dotnet-csharp

name	dotnet-csharp
description	Build .NET 8+ applications with C#. Covers ASP.NET Core, Minimal APIs, Entity Framework Core, dependency injection, and testing. Use for .NET development, C# programming, and Azure backend services.

.NET & C# Development Skill

Expert guidance for building modern .NET 8+ applications with C#, ASP.NET Core, Entity Framework Core, and best practices.

Triggers

Use this skill when:

• Building .NET 8+ applications with C#
• Creating ASP.NET Core Web APIs or MVC applications
• Working with Minimal APIs for lightweight endpoints
• Implementing Entity Framework Core for data access
• Setting up dependency injection and service registration
• Configuring authentication and authorization (JWT, policies)
• Writing background services and hosted services
• Testing with xUnit, NUnit, Moq, and FluentAssertions
• Keywords: dotnet, csharp, asp.net core, minimal api, entity framework, ef core, dependency injection, jwt authentication, xunit, nuget

---

Quick Reference

.NET CLI Commands

Command	Description
dotnet new webapi	Create new Web API project
dotnet new mvc	Create new MVC project
dotnet new classlib	Create class library
dotnet new xunit	Create xUnit test project
dotnet build	Build the project
dotnet run	Run the application
dotnet test	Run tests
dotnet publish -c Release	Publish for deployment
dotnet ef migrations add <Name>	Create EF migration
dotnet ef database update	Apply migrations

Project Templates

Template	Use Case
webapi	REST API with controllers or minimal APIs
web	Empty ASP.NET Core project
mvc	MVC web application
razor	Razor Pages application
blazorserver	Blazor Server app
blazorwasm	Blazor WebAssembly app
worker	Background service
grpc	gRPC service

C# 12 Features (.NET 8+)

Feature	Example
Primary Constructors	class Person(string name, int age)
Collection Expressions	int[] nums = [1, 2, 3];
Alias Any Type	using Point = (int X, int Y);
Default Lambda Parameters	var add = (int a, int b = 1) => a + b;
Inline Arrays	[InlineArray(10)] struct Buffer { int _element; }

---

Project Setup

Solution Structure

MySolution/
├── src/
│   ├── MyApp.Api/              # Web API project
│   │   ├── Controllers/
│   │   ├── Endpoints/          # Minimal API endpoints
│   │   ├── Middleware/
│   │   ├── Program.cs
│   │   └── appsettings.json
│   ├── MyApp.Core/             # Domain/business logic
│   │   ├── Entities/
│   │   ├── Interfaces/
│   │   └── Services/
│   ├── MyApp.Infrastructure/   # Data access, external services
│   │   ├── Data/
│   │   ├── Repositories/
│   │   └── Services/
│   └── MyApp.Shared/           # Shared DTOs, utilities
│       ├── DTOs/
│       └── Extensions/
├── tests/
│   ├── MyApp.UnitTests/
│   ├── MyApp.IntegrationTests/
│   └── MyApp.FunctionalTests/
├── Directory.Build.props       # Shared build properties
├── Directory.Packages.props    # Central package management
└── MySolution.sln

Directory.Build.props

<Project>
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <AnalysisLevel>latest-recommended</AnalysisLevel>
  </PropertyGroup>
</Project>

Central Package Management (Directory.Packages.props)

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup>
    <PackageVersion Include="Microsoft.EntityFrameworkCore" Version="8.0.0" />
    <PackageVersion Include="Microsoft.EntityFrameworkCore.SqlServer" Version="8.0.0" />
    <PackageVersion Include="Swashbuckle.AspNetCore" Version="6.5.0" />
    <PackageVersion Include="FluentValidation" Version="11.9.0" />
    <PackageVersion Include="Serilog.AspNetCore" Version="8.0.0" />
    <PackageVersion Include="xunit" Version="2.6.4" />
    <PackageVersion Include="Moq" Version="4.20.70" />
  </ItemGroup>
</Project>

---

ASP.NET Core

Program.cs (Modern Setup)

using MyApp.Api.Middleware;
using MyApp.Core.Interfaces;
using MyApp.Infrastructure.Data;
using MyApp.Infrastructure.Services;

var builder = WebApplication.CreateBuilder(args);

// Configuration
builder.Configuration
    .AddJsonFile("appsettings.json", optional: false)
    .AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true)
    .AddEnvironmentVariables()
    .AddUserSecrets<Program>(optional: true);

// Services
builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

// Database
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));

