feat: add multi-tenancy builder

Inspired by how Microsoft does it in ASP.NET Core. It's
the main entry point on configuring multi-tenancy services.

src/Ayaka.MultiTenancy.Abstractions/Ayaka.MultiTenancy.Abstractions.csproj

@@ -14,4 +14,8 @@
     <PackageTags>$(PackageCommonTags);multi-tenancy</PackageTags>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="8.0.2" />
+  </ItemGroup>
+
 </Project>

src/Ayaka.MultiTenancy.Abstractions/DependencyInjection/IMultiTenancyBuilder.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Raphael Strotz. All rights reserved.
+
+namespace Ayaka.MultiTenancy.DependencyInjection;
+
+using Microsoft.Extensions.DependencyInjection;
+
+/// <summary>
+/// Provides functionality to configure multi-tenancy services.
+/// </summary>
+public interface IMultiTenancyBuilder
+{
+    /// <summary>
+    /// Gets the <see cref="IServiceCollection"/> where multi-tenancy services are configured.
+    /// </summary>
+    IServiceCollection Services { get; }
+}

src/Ayaka.MultiTenancy.Abstractions/PublicAPI.Unshipped.txt

@@ -1,4 +1,6 @@
 #nullable enable
+Ayaka.MultiTenancy.DependencyInjection.IMultiTenancyBuilder
+Ayaka.MultiTenancy.DependencyInjection.IMultiTenancyBuilder.Services.get -> Microsoft.Extensions.DependencyInjection.IServiceCollection!
 Ayaka.MultiTenancy.ITenantContextAccessor
 Ayaka.MultiTenancy.ITenantContextAccessor.TenantContext.get -> Ayaka.MultiTenancy.TenantContext?
 Ayaka.MultiTenancy.ITenantContextAccessor.TenantContext.set -> void

src/Ayaka.MultiTenancy/Ayaka.MultiTenancy.csproj

@@ -4,15 +4,15 @@
     <TargetFramework>$(DotNetTargetFramework)</TargetFramework>
   </PropertyGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="Ayaka.MultiTenancy.Tests" />
+  </ItemGroup>
+
   <PropertyGroup>
     <PackageDescription>Provides functionality for creating multi-tenanted applications. $(PackageDescriptionAppendix)</PackageDescription>
     <PackageTags>$(PackageCommonTags);multi-tenancy</PackageTags>
   </PropertyGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="8.0.2" />
-  </ItemGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\Ayaka.MultiTenancy.Abstractions\Ayaka.MultiTenancy.Abstractions.csproj" />
   </ItemGroup>

src/Ayaka.MultiTenancy/DependencyInjection/MultiTenancyBuilder.cs

@@ -0,0 +1,23 @@
+// Copyright (c) Raphael Strotz. All rights reserved.
+
+namespace Ayaka.MultiTenancy.DependencyInjection;
+
+using Microsoft.Extensions.DependencyInjection;
+
+/// <summary>
+/// Allows configuration of multi-tenancy services.
+/// </summary>
+internal sealed class MultiTenancyBuilder : IMultiTenancyBuilder
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="MultiTenancyBuilder"/> class.
+    /// </summary>
+    /// <param name="services">The <see cref="IServiceCollection" /> to add services to.</param>
+    public MultiTenancyBuilder(IServiceCollection services)
+    {
+        Services = services;
+    }
+
+    /// <inheritdoc />
+    public IServiceCollection Services { get; }
+}

src/Ayaka.MultiTenancy/PublicAPI.Unshipped.txt

@@ -4,4 +4,5 @@ Ayaka.MultiTenancy.AsyncLocalTenantContextAccessor.AsyncLocalTenantContextAccess
 Ayaka.MultiTenancy.AsyncLocalTenantContextAccessor.TenantContext.get -> Ayaka.MultiTenancy.TenantContext?
 Ayaka.MultiTenancy.AsyncLocalTenantContextAccessor.TenantContext.set -> void
 Ayaka.MultiTenancy.ServiceCollectionExtensions
+static Ayaka.MultiTenancy.ServiceCollectionExtensions.AddMultiTenancy(this Microsoft.Extensions.DependencyInjection.IServiceCollection! services) -> Ayaka.MultiTenancy.DependencyInjection.IMultiTenancyBuilder!
 static Ayaka.MultiTenancy.ServiceCollectionExtensions.AddTenantContextAccessor(this Microsoft.Extensions.DependencyInjection.IServiceCollection! services) -> Microsoft.Extensions.DependencyInjection.IServiceCollection!

