csharp-patterns

C# / .NET Patterns

Project Structure

Solution Layout

MyApp.sln
Directory.Build.props              # Shared build properties
Directory.Packages.props           # Central package management
src/
  MyApp.Api/                       # ASP.NET Core host (Controllers, Middleware, Program.cs)
  MyApp.Application/               # Use cases, MediatR handlers, Behaviors
  MyApp.Domain/                    # Entities, value objects, domain events
  MyApp.Infrastructure/            # EF Core, external services
tests/
  MyApp.UnitTests/
  MyApp.IntegrationTests/

Directory.Build.props

<Project>
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
    <AnalysisLevel>latest-recommended</AnalysisLevel>
  </PropertyGroup>
</Project>

Central Package Management (Directory.Packages.props)

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup>
    <PackageVersion Include="MediatR" Version="12.4.1" />
    <PackageVersion Include="FluentValidation" Version="11.11.0" />
    <PackageVersion Include="Microsoft.EntityFrameworkCore" Version="9.0.0" />
  </ItemGroup>
</Project>

---

Idioms / Code Style

Nullable Reference Types

public class Order
{
    public required string Id { get; init; }
    public string? Notes { get; set; }               // explicitly nullable
    public required Customer Customer { get; init; }  // never null
    public string Summary => $"Order {Id}: {Notes ?? "no notes"}";
}

Records

public record Money(decimal Amount, string Currency)
{
    public Money Add(Money other) => Currency != other.Currency
        ? throw new InvalidOperationException("Currency mismatch")
        : this with { Amount = Amount + other.Amount };
}

public record CreateOrderRequest(string CustomerId, List<OrderLineDto> Lines);
public record OrderLineDto(string ProductId, int Quantity);