custom-bind-attribute.md

Custom bind attribute

Shows how to declare a binding directly on an implementation type with a custom attribute. A custom attribute can combine contract type, lifetime, and tag metadata by registering argument positions in the composition setup.

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .TypeAttribute<ServiceAttribute<TT>>()
    .LifetimeAttribute<ServiceAttribute<TT>>()
    .TagAttribute<ServiceAttribute<TT>>(1)

    // Composition root
    .Root<IMessageWriter>("Writer", "console");

var composition = new Composition();
var writer = composition.Writer;
writer.Write("Pure.DI");

writer.ShouldBeOfType<ConsoleMessageWriter>();
writer.ShouldBeSameAs(composition.Writer);

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
class ServiceAttribute<T> : Attribute
{
    public ServiceAttribute(
        Lifetime lifetime = Lifetime.Transient,
        object? tag = null)
    {
    }
}

interface IMessageWriter
{
    void Write(string message);
}

[Service<IMessageWriter>(Lifetime.Singleton, "console")]
class ConsoleMessageWriter : IMessageWriter
{
    public void Write(string message) => Console.WriteLine(message);
}

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

Note

Implementation-level binding attributes are useful when the implementation assembly should describe its DI role while keeping the composition concise. Custom attributes participate in the same merge rules as the built-in Bind, Type, Tag, and Lifetime attributes: attributes in one square-bracket group form one binding, while separate Bind groups create separate bindings.

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 ConsoleMessageWriter? _singletonConsoleMessageWriter1;

  public IMessageWriter Writer
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      if (_singletonConsoleMessageWriter1 is null)
        lock (_lock)
          if (_singletonConsoleMessageWriter1 is null)
          {
            _singletonConsoleMessageWriter1 = new ConsoleMessageWriter();
          }

      return _singletonConsoleMessageWriter1;
    }
  }
}

Class diagram:

---
 config:
  class:
   hideEmptyMembersBox: true
---
classDiagram
	ConsoleMessageWriter --|> IMessageWriter : "console"
	Composition ..> ConsoleMessageWriter : IMessageWriter Writer
	namespace Pure.DI.UsageTests.Attributes.CustomBindAttributeScenario {
		class Composition {
		<<partial>>
		+IMessageWriter Writer
		}
		class ConsoleMessageWriter {
				<<class>>
			+ConsoleMessageWriter()
		}
		class IMessageWriter {
			<<interface>>
		}
	}

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See also:

• Bind attribute