home Component</h1>
  `
})
export class HomeComponent { }

@Component({
  selector: 'app-about',
  template: `
    <h1>About Component</h1>
  `
})
export class AboutComponent { }

const routes: Routes = [
  { path: 'home', component: HomeComponent },
  { path: 'about', component: AboutComponent }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

@Component({
  selector: 'app-root',
  template: `
    <nav>
      <a routerLink="/home">Home</a>
      <a routerLink="/about">About</a>
    </nav>
    <router-outlet></router-outlet>
  `
})
export class AppComponent { }

Can you explain how to use the Angular HTTP Client to make HTTP requests?¶

Answer: The Angular HTTP Client allows us to make HTTP requests to a server. To use the HTTP Client, we first need to import the HttpClientModule in our application module. Then, we can use the HttpClient service to make HTTP requests. The HTTP Client supports various methods such as get, post, put, delete, etc. to make different types of HTTP requests.

Code sample: Here's an example of using the Angular HTTP Client to make a GET request:

import { HttpClientModule } from '@angular/common/http';
import { HttpClient } from '@angular/common/http';
import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-http',
  template: `
    <h1>{{ data }}</h1>
  `
})
export class HttpComponent implements OnInit {
  data: any;

  constructor(private http: HttpClient) { }

  ngOnInit() {
    this.http.get('https://jsonplaceholder.typicode.com/posts/1')
      .subscribe(data => {
        this.data = data;
      });
  }
}

@NgModule({
  imports: [
    HttpClientModule
  ],
  declarations: [
    HttpComponent
  ],
  providers: [],
  bootstrap: [HttpComponent]
})
export class AppModule { }

Can you explain how to use the Angular Forms to create a form and handle form submissions?¶

Answer: The Angular Forms allow us to create forms and handle form submissions in our application. To use the Forms, we first need to import the FormsModule in our application module. Then, we can use the FormGroup and FormControl classes to define the form structure and form controls. We can use the ngModel directive to bind form controls to our template, and the ngSubmit directive to handle form submissions.

Code sample: Here's an example of using the Angular Forms to create a form:

import { FormsModule, FormControl, FormGroup } from '@angular/forms';
import { Component } from '@angular/core';

@Component({
  selector: 'app-forms',
  template: `
    <form [formGroup]="form" (ngSubmit)="onSubmit()">
      <input formControlName="name" type="text">
      <button type="submit">Submit</button>
    </form>
  `
})
export class FormsComponent {
  form = new FormGroup({
    name: new FormControl('')
  });

  onSubmit() {
    console.log(this.form.value);
  }
}

@NgModule({
  imports: [
    FormsModule
  ],
  declarations: [
    FormsComponent
  ],
  providers: [],
  bootstrap: [FormsComponent]
})
export class AppModule { }

Can you explain how to use the Angular Pipes to format data in the template?¶

Answer: The Angular Pipes allow us to format data in the template. To use the Pipes, we can use the pipe operator (|) in our template to apply a pipe to an expression. Angular provides many built-in pipes such as DatePipe, UpperCasePipe, LowerCasePipe, etc. We can also create our custom pipes.

Code sample: Here's an example of using the Angular Pipes to format a date in the template:

import { DatePipe } from '@angular/common';
import { Component } from '@angular/core';

@Component({
  selector: 'app-pipes',
  template: `
    <h1>{{ date | date: 'short' }}</h1>
  `
})
export class PipesComponent {
  date = new Date();
}

@NgModule({
  imports: [],
  declarations: [
    PipesComponent
  ],
  providers: [
    DatePipe
  ],
  bootstrap: [PipesComponent]
})
export class AppModule { }

Can you explain how to use the @Input decorator to pass data from a parent component to a child component?¶

Answer: The @Input decorator is used to pass data from a parent component to a child component. To use the @Input decorator, we need to define a property with the @Input decorator in the child component and use the property binding [] in the template of the parent component.

