default-values.md

Default values

This example demonstrates how to use default values in dependency injection when explicit injection is not possible.

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .Bind<IDependency>().To<Dependency>()
    .Bind<IService>().To<Service>()

    // Composition root
    .Root<IService>("Root");

var composition = new Composition();
var service = composition.Root;
service.Dependency.ShouldBeOfType<Dependency>();
service.Name.ShouldBe("My Service");

interface IDependency;

class Dependency : IDependency;

interface IService
{
    string Name { get; }

    IDependency Dependency { get; }
}

// If injection cannot be performed explicitly,
// the default value will be used
class Service(string name = "My Service") : IService
{
    public string Name { get; } = name;

    public required IDependency Dependency { get; init; } = new Dependency();
}

Running this code sample locally

• Make sure you have the .NET SDK 9.0 or later is installed

dotnet --list-sdk

• Create a net9.0 (or later) console application

dotnet new console -n Sample

• Add references to 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

The key points are:

• Default constructor arguments can be used for simple values
• The DI container will use these defaults if no explicit bindings are provided

This example illustrates how to handle default values in a dependency injection scenario:

• Constructor Default Argument