// Application services
builder.Services.AddScoped<IUserService, UserService>();
builder.Services.AddScoped<IEmailService, EmailService>();

// Health checks
builder.Services.AddHealthChecks()
    .AddDbContextCheck<AppDbContext>();

var app = builder.Build();

// Middleware pipeline
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();
app.UseAuthentication();
app.UseAuthorization();

app.UseMiddleware<ExceptionHandlingMiddleware>();

app.MapControllers();
app.MapHealthChecks("/health");

app.Run();

// Make Program class accessible for integration tests
public partial class Program { }

Controller Pattern

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Api.Controllers;

[ApiController]
[Route("api/[controller]")]
[Produces("application/json")]
public class UsersController : ControllerBase
{
    private readonly IUserService _userService;
    private readonly ILogger<UsersController> _logger;

    public UsersController(IUserService userService, ILogger<UsersController> logger)
    {
        _userService = userService;
        _logger = logger;
    }

    /// <summary>
    /// Gets all users with optional filtering
    /// </summary>
    [HttpGet]
    [ProducesResponseType(typeof(IEnumerable<UserDto>), StatusCodes.Status200OK)]
    public async Task<ActionResult<IEnumerable<UserDto>>> GetUsers(
        [FromQuery] string? search = null,
        [FromQuery] int page = 1,
        [FromQuery] int pageSize = 10,
        CancellationToken cancellationToken = default)
    {
        var users = await _userService.GetUsersAsync(search, page, pageSize, cancellationToken);
        return Ok(users);
    }

    /// <summary>
    /// Gets a specific user by ID
    /// </summary>
    [HttpGet("{id:guid}")]
    [ProducesResponseType(typeof(UserDto), StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<ActionResult<UserDto>> GetUser(
        Guid id,
        CancellationToken cancellationToken = default)
    {
        var user = await _userService.GetByIdAsync(id, cancellationToken);

        if (user is null)
        {
            return NotFound();
        }

        return Ok(user);
    }

    /// <summary>
    /// Creates a new user
    /// </summary>
    [HttpPost]
    [ProducesResponseType(typeof(UserDto), StatusCodes.Status201Created)]
    [ProducesResponseType(typeof(ValidationProblemDetails), StatusCodes.Status400BadRequest)]
    public async Task<ActionResult<UserDto>> CreateUser(
        [FromBody] CreateUserRequest request,
        CancellationToken cancellationToken = default)
    {
        var user = await _userService.CreateAsync(request, cancellationToken);

        _logger.LogInformation("Created user {UserId}", user.Id);

        return CreatedAtAction(
            nameof(GetUser),
            new { id = user.Id },
            user);
    }

    /// <summary>
    /// Updates an existing user
    /// </summary>
    [HttpPut("{id:guid}")]
    [ProducesResponseType(typeof(UserDto), StatusCodes.Status200OK)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<ActionResult<UserDto>> UpdateUser(
        Guid id,
        [FromBody] UpdateUserRequest request,
        CancellationToken cancellationToken = default)
    {
        var user = await _userService.UpdateAsync(id, request, cancellationToken);

        if (user is null)
        {
            return NotFound();
        }

        return Ok(user);
    }

    /// <summary>
    /// Deletes a user
    /// </summary>
    [HttpDelete("{id:guid}")]
    [ProducesResponseType(StatusCodes.Status204NoContent)]
    [ProducesResponseType(StatusCodes.Status404NotFound)]
    public async Task<IActionResult> DeleteUser(
        Guid id,
        CancellationToken cancellationToken = default)
    {
        var deleted = await _userService.DeleteAsync(id, cancellationToken);

        if (!deleted)
        {
            return NotFound();
        }

        return NoContent();
    }
}

---

Minimal APIs

Basic Setup

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddScoped<IUserService, UserService>();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

// Map endpoints
app.MapUserEndpoints();
app.MapProductEndpoints();

app.Run();