Code sample: Here's an example of using the @Input decorator to pass data from a parent component to a child component:

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { map } from 'rxjs/operators';

@Component({
  selector: 'app-http',
  template: `
    <ul *ngFor="let post of posts">
      <li>{{ post.title }}</li>
    </ul>
  `
})
export class HttpComponent {
  posts: any;

  constructor(private http: HttpClient) {}

  ngOnInit() {
    this.http.get('https://jsonplaceholder.typicode.com/posts')
      .pipe(
        map((response: any) => response.map(post => post.title))
      )
      .subscribe(posts => this.posts = posts);
  }
}

@NgModule({
  imports: [
    HttpClientModule
  ],
  declarations: [
    HttpComponent
  ],
  providers: [
    HttpClient
  ],
  bootstrap: [HttpComponent]
})
export class AppModule { }

Can you explain how to make an HTTP request with the Angular HttpClient?¶

Answer: The Angular HttpClient allows us to make HTTP requests. To use the HttpClient, we need to import it in our component and inject it in the constructor. We can use the http.get() or http.post() method to make an HTTP GET or POST request respectively.

Can you explain how to use the ngOnChanges lifecycle hook in Angular?¶

Answer: The ngOnChanges lifecycle hook is called when an input of a component changes. This hook allows us to react to changes in the input properties of a component. To use the ngOnChanges lifecycle hook, we need to implement the ngOnChanges method in our component class.

Code sample: Here's an example of using the ngOnChanges lifecycle hook to log the changes in the input properties of a component:

import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-on-changes',
  template: `
    <p>{{ message }}</p>
  `
})
export class OnChangesComponent implements OnChanges {
  @Input() message: string;

  ngOnChanges(changes: SimpleChanges) {
    console.log(changes);
  }
}

@NgModule({
  imports: [],
  declarations: [
    OnChangesComponent
  ],
  providers: [],
  bootstrap: [OnChangesComponent]
})
export class AppModule { }

Can you explain how to use the ngDoCheck lifecycle hook in Angular?¶

Answer: The ngDoCheck lifecycle hook is called whenever Angular performs a check for changes. This hook allows us to implement our own custom change detection logic. To use the ngDoCheck lifecycle hook, we need to implement the ngDoCheck method in our component class.

Code sample: Here's an example of using the ngDoCheck lifecycle hook to log the changes in the input properties of a component:

import { Component, Input, DoCheck } from '@angular/core';

@Component({
  selector: 'app-do-check',
  template: `
    <p>{{ message }}</p>
  `
})
export class DoCheckComponent implements DoCheck {
  @Input() message: string;
  previousMessage: string;

  ngDoCheck() {
    if (this.message !== this.previousMessage) {
      console.log(this.message);
      this.previousMessage = this.message;
    }
  }
}

@NgModule({
  imports: [],
  declarations: [
    DoCheckComponent
  ],
  providers: [],
  bootstrap: [DoCheckComponent]
})
export class AppModule { }

Can you explain how to use the ngOnDestroy lifecycle hook in Angular?¶

Answer: The ngOnDestroy lifecycle hook is called when a component is being destroyed. This hook allows us to clean up any resources or subscriptions before the component is destroyed. To use the ngOnDestroy lifecycle hook, we need to implement the ngOnDestroy method in our component class.

Code sample: Here's an example of using the ngOnDestroy lifecycle hook to log a message when a component is being destroyed:

import { Component, OnDestroy } from '@angular/core';

@Component({
  selector: 'app-on-destroy',
  template: `
    <p>{{ message }}</p>
  `
})
export class OnDestroyComponent implements OnDestroy {
  message: string = 'Hello, World!';

  ngOnDestroy() {
    console.log('Component destroyed!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    OnDestroyComponent
  ],
  providers: [],
  bootstrap: [OnDestroyComponent]
})
export class AppModule { }

Can you explain how to use the ngAfterViewInit lifecycle hook in Angular?¶

Answer: The ngAfterViewInit lifecycle hook is called after the component's view has been fully initialized. This hook allows us to interact with the component's view and child views. To use the ngAfterViewInit lifecycle hook, we need to implement the ngAfterViewInit method in our component class.