src/Ayaka.MultiTenancy/ServiceCollectionExtensions.cs

@@ -2,6 +2,7 @@
 
 namespace Ayaka.MultiTenancy;
 
+using Ayaka.MultiTenancy.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
 
@@ -10,6 +11,22 @@ namespace Ayaka.MultiTenancy;
 /// </summary>
 public static class ServiceCollectionExtensions
 {
+    /// <summary>
+    ///     Adds multi-tenancy services to the specified <see cref="IServiceCollection"/>.
+    /// </summary>
+    /// <remarks>
+    ///     In order to fine-tune the multi-tenancy configuration, use the <see cref="IMultiTenancyBuilder"/> returned by
+    ///     this method.
+    /// </remarks>
+    /// <param name="services">The <see cref="IServiceCollection"/> to add the service to.</param>
+    /// <returns>A <see cref="IMultiTenancyBuilder"/> that can be used to further configure multi-tenancy.</returns>
+    public static IMultiTenancyBuilder AddMultiTenancy(this IServiceCollection services)
+    {
+        ConfigureDefaultServices(services);
+
+        return new MultiTenancyBuilder(services);
+    }
+
     /// <summary>
     ///     Adds the default implementation for the <see cref="ITenantContextAccessor"/> service.
     /// </summary>
@@ -20,4 +37,11 @@ public static IServiceCollection AddTenantContextAccessor(this IServiceCollectio
         services.TryAddSingleton<ITenantContextAccessor, AsyncLocalTenantContextAccessor>();
         return services;
     }
+
+    [SuppressMessage("Style", "IDE0058:Expression value is never used")]
+    private static void ConfigureDefaultServices(this IServiceCollection services)
+    {
+        // The heart of multi-tenancy
+        services.AddTenantContextAccessor();
+    }
 }

test/Ayaka.MultiTenancy.Tests/ServiceCollectionExtensionsTest.cs

@@ -2,10 +2,57 @@
 
 namespace Ayaka.MultiTenancy.Tests;
 
+using Ayaka.MultiTenancy.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection;
 
 public sealed class ServiceCollectionExtensionsTest
 {
+    public sealed class AddMultiTenancy
+    {
+        [Fact]
+        public void Does_return_instance_of_MultiTenancyBuilder()
+        {
+            var services = new ServiceCollection();
+
+            var builder = services.AddMultiTenancy();
+
+            builder.Should().NotBeNull();
+            builder.Should().BeOfType<MultiTenancyBuilder>();
+        }
+
+        [Fact]
+        public void Does_use_specified_service_collection()
+        {
+            var services = new ServiceCollection();
+
+            var builder = services.AddMultiTenancy();
+
+            builder.Services.Should().BeSameAs(services);
+        }
+
+        [Fact]
+        public void Does_add_default_services()
+        {
+            var services = new ServiceCollection();
+
+            services.AddMultiTenancy();
+
+            var accessor = services.FirstOrDefault(x => x.ServiceType == typeof(ITenantContextAccessor));
+            accessor.Should().NotBeNull("ITenantContextAccessor should be registered");
+        }
+
+        [Fact]
+        public void Does_not_add_default_services_twice()
+        {
+            var services = new ServiceCollection();
+
+            services.AddMultiTenancy();
+            services.AddMultiTenancy();
+
+            services.Count(x => x.ServiceType == typeof(ITenantContextAccessor)).Should().Be(1);
+        }
+    }
+
     public sealed class AddTenantContextAccessor
     {
         [Fact]