Organized Endpoints

namespace MyApp.Api.Endpoints;

public static class UserEndpoints
{
    public static void MapUserEndpoints(this IEndpointRouteBuilder routes)
    {
        var group = routes.MapGroup("/api/users")
            .WithTags("Users")
            .WithOpenApi();

        group.MapGet("/", GetUsers)
            .WithName("GetUsers")
            .WithSummary("Gets all users");

        group.MapGet("/{id:guid}", GetUserById)
            .WithName("GetUserById")
            .WithSummary("Gets a user by ID");

        group.MapPost("/", CreateUser)
            .WithName("CreateUser")
            .WithSummary("Creates a new user")
            .AddEndpointFilter<ValidationFilter<CreateUserRequest>>();

        group.MapPut("/{id:guid}", UpdateUser)
            .WithName("UpdateUser")
            .RequireAuthorization();

        group.MapDelete("/{id:guid}", DeleteUser)
            .WithName("DeleteUser")
            .RequireAuthorization("AdminOnly");
    }

    private static async Task<IResult> GetUsers(
        IUserService userService,
        [AsParameters] PaginationQuery pagination,
        CancellationToken cancellationToken)
    {
        var users = await userService.GetUsersAsync(
            pagination.Page,
            pagination.PageSize,
            cancellationToken);

        return Results.Ok(users);
    }

    private static async Task<IResult> GetUserById(
        Guid id,
        IUserService userService,
        CancellationToken cancellationToken)
    {
        var user = await userService.GetByIdAsync(id, cancellationToken);

        return user is null
            ? Results.NotFound()
            : Results.Ok(user);
    }

    private static async Task<IResult> CreateUser(
        CreateUserRequest request,
        IUserService userService,
        CancellationToken cancellationToken)
    {
        var user = await userService.CreateAsync(request, cancellationToken);

        return Results.CreatedAtRoute(
            "GetUserById",
            new { id = user.Id },
            user);
    }

    private static async Task<IResult> UpdateUser(
        Guid id,
        UpdateUserRequest request,
        IUserService userService,
        CancellationToken cancellationToken)
    {
        var user = await userService.UpdateAsync(id, request, cancellationToken);

        return user is null
            ? Results.NotFound()
            : Results.Ok(user);
    }

    private static async Task<IResult> DeleteUser(
        Guid id,
        IUserService userService,
        CancellationToken cancellationToken)
    {
        var deleted = await userService.DeleteAsync(id, cancellationToken);

        return deleted
            ? Results.NoContent()
            : Results.NotFound();
    }
}

// Parameter binding class
public record PaginationQuery(
    [FromQuery] int Page = 1,
    [FromQuery] int PageSize = 10);

Typed Results (Better OpenAPI)

public static class UserEndpoints
{
    public static void MapUserEndpoints(this IEndpointRouteBuilder routes)
    {
        routes.MapGet("/api/users/{id:guid}", GetUserById);
    }

    private static async Task<Results<Ok<UserDto>, NotFound>> GetUserById(
        Guid id,
        IUserService userService,
        CancellationToken cancellationToken)
    {
        var user = await userService.GetByIdAsync(id, cancellationToken);

        return user is null
            ? TypedResults.NotFound()
            : TypedResults.Ok(user);
    }
}

---

Entity Framework Core

DbContext

using Microsoft.EntityFrameworkCore;

namespace MyApp.Infrastructure.Data;

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options)
        : base(options)
    {
    }

    public DbSet<User> Users => Set<User>();
    public DbSet<Order> Orders => Set<Order>();
    public DbSet<Product> Products => Set<Product>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Apply all configurations from assembly
        modelBuilder.ApplyConfigurationsFromAssembly(
            typeof(AppDbContext).Assembly);

        // Global query filters
        modelBuilder.Entity<User>()
            .HasQueryFilter(u => !u.IsDeleted);

        base.OnModelCreating(modelBuilder);
    }

    public override async Task<int> SaveChangesAsync(
        CancellationToken cancellationToken = default)
    {
        // Audit trail
        foreach (var entry in ChangeTracker.Entries<IAuditable>())
        {
            switch (entry.State)
            {
                case EntityState.Added:
                    entry.Entity.CreatedAt = DateTime.UtcNow;
                    break;
                case EntityState.Modified:
                    entry.Entity.UpdatedAt = DateTime.UtcNow;
                    break;
            }
        }

        return await base.SaveChangesAsync(cancellationToken);
    }
}

Entity Configuration

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;

namespace MyApp.Infrastructure.Data.Configurations;

public class UserConfiguration : IEntityTypeConfiguration<User>
{
    public void Configure(EntityTypeBuilder<User> builder)
    {
        builder.ToTable("Users");

        builder.HasKey(u => u.Id);

        builder.Property(u => u.Id)
            .ValueGeneratedOnAdd();

        builder.Property(u => u.Email)
            .IsRequired()
            .HasMaxLength(256);

        builder.Property(u => u.FirstName)
            .IsRequired()
            .HasMaxLength(100);

        builder.Property(u => u.LastName)
            .IsRequired()
            .HasMaxLength(100);

        builder.Property(u => u.PasswordHash)
            .IsRequired();