Code sample: Here's an example of using the ngAfterViewInit lifecycle hook to log a message after the component's view has been fully initialized:

import { Component, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-after-view-init',
  template: `
    <p>{{ message }}</p>
  `
})
export class AfterViewInitComponent implements AfterViewInit {
  message: string = 'Hello, World!';

  ngAfterViewInit() {
    console.log('View initialized!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    AfterViewInitComponent
  ],
  providers: [],
  bootstrap: [AfterViewInitComponent]
})
export class AppModule { }

Can you explain how to use the ngAfterViewChecked lifecycle hook in Angular?¶

Answer: The ngAfterViewChecked lifecycle hook is called after Angular has finished checking the component's view and child views for changes. This hook allows us to perform any additional actions after the view has been checked for changes. To use the ngAfterViewChecked lifecycle hook, we need to implement the ngAfterViewChecked method in our component class.

Code sample: Here's an example of using the ngAfterViewChecked lifecycle hook to log a message after Angular has finished checking the component's view for changes:

import { Component, AfterViewChecked } from '@angular/core';

@Component({
  selector: 'app-after-view-checked',
  template: `
    <p>{{ message }}</p>
  `
})
export class AfterViewCheckedComponent implements AfterViewChecked {
  message: string = 'Hello, World!';

  ngAfterViewChecked() {
    console.log('View checked for changes!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    AfterViewCheckedComponent
  ],
  providers: [],
  bootstrap: [AfterViewCheckedComponent]
})
export class AppModule { }

Can you explain how to use the `ngOnChanges lifecycle hook in Angular?¶

Answer: The ngOnChanges lifecycle hook is called whenever a data-bound input property changes. This hook allows us to detect and respond to changes in the component's input properties. To use the ngOnChanges lifecycle hook, we need to implement the ngOnChanges method in our component class.

Code sample: Here's an example of using the ngOnChanges lifecycle hook to log a message whenever the component's input property changes:

import { Component, Input, OnChanges } from '@angular/core';

@Component({
  selector: 'app-on-changes',
  template: `
    <p>{{ message }}</p>
  `
})
export class OnChangesComponent implements OnChanges {
  @Input() message: string = 'Hello, World!';

  ngOnChanges() {
    console.log('Input property changed!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    OnChangesComponent
  ],
  providers: [],
  bootstrap: [OnChangesComponent]
})
export class AppModule { }

Can you explain how to use the ngAfterContentInit lifecycle hook in Angular?¶

Answer: The ngAfterContentInit lifecycle hook is called after Angular has finished initializing the component's content projected from a host element. This hook allows us to interact with the component's projected content. To use the ngAfterContentInit lifecycle hook, we need to implement the ngAfterContentInit method in our component class.

Code sample: Here's an example of using the ngAfterContentInit lifecycle hook to log a message after Angular has finished initializing the component's projected content:

import { Component, AfterContentInit } from '@angular/core';

@Component({
  selector: 'app-after-content-init',
  template: `
    <ng-content></ng-content>
  `
})
export class AfterContentInitComponent implements AfterContentInit {
  ngAfterContentInit() {
    console.log('Content initialized!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    AfterContentInitComponent
  ],
  providers: [],
  bootstrap: [AfterContentInitComponent]
})
export class AppModule { }

Can you explain how to use the ngAfterContentChecked lifecycle hook in Angular?¶

Answer: The ngAfterContentChecked lifecycle hook is called after Angular has finished checking the component's content projected from a host element. This hook allows us to interact with the component's projected content after Angular has finished checking it. To use the ngAfterContentChecked lifecycle hook, we need to implement the ngAfterContentChecked method in our component class.

