singleton.md

Singleton

The Singleton lifetime ensures that there will be a single instance of the dependency for each composition.

using Shouldly;
using Pure.DI;
using System.Diagnostics.CodeAnalysis;
using static Pure.DI.Lifetime;

DI.Setup(nameof(Composition))
    // Bind the cache as Singleton to share it across all services
    .Bind().As(Singleton).To<Cache>()
    // Bind the order service as Transient (default) for per-request instances
    .Bind().To<OrderService>()
    .Root<IOrderService>("OrderService");

var composition = new Composition();
var orderService1 = composition.OrderService; // First order service instance
var orderService2 = composition.OrderService; // Second order service instance

// Verify that both services share the same cache instance (Singleton behavior)
orderService1.Cache.ShouldBe(orderService2.Cache);
// Simulate real-world usage: add data to cache via one service and check via another
orderService1.AddToCache("Order123", "Processed");
orderService2.GetFromCache("Order123").ShouldBe("Processed");

// Interface for a shared cache (e.g., for storing order statuses)
interface ICache
{
    void Add(string key, string value);

    bool TryGet(string key, [MaybeNullWhen(false)] out string value);
}

// Implementation of a simple in-memory cache (must be thread-safe in real apps)
class Cache : ICache
{
    private readonly Dictionary<string, string> _data = new();

    public void Add(string key, string value) =>
        _data[key] = value;

    public bool TryGet(string key, [MaybeNullWhen(false)] out string value) =>
        _data.TryGetValue(key, out value);
}

// Interface for order processing service
interface IOrderService
{
    ICache Cache { get; }

    void AddToCache(string orderId, string status);

    string GetFromCache(string orderId);
}

// Order service that uses the shared cache
class OrderService(ICache cache) : IOrderService
{
    // The cache is injected and shared (Singleton)
    public ICache Cache { get; } = cache;

    // Real-world method: add order status to cache
    public void AddToCache(string orderId, string status) =>
        Cache.Add(orderId, status);

    // Real-world method: retrieve order status from cache
    public string GetFromCache(string orderId) =>
        Cache.TryGet(orderId, out var status) ? status : "unknown";
}

Running this code sample locally

• Make sure you have the .NET SDK 10.0 or later installed

dotnet --list-sdk

• Create a net10.0 (or later) console application

dotnet new console -n Sample

• Add references to the NuGet packages
  • Pure.DI
  • Shouldly

dotnet add package Pure.DI
dotnet add package Shouldly

• Copy the example code into the Program.cs file

You are ready to run the example 🚀

dotnet run

Some articles advise using objects with a Singleton lifetime as often as possible, but the following details must be considered:

• For .NET the default behavior is to create a new instance of the type each time it is needed, other behavior requires, additional logic that is not free and requires additional resources.

• The use of Singleton adds a requirement for thread-safety controls on their use, since singletons are more likely to share their state between different threads without even realizing it.

• The thread-safety control should be automatically extended to all dependencies that Singleton uses, since their state is also now shared.

• Logic for thread-safety control can be resource-costly, error-prone, interlocking, and difficult to test.

• Singleton can retain dependency references longer than their expected lifetime, this is especially significant for objects that hold "non-renewable" resources, such as the operating system Handler.

• Sometimes additional logic is required to dispose of Singleton.

The following partial class will be generated:

partial class Composition
{
#if NET9_0_OR_GREATER
  private readonly Lock _lock = new Lock();
#else
  private readonly Object _lock = new Object();
#endif

  private Cache? _singletonCache62;

  public IOrderService OrderService
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      if (_singletonCache62 is null)
        lock (_lock)
          if (_singletonCache62 is null)
          {
            _singletonCache62 = new Cache();
          }

      return new OrderService(_singletonCache62);
    }
  }
}

Class diagram:

---
 config:
  maxTextSize: 2147483647
  maxEdges: 2147483647
  class:
   hideEmptyMembersBox: true
---
classDiagram
	Cache --|> ICache
	OrderService --|> IOrderService
	Composition ..> OrderService : IOrderService OrderService
	OrderService o-- "Singleton" Cache : ICache
	namespace Pure.DI.UsageTests.Lifetimes.SingletonScenario {
		class Cache {
				<<class>>
			+Cache()
		}
		class Composition {
		<<partial>>
		+IOrderService OrderService
		}
		class ICache {
			<<interface>>
		}
		class IOrderService {
			<<interface>>
		}
		class OrderService {
				<<class>>
			+OrderService(ICache cache)
		}
	}

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