        // Indexes
        builder.HasIndex(u => u.Email)
            .IsUnique();

        // Relationships
        builder.HasMany(u => u.Orders)
            .WithOne(o => o.User)
            .HasForeignKey(o => o.UserId)
            .OnDelete(DeleteBehavior.Cascade);

        // Value objects
        builder.OwnsOne(u => u.Address, address =>
        {
            address.Property(a => a.Street).HasMaxLength(200);
            address.Property(a => a.City).HasMaxLength(100);
            address.Property(a => a.ZipCode).HasMaxLength(20);
            address.Property(a => a.Country).HasMaxLength(100);
        });

        // Seed data
        builder.HasData(
            new User
            {
                Id = Guid.Parse("11111111-1111-1111-1111-111111111111"),
                Email = "[e-mail]",
                FirstName = "Admin",
                LastName = "User"
            });
    }
}

Repository Pattern

namespace MyApp.Core.Interfaces;

public interface IRepository<T> where T : class
{
    Task<T?> GetByIdAsync(Guid id, CancellationToken cancellationToken = default);
    Task<IReadOnlyList<T>> GetAllAsync(CancellationToken cancellationToken = default);
    Task<T> AddAsync(T entity, CancellationToken cancellationToken = default);
    Task UpdateAsync(T entity, CancellationToken cancellationToken = default);
    Task DeleteAsync(T entity, CancellationToken cancellationToken = default);
}

public interface IUserRepository : IRepository<User>
{
    Task<User?> GetByEmailAsync(string email, CancellationToken cancellationToken = default);
    Task<IReadOnlyList<User>> GetActiveUsersAsync(CancellationToken cancellationToken = default);
}

namespace MyApp.Infrastructure.Repositories;

public class Repository<T> : IRepository<T> where T : class
{
    protected readonly AppDbContext _context;
    protected readonly DbSet<T> _dbSet;

    public Repository(AppDbContext context)
    {
        _context = context;
        _dbSet = context.Set<T>();
    }

    public virtual async Task<T?> GetByIdAsync(
        Guid id,
        CancellationToken cancellationToken = default)
    {
        return await _dbSet.FindAsync([id], cancellationToken);
    }

    public virtual async Task<IReadOnlyList<T>> GetAllAsync(
        CancellationToken cancellationToken = default)
    {
        return await _dbSet.ToListAsync(cancellationToken);
    }

    public virtual async Task<T> AddAsync(
        T entity,
        CancellationToken cancellationToken = default)
    {
        await _dbSet.AddAsync(entity, cancellationToken);
        await _context.SaveChangesAsync(cancellationToken);
        return entity;
    }

    public virtual async Task UpdateAsync(
        T entity,
        CancellationToken cancellationToken = default)
    {
        _dbSet.Update(entity);
        await _context.SaveChangesAsync(cancellationToken);
    }

    public virtual async Task DeleteAsync(
        T entity,
        CancellationToken cancellationToken = default)
    {
        _dbSet.Remove(entity);
        await _context.SaveChangesAsync(cancellationToken);
    }
}

public class UserRepository : Repository<User>, IUserRepository
{
    public UserRepository(AppDbContext context) : base(context)
    {
    }

    public async Task<User?> GetByEmailAsync(
        string email,
        CancellationToken cancellationToken = default)
    {
        return await _dbSet
            .FirstOrDefaultAsync(u => u.Email == email, cancellationToken);
    }

    public async Task<IReadOnlyList<User>> GetActiveUsersAsync(
        CancellationToken cancellationToken = default)
    {
        return await _dbSet
            .Where(u => u.IsActive)
            .OrderBy(u => u.LastName)
            .ThenBy(u => u.FirstName)
            .ToListAsync(cancellationToken);
    }
}

---

Dependency Injection

Service Registration

namespace MyApp.Api.Extensions;

public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddApplicationServices(
        this IServiceCollection services)
    {
        // Transient - new instance every time
        services.AddTransient<IEmailService, EmailService>();

        // Scoped - one instance per request
        services.AddScoped<IUserService, UserService>();
        services.AddScoped<IOrderService, OrderService>();
        services.AddScoped(typeof(IRepository<>), typeof(Repository<>));