Code sample: Here's an example of using the ngAfterContentChecked lifecycle hook to log a message after Angular has finished checking the component's projected content:

import { Component, AfterContentChecked } from '@angular/core';

@Component({
  selector: 'app-after-content-checked',
  template: `
    <ng-content></ng-content>
  `
})
export class AfterContentCheckedComponent implements AfterContentChecked {
  ngAfterContentChecked() {
    console.log('Content checked!');
  }
}

@NgModule({
  imports: [],
  declarations: [
    AfterContentCheckedComponent
  ],
  providers: [],
  bootstrap: [AfterContentCheckedComponent]
})
export class AppModule { }

How do you implement dynamic forms in Angular?¶

Answer: To implement dynamic forms in Angular, we can use the FormBuilder and FormArray classes from the angular/forms module. The FormBuilder class provides a convenient way to build and manage complex forms, while the FormArray class allows us to create dynamic form arrays that can be added to and removed from at runtime.

Code sample: Here's an example of implementing a dynamic form in Angular:

import { FormBuilder, FormGroup, FormArray } from '@angular/forms';

@Component({
  selector: 'app-dynamic-form',
  template: `
    <form [formGroup]="form">
      <div formArrayName="items">
        <div *ngFor="let item of form.get('items').controls; let i = index">
          <input formControlName="{{ i }}">
        </div>
      </div>
      <button (click)="addItem()">Add Item</button>
    </form>
  `
})
export class DynamicFormComponent {
  form: FormGroup;

  constructor(private fb: FormBuilder){
    this.form = this.fb.group({
      items: this.fb.array([])
    });
  }

  get items(): FormArray {
    return this.form.get('items') as FormArray;
  }

  addItem(): void {
    this.items.push(this.fb.control(''));
  }
}

C# Dotnet API¶

How can you implement JWT token-based authentication in C# API services?¶

using System;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using Microsoft.IdentityModel.Tokens;

// Generate a token
private static string GenerateToken(string userId)
{
    var claims = new[]
    {
        new Claim(JwtRegisteredClaimNames.Sub, userId),
        new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
    };

    var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("secret_key_that_should_be_long_and_secure"));
    var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);

    var token = new JwtSecurityToken(
        issuer: "https://example.com",
        audience: "https://example.com",
        claims: claims,
        expires: DateTime.Now.AddMinutes(30),
        signingCredentials: creds);

    return new JwtSecurityTokenHandler().WriteToken(token);
}

// Validate a token
private static ClaimsPrincipal ValidateToken(string token)
{
    var handler = new JwtSecurityTokenHandler();
    var validationParameters = new TokenValidationParameters
    {
        IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("secret_key_that_should_be_long_and_secure")),
        ValidIssuer = "https://example.com",
        ValidAudience = "https://example.com",
        ValidateIssuerSigningKey = true,
        ValidateLifetime = true,
    };

    return handler.ValidateToken(token, validationParameters, out SecurityToken securityToken);
}

How can you implement versioning in C# API services?¶

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/v{version:apiVersion}/[controller]")]
public class MyController : ControllerBase
{
    [HttpGet]
    public IActionResult GetData(ApiVersion version)
    {
        if (version.MajorVersion == 1)
        {
            return Ok("Data from version 1");
        }
        else
        {
            return Ok("Data from version 2");
        }
    }
}

How can you implement background tasks in C# API services?¶

using Microsoft.Extensions.Hosting;
using System.Threading;
using System.Threading.Tasks;

public class BackgroundTask : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            // Do the background task here
            await Task.Delay(10000, stoppingToken);
        }
    }
}

// Add to Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddHostedService<BackgroundTask>();
}

