Project Structure

Core Principles

1. Central package management — Use Directory.Packages.props to manage NuGet package versions in one place. No version numbers in individual .csproj files.
2. Shared build properties — Use Directory.Build.props for common settings (target framework, nullable, implicit usings). Don't repeat in every project.
3. .slnx for solutions — The new XML-based solution format is cleaner and more merge-friendly than the legacy .sln format.
4. src/tests separation — Source projects in src/, test projects in tests/. Clear boundary.

Patterns

Solution Layout

MyApp/
├── MyApp.slnx                       # Solution file
├── Directory.Build.props             # Shared MSBuild properties
├── Directory.Packages.props          # Central package management
├── .editorconfig                     # Code style rules
├── .gitignore
├── global.json                       # SDK version pinning
├── src/
│   ├── MyApp.Api/                    # Web API (entry point)
│   │   ├── MyApp.Api.csproj
│   │   ├── Program.cs
│   │   └── Features/
│   ├── MyApp.Domain/                 # Domain entities, value objects (optional)
│   │   └── MyApp.Domain.csproj
│   └── MyApp.Infrastructure/         # EF Core, external services (optional)
│       └── MyApp.Infrastructure.csproj
└── tests/
    └── MyApp.Api.Tests/
        └── MyApp.Api.Tests.csproj

Directory.Build.props

<Project>
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14</LangVersion>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
  </PropertyGroup>
</Project>

Directory.Packages.props (Central Package Management)

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>

  <ItemGroup>
    <!-- Versions below are illustrative — resolve the current stable versions
         with `dotnet add package <name>` (no --version flag); see the packages rule -->
    <!-- ASP.NET Core -->
    <PackageVersion Include="Mediator.Abstractions" Version="3.0.0" />
    <PackageVersion Include="Mediator.SourceGenerator" Version="3.0.0" />
    <PackageVersion Include="FluentValidation.DependencyInjectionExtensions" Version="12.0.0" />

    <!-- Data -->
    <PackageVersion Include="Microsoft.EntityFrameworkCore" Version="10.0.10" />
    <PackageVersion Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="10.0.10" />

    <!-- Observability -->
    <PackageVersion Include="Serilog.AspNetCore" Version="10.0.0" />
    <PackageVersion Include="OpenTelemetry.Extensions.Hosting" Version="1.17.0" />

    <!-- Testing -->
    <PackageVersion Include="xunit.v3" Version="3.2.2" />
    <PackageVersion Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.10" />
    <PackageVersion Include="Testcontainers.PostgreSql" Version="4.13.0" />
  </ItemGroup>
</Project>

Project File (.csproj) with Central Package Management

<Project Sdk="Microsoft.NET.Sdk.Web">
  <!-- No TargetFramework here — inherited from Directory.Build.props -->

  <ItemGroup>
    <!-- No Version attribute — managed centrally -->
    <PackageReference Include="Mediator.Abstractions" />
    <PackageReference Include="Mediator.SourceGenerator" />
    <PackageReference Include="FluentValidation.DependencyInjectionExtensions" />
    <PackageReference Include="Microsoft.EntityFrameworkCore" />
    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" />
    <PackageReference Include="Serilog.AspNetCore" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\MyApp.Domain\MyApp.Domain.csproj" />
    <ProjectReference Include="..\MyApp.Infrastructure\MyApp.Infrastructure.csproj" />
  </ItemGroup>
</Project>

global.json (SDK Pinning)

{
  "sdk": {
    "version": "10.0.100",
    "rollForward": "latestFeature"
  }
}

.slnx Solution Format

<Solution>
  <Folder Name="/src/">
    <Project Path="src/MyApp.Api/MyApp.Api.csproj" />
    <Project Path="src/MyApp.Domain/MyApp.Domain.csproj" />
    <Project Path="src/MyApp.Infrastructure/MyApp.Infrastructure.csproj" />
  </Folder>
  <Folder Name="/tests/">
    <Project Path="tests/MyApp.Api.Tests/MyApp.Api.Tests.csproj" />
  </Folder>
</Solution>

Naming Conventions

Element	Convention	Example
Solution	CompanyName.AppName or AppName	MyApp.slnx
Project	AppName.Layer	MyApp.Api, MyApp.Domain
Namespace	Matches folder path	MyApp.Api.Features.Orders
Feature folder	PascalCase, plural	Features/Orders/
Test project	ProjectName.Tests	MyApp.Api.Tests

Anti-patterns

Don't Scatter Package Versions

<!-- BAD — version in every .csproj, version drift -->
<PackageReference Include="Mediator.Abstractions" Version="2.0.0" />  <!-- in Project A -->
<PackageReference Include="Mediator.Abstractions" Version="3.0.0" />  <!-- in Project B -->

<!-- GOOD — central management, one version -->
<!-- Directory.Packages.props: <PackageVersion Include="Mediator.Abstractions" Version="3.0.0" /> -->
<!-- .csproj: <PackageReference Include="Mediator.Abstractions" /> -->

Don't Repeat Build Properties

<!-- BAD — same properties in every .csproj -->
<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
  <Nullable>enable</Nullable>
  <ImplicitUsings>enable</ImplicitUsings>
</PropertyGroup>

<!-- GOOD — once in Directory.Build.props, inherited everywhere -->

Don't Mix Source and Test Projects

# BAD — tests mixed with source
src/
  MyApp.Api/
  MyApp.Api.Tests/    # test project in src/

# GOOD — clear separation
src/
  MyApp.Api/
tests/
  MyApp.Api.Tests/

Decision Guide

Scenario	Recommendation
New solution	.slnx format
Package version management	Directory.Packages.props (central)
Shared build settings	Directory.Build.props
SDK version pinning	global.json
Common using directives	Global usings in Directory.Build.props
Small API (1-2 devs)	Single project (MyApp.Api)
Medium API (3-5 devs)	2-3 projects (Api, Domain, Infrastructure)
Large / modular app	Module-per-project with shared Contracts