        // Singleton - one instance for app lifetime
        services.AddSingleton<ICacheService, MemoryCacheService>();

        return services;
    }

    public static IServiceCollection AddInfrastructure(
        this IServiceCollection services,
        IConfiguration configuration)
    {
        // Database
        services.AddDbContext<AppDbContext>(options =>
            options.UseSqlServer(
                configuration.GetConnectionString("DefaultConnection"),
                sqlOptions =>
                {
                    sqlOptions.EnableRetryOnFailure(
                        maxRetryCount: 3,
                        maxRetryDelay: TimeSpan.FromSeconds(30),
                        errorNumbersToAdd: null);
                    sqlOptions.CommandTimeout(30);
                }));

        // Repositories
        services.AddScoped<IUserRepository, UserRepository>();
        services.AddScoped<IOrderRepository, OrderRepository>();

        // HTTP clients
        services.AddHttpClient<IExternalApiClient, ExternalApiClient>(client =>
        {
            client.BaseAddress = new Uri(configuration["ExternalApi:BaseUrl"]!);
            client.Timeout = TimeSpan.FromSeconds(30);
        })
        .AddPolicyHandler(GetRetryPolicy())
        .AddPolicyHandler(GetCircuitBreakerPolicy());

        return services;
    }

    private static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
    {
        return HttpPolicyExtensions
            .HandleTransientHttpError()
            .WaitAndRetryAsync(3, retryAttempt =>
                TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
    }

    private static IAsyncPolicy<HttpResponseMessage> GetCircuitBreakerPolicy()
    {
        return HttpPolicyExtensions
            .HandleTransientHttpError()
            .CircuitBreakerAsync(5, TimeSpan.FromSeconds(30));
    }
}

Keyed Services (.NET 8+)

// Registration
services.AddKeyedScoped<INotificationService, EmailNotificationService>("email");
services.AddKeyedScoped<INotificationService, SmsNotificationService>("sms");
services.AddKeyedScoped<INotificationService, PushNotificationService>("push");

// Injection
public class NotificationController : ControllerBase
{
    public NotificationController(
        [FromKeyedServices("email")] INotificationService emailService,
        [FromKeyedServices("sms")] INotificationService smsService)
    {
        // Use specific implementations
    }
}

---

Authentication & Authorization

JWT Authentication

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.IdentityModel.Tokens;

public static class AuthenticationExtensions
{
    public static IServiceCollection AddJwtAuthentication(
        this IServiceCollection services,
        IConfiguration configuration)
    {
        var jwtOptions = configuration
            .GetSection(JwtOptions.SectionName)
            .Get<JwtOptions>()!;

        services.AddAuthentication(options =>
        {
            options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
            options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
        })
        .AddJwtBearer(options =>
        {
            options.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = jwtOptions.Issuer,
                ValidAudience = jwtOptions.Audience,
                IssuerSigningKey = new SymmetricSecurityKey(
                    Encoding.UTF8.GetBytes(jwtOptions.Secret)),
                ClockSkew = TimeSpan.Zero
            };

            options.Events = new JwtBearerEvents
            {
                OnAuthenticationFailed = context =>
                {
                    if (context.Exception is SecurityTokenExpiredException)
                    {
                        context.Response.Headers.Append(
                            "Token-Expired", "true");
                    }
                    return Task.CompletedTask;
                }
            };
        });

        return services;
    }
}

Token Service

public interface ITokenService
{
    string GenerateAccessToken(User user);
    string GenerateRefreshToken();
    ClaimsPrincipal? ValidateToken(string token);
}

public class TokenService : ITokenService
{
    private readonly JwtOptions _options;

    public TokenService(IOptions<JwtOptions> options)
    {
        _options = options.Value;
    }

    public string GenerateAccessToken(User user)
    {
        var claims = new List<Claim>
        {
            new(ClaimTypes.NameIdentifier, user.Id.ToString()),
            new(ClaimTypes.Email, user.Email),
            new(ClaimTypes.Name, $"{user.FirstName} {user.LastName}"),
            new("role", user.Role.ToString())
        };

        var key = new SymmetricSecurityKey(
            Encoding.UTF8.GetBytes(_options.Secret));
        var credentials = new SigningCredentials(
            key, SecurityAlgorithms.HmacSha256);

        var token = new JwtSecurityToken(
            issuer: _options.Issuer,
            audience: _options.Audience,
            claims: claims,
            expires: DateTime.UtcNow.AddMinutes(_options.ExpirationMinutes),
            signingCredentials: credentials);