How can you implement custom exception handling in C# API services?¶

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;

public class CustomExceptionFilterAttribute : ExceptionFilterAttribute
{
    public override void OnException(ExceptionContext context)
    {
        var result = new ObjectResult("An error occurred. Please try again later.")
        {
            StatusCode = 500
        };

        context.Result = result;
    }
}

// Add to Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
    {
        options.Filters.Add<CustomExceptionFilterAttribute>();
    });
}

How can you implement request logging in C# API services?¶

using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using System.IO;
using System.Text;
using System.Threading.Tasks;

public class RequestLoggingMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger _logger;

    public RequestLoggingMiddleware(RequestDelegate next, ILogger<RequestLoggingMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var request = context.Request;

        var requestBody = await ReadRequestBodyAsync(request);

        _logger.LogInformation($"Method: {request.Method} | URL: {request.Path} | Request Body: {requestBody}");

        await _next(context);
    }

    private async Task<string> ReadRequestBodyAsync(HttpRequest request)
    {
        request.EnableBuffering();

        using (var reader = new StreamReader(request.Body, Encoding.UTF8, true, 1024, true))
        {
            return await reader.ReadToEndAsync();
        }
    }
}

// Add to Startup.cs
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseMiddleware<RequestLoggingMiddleware>();
}

How can we create a custom authentication process in C# API?¶

public class CustomAuthenticationHandler : AuthenticationHandler<AuthenticationSchemeOptions>
{
    private readonly IUserService _userService;

    public CustomAuthenticationHandler(
        IOptionsMonitor<AuthenticationSchemeOptions> options,
        ILoggerFactory logger,
        UrlEncoder encoder,
        ISystemClock clock,
        IUserService userService)
        : base(options, logger, encoder, clock)
    {
        _userService = userService;
    }

    protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        if (!Request.Headers.ContainsKey("Authorization"))
        {
            return AuthenticateResult.Fail("Missing Authorization Header");
        }

        try
        {
            var authHeader = AuthenticationHeaderValue.Parse(Request.Headers["Authorization"]);
            var credentials = Encoding.UTF8.GetString(Convert.FromBase64String(authHeader.Parameter)).Split(':');
            var username = credentials[0];
            var password = credentials[1];

            var user = await _userService.Authenticate(username, password);
            if (user == null)
            {
                return AuthenticateResult.Fail("Invalid Username or Password");
            }

            var claims = new[] {
                new Claim(ClaimTypes.NameIdentifier, user.Id.ToString()),
                new Claim(ClaimTypes.Name, user.Username),
            };
            var identity = new ClaimsIdentity(claims, Scheme.Name);
            var principal = new ClaimsPrincipal(identity);
            var ticket = new AuthenticationTicket(principal, Scheme.Name);

            return AuthenticateResult.Success(ticket);
        }
        catch
        {
            return AuthenticateResult.Fail("Invalid Authorization Header");
        }
    }
}

Answer: In the example above, we created a custom authentication handler CustomAuthenticationHandler that implements the AuthenticationHandler class from the Microsoft.AspNetCore.Authentication namespace. The handler overrides the HandleAuthenticateAsync method and implements custom logic for parsing the Authorization header, validating the credentials, and returning an AuthenticateResult instance. To use this custom authentication, we need to register it in the Startup.ConfigureServices method:

services.AddAuthentication("Custom")
    .AddScheme<AuthenticationSchemeOptions, CustomAuthenticationHandler>("Custom", options => { });

How can you use the DelegatingHandler class in C# to create custom message handlers for an API service?¶

Answer: You can create custom message handlers by creating a class that derives from the DelegatingHandler class and overriding its SendAsync method. In this method, you can add any custom logic, such as authentication or logging, that you want to be performed for each incoming request.