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

    public string GenerateRefreshToken()
    {
        var randomBytes = new byte[64];
        using var rng = RandomNumberGenerator.Create();
        rng.GetBytes(randomBytes);
        return Convert.ToBase64String(randomBytes);
    }

    public ClaimsPrincipal? ValidateToken(string token)
    {
        var tokenHandler = new JwtSecurityTokenHandler();
        var key = Encoding.UTF8.GetBytes(_options.Secret);

        try
        {
            return tokenHandler.ValidateToken(token,
                new TokenValidationParameters
                {
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(key),
                    ValidateIssuer = true,
                    ValidIssuer = _options.Issuer,
                    ValidateAudience = true,
                    ValidAudience = _options.Audience,
                    ValidateLifetime = false // Allow expired tokens for refresh
                }, out _);
        }
        catch
        {
            return null;
        }
    }
}

Authorization Policies

public static class AuthorizationExtensions
{
    public static IServiceCollection AddAuthorizationPolicies(
        this IServiceCollection services)
    {
        services.AddAuthorization(options =>
        {
            // Role-based policies
            options.AddPolicy("AdminOnly", policy =>
                policy.RequireRole("Admin"));

            options.AddPolicy("ManagerOrAdmin", policy =>
                policy.RequireRole("Manager", "Admin"));

            // Claim-based policies
            options.AddPolicy("VerifiedEmail", policy =>
                policy.RequireClaim("email_verified", "true"));

            // Custom requirement policies
            options.AddPolicy("MinimumAge", policy =>
                policy.AddRequirements(new MinimumAgeRequirement(18)));

            // Resource-based authorization
            options.AddPolicy("OwnerOnly", policy =>
                policy.AddRequirements(new ResourceOwnerRequirement()));

            // Combined policies
            options.AddPolicy("PremiumFeature", policy =>
                policy.RequireRole("Premium", "Admin")
                      .RequireClaim("subscription_active", "true"));
        });

        // Register handlers
        services.AddScoped<IAuthorizationHandler, MinimumAgeHandler>();
        services.AddScoped<IAuthorizationHandler, ResourceOwnerHandler>();

        return services;
    }
}

---

Background Services

Hosted Service

namespace MyApp.Api.Services;

public class CleanupBackgroundService : BackgroundService
{
    private readonly IServiceScopeFactory _scopeFactory;
    private readonly ILogger<CleanupBackgroundService> _logger;
    private readonly TimeSpan _period = TimeSpan.FromHours(1);

    public CleanupBackgroundService(
        IServiceScopeFactory scopeFactory,
        ILogger<CleanupBackgroundService> logger)
    {
        _scopeFactory = scopeFactory;
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Cleanup service starting");

        using var timer = new PeriodicTimer(_period);

        while (!stoppingToken.IsCancellationRequested &&
               await timer.WaitForNextTickAsync(stoppingToken))
        {
            try
            {
                await DoCleanupAsync(stoppingToken);
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Error during cleanup");
            }
        }
    }

    private async Task DoCleanupAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation("Starting cleanup task");

        using var scope = _scopeFactory.CreateScope();
        var dbContext = scope.ServiceProvider.GetRequiredService<AppDbContext>();

        var cutoffDate = DateTime.UtcNow.AddDays(-30);

        var deletedCount = await dbContext.AuditLogs
            .Where(l => l.CreatedAt < cutoffDate)
            .ExecuteDeleteAsync(stoppingToken);

        _logger.LogInformation("Deleted {Count} old audit logs", deletedCount);
    }

    public override async Task StopAsync(CancellationToken cancellationToken)
    {
        _logger.LogInformation("Cleanup service stopping");
        await base.StopAsync(cancellationToken);
    }
}

// Registration
services.AddHostedService<CleanupBackgroundService>();

---

Testing

Unit Test Structure (xUnit)

using Xunit;
using Moq;
using FluentAssertions;

namespace MyApp.UnitTests.Services;

public class UserServiceTests
{
    private readonly Mock<IUserRepository> _userRepositoryMock;
    private readonly Mock<IEmailService> _emailServiceMock;
    private readonly Mock<ILogger<UserService>> _loggerMock;
    private readonly UserService _sut; // System Under Test

    public UserServiceTests()
    {
        _userRepositoryMock = new Mock<IUserRepository>();
        _emailServiceMock = new Mock<IEmailService>();
        _loggerMock = new Mock<ILogger<UserService>>();

        _sut = new UserService(
            _userRepositoryMock.Object,
            _emailServiceMock.Object,
            _loggerMock.Object);
    }

    [Fact]
    public async Task GetByIdAsync_WithValidId_ReturnsUser()
    {
        // Arrange
        var userId = Guid.NewGuid();
        var expectedUser = new User
        {
            Id = userId,
            Email = "[e-mail]",
            FirstName = "John",
            LastName = "Doe"
        };

        _userRepositoryMock
            .Setup(r => r.GetByIdAsync(userId, It.IsAny<CancellationToken>()))
            .ReturnsAsync(expectedUser);

        // Act
        var result = await _sut.GetByIdAsync(userId);

        // Assert
        result.Should().NotBeNull();
        result!.Id.Should().Be(userId);
        result.Email.Should().Be("[e-mail]");

        _userRepositoryMock.Verify(
            r => r.GetByIdAsync(userId, It.IsAny<CancellationToken>()),
            Times.Once);
    }

    [Fact]
    public async Task GetByIdAsync_WithInvalidId_ReturnsNull()
    {
        // Arrange
        var userId = Guid.NewGuid();

        _userRepositoryMock
            .Setup(r => r.GetByIdAsync(userId, It.IsAny<CancellationToken>()))
            .ReturnsAsync((User?)null);

        // Act
        var result = await _sut.GetByIdAsync(userId);

        // Assert
        result.Should().BeNull();
    }

    [Theory]
    [InlineData("")]
    [InlineData(" ")]
    [InlineData(null)]
    public async Task CreateAsync_WithInvalidEmail_ThrowsValidationException(
        string? email)
    {
        // Arrange
        var request = new CreateUserRequest
        {
            Email = email!,
            FirstName = "John",
            LastName = "Doe"
        };

        // Act
        var act = () => _sut.CreateAsync(request);

        // Assert
        await act.Should().ThrowAsync<ValidationException>()
            .WithMessage("*email*");
    }

    [Fact]
    public async Task CreateAsync_WithValidRequest_SendsWelcomeEmail()
    {
        // Arrange
        var request = new CreateUserRequest
        {
            Email = "[e-mail]",
            FirstName = "John",
            LastName = "Doe"
        };

        _userRepositoryMock
            .Setup(r => r.AddAsync(It.IsAny<User>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync((User u, CancellationToken _) => u);

        // Act
        await _sut.CreateAsync(request);

        // Assert
        _emailServiceMock.Verify(
            e => e.SendWelcomeEmailAsync(
                request.Email,
                It.IsAny<CancellationToken>()),
            Times.Once);
    }
}

Integration Tests with WebApplicationFactory

using Microsoft.AspNetCore.Mvc.Testing;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;

namespace MyApp.IntegrationTests;

public class CustomWebApplicationFactory<TProgram>
    : WebApplicationFactory<TProgram> where TProgram : class
{
    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureServices(services =>
        {
            // Remove existing DbContext registration
            var descriptor = services.SingleOrDefault(
                d => d.ServiceType == typeof(DbContextOptions<AppDbContext>));

            if (descriptor != null)
            {
                services.Remove(descriptor);
            }

            // Add in-memory database
            services.AddDbContext<AppDbContext>(options =>
            {
                options.UseInMemoryDatabase("TestDb");
            });

            // Replace external services with mocks
            services.AddScoped<IEmailService, FakeEmailService>();

            // Build service provider and ensure database is created
            var sp = services.BuildServiceProvider();
            using var scope = sp.CreateScope();
            var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
            db.Database.EnsureCreated();

            // Seed test data
            SeedTestData(db);
        });

        builder.UseEnvironment("Testing");
    }

    private static void SeedTestData(AppDbContext context)
    {
        context.Users.AddRange(
            new User
            {
                Id = Guid.Parse("11111111-1111-1111-1111-111111111111"),
                Email = "[e-mail]",
                FirstName = "Admin",
                LastName = "User"
            },
            new User
            {
                Id = Guid.Parse("22222222-2222-2222-2222-222222222222"),
                Email = "[e-mail]",
                FirstName = "Regular",
                LastName = "User"
            });

        context.SaveChanges();
    }
}

public class UsersControllerTests : IClassFixture<CustomWebApplicationFactory<Program>>
{
    private readonly HttpClient _client;
    private readonly CustomWebApplicationFactory<Program> _factory;

    public UsersControllerTests(CustomWebApplicationFactory<Program> factory)
    {
        _factory = factory;
        _client = factory.CreateClient(new WebApplicationFactoryClientOptions
        {
            AllowAutoRedirect = false
        });
    }