public class AuthenticationHandler : DelegatingHandler
{
    protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var token = request.Headers.Authorization?.Parameter;

        if (!string.IsNullOrEmpty(token))
        {
            try
            {
                // Validate the token here
                var claimsPrincipal = JwtHelper.ValidateToken(token);
                request.GetRequestContext().Principal = claimsPrincipal;
            }
            catch (Exception ex)
            {
                return request.CreateErrorResponse(HttpStatusCode.Unauthorized, ex.Message);
            }
        }

        return await base.SendAsync(request, cancellationToken);
    }
}

How can you implement rate limiting for an API service in C#?¶

Answer: You can implement rate limiting by creating a custom message handler that uses a ConcurrentDictionary to keep track of the number of requests made by each client IP address. The handler uses a SemaphoreSlim to limit the number of concurrent requests allowed for each IP address to a specified number. If the limit is exceeded, the handler returns a TooManyRequests response.

public class RateLimitHandler : DelegatingHandler
{
    private static readonly ConcurrentDictionary<string, SemaphoreSlim> _limits =
        new ConcurrentDictionary<string, SemaphoreSlim>();

    private readonly int _maxRequestsPerInterval;
    private readonly TimeSpan _interval;

    public RateLimitHandler(int maxRequestsPerInterval, TimeSpan interval)
    {
        _maxRequestsPerInterval = maxRequestsPerInterval;
        _interval = interval;
    }

    protected async override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var key = request.GetClientIpAddress();
        var limit = _limits.GetOrAdd(key, k => new SemaphoreSlim(_maxRequestsPerInterval, _maxRequestsPerInterval));

        if (!await limit.WaitAsync(0))
        {
            return request.CreateErrorResponse(HttpStatusCode.TooManyRequests, "Rate limit exceeded");
        }

        var response = await base.SendAsync(request, cancellationToken);

        if (response.StatusCode != HttpStatusCode.OK)
        {
            limit.Release();
        }

        return response;
    }
}

What is the difference between async and sync programming in C#?¶

Async programming is a way of writing non-blocking code in C#, while sync programming is a blocking code. Async programming allows the execution of multiple tasks at the same time, making it more efficient, while sync programming requires tasks to be completed one after another.

public async Task<int> GetDataAsync()
{
    HttpClient client = new HttpClient();
    var result = await client.GetAsync("https://www.example.com/data");
    return result.StatusCode;
}

public int GetDataSync()
{
    HttpClient client = new HttpClient();
    var result = client.GetAsync("https://www.example.com/data").Result;
    return result.StatusCode;
}

What is the use of the await keyword in C# async programming?¶

The await keyword is used to indicate that the method or task it is applied to should be executed asynchronously. It is used to wait for an asynchronous operation to complete without blocking the main thread.

public async Task<int> GetDataAsync()
{
    HttpClient client = new HttpClient();
    var result = await client.GetAsync("https://www.example.com/data");
    return result.StatusCode;
}

What is the difference between Task and Task in C# async programming?¶

The Task class represents an asynchronous operation that does not return a value, while the Task class represents an asynchronous operation that returns a value of type TResult.

public async Task<int> GetDataAsync()
{
    HttpClient client = new HttpClient();
    var result = await client.GetAsync("https://www.example.com/data");
    return result.StatusCode;
}

public async Task ProcessDataAsync()
{
    int statusCode = await GetDataAsync();
    Console.WriteLine("Status code: " + statusCode);
}

What is the use of the ConfigureAwait method in C# async programming?¶

The ConfigureAwait method is used to specify whether the continuation after an await should be executed on the original context or not. By default, the continuation is executed on the original context.

public async Task<int> GetDataAsync()
{
    HttpClient client = new HttpClient();
    var result = await client.GetAsync("https://www.example.com/data").ConfigureAwait(false);
    return result.StatusCode;
}

What is the use of the ValueTask class in C# async programming?¶

The ValueTask class represents a task that can be returned as a value. It is used to optimize asynchronous operations that return a small amount of data, as it reduces the overhead of creating a new task for every asynchronous operation.

public async ValueTask<int> GetDataAsync()
{
    HttpClient client = new HttpClient();
    var result = await client.GetAsync("https://www.example.com/data");
    return result.StatusCode;
}