    [Fact]
    public async Task GetUsers_ReturnsSuccessAndUsers()
    {
        // Act
        var response = await _client.GetAsync("/api/users");

        // Assert
        response.EnsureSuccessStatusCode();

        var users = await response.Content
            .ReadFromJsonAsync<IEnumerable<UserDto>>();

        users.Should().NotBeNull();
        users.Should().HaveCountGreaterThan(0);
    }

    [Fact]
    public async Task GetUserById_WithValidId_ReturnsUser()
    {
        // Arrange
        var userId = "11111111-1111-1111-1111-111111111111";

        // Act
        var response = await _client.GetAsync($"/api/users/{userId}");

        // Assert
        response.EnsureSuccessStatusCode();

        var user = await response.Content.ReadFromJsonAsync<UserDto>();

        user.Should().NotBeNull();
        user!.Email.Should().Be("[e-mail]");
    }

    [Fact]
    public async Task GetUserById_WithInvalidId_ReturnsNotFound()
    {
        // Arrange
        var userId = Guid.NewGuid();

        // Act
        var response = await _client.GetAsync($"/api/users/{userId}");

        // Assert
        response.StatusCode.Should().Be(HttpStatusCode.NotFound);
    }
}

---

Best Practices

Code Organization

Practice	Description
Clean Architecture	Separate concerns into layers (Core, Infrastructure, API)
Vertical Slices	Organize by feature for complex applications
SOLID Principles	Single responsibility, dependency injection, etc.
DTOs	Never expose entities directly to API responses

Performance

Practice	Description
Async/Await	Use async for I/O operations
CancellationToken	Pass through all async chains
IQueryable vs IEnumerable	Filter at database level
Compiled Queries	For frequently executed queries
Caching	Use IMemoryCache or IDistributedCache

Security

Practice	Description
Input Validation	Validate all user input
Parameterized Queries	Prevent SQL injection
HTTPS	Always use HTTPS in production
Secrets Management	Use User Secrets/Key Vault
CORS	Configure properly for your domains

Error Handling

// Custom exception types
public class NotFoundException : Exception
{
    public NotFoundException(string message) : base(message) { }
    public NotFoundException(string name, object key)
        : base($"Entity \"{name}\" ({key}) was not found.") { }
}

public class ValidationException : Exception
{
    public IReadOnlyList<string> Errors { get; }

    public ValidationException(IEnumerable<string> errors)
        : base("One or more validation errors occurred.")
    {
        Errors = errors.ToList();
    }
}

public class ForbiddenException : Exception
{
    public ForbiddenException(string message = "Access denied")
        : base(message) { }
}

Logging Best Practices

// Structured logging with semantic values
_logger.LogInformation(
    "User {UserId} created order {OrderId} for {Amount:C}",
    userId, orderId, amount);

// Log levels
_logger.LogTrace("Detailed debugging info");
_logger.LogDebug("Debugging info");
_logger.LogInformation("General info");
_logger.LogWarning("Something unexpected but not an error");
_logger.LogError(ex, "An error occurred");
_logger.LogCritical(ex, "A critical failure");

// Scopes for correlation
using (_logger.BeginScope(new Dictionary<string, object>
{
    ["UserId"] = userId,
    ["OrderId"] = orderId
}))
{
    _logger.LogInformation("Processing order");
    // All logs within scope include UserId and OrderId
}

Health Checks

services.AddHealthChecks()
    .AddDbContextCheck<AppDbContext>("database")
    .AddRedis(connectionString, "redis")
    .AddUrlGroup(new Uri("https://api.example.com/health"), "external-api")
    .AddCheck<CustomHealthCheck>("custom");

app.MapHealthChecks("/health", new HealthCheckOptions
{
    ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
});

app.MapHealthChecks("/health/ready", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("ready")
});

app.MapHealthChecks("/health/live", new HealthCheckOptions
{
    Predicate = _ => false // Just returns healthy if app is running
});

---

Additional Resources

• .NET Documentation
• ASP.NET Core Documentation
• EF Core Documentation
• C# Language Reference
• .NET Architecture Guides

---

Note: This skill covers .NET 8+ patterns and best practices. For earlier versions, some features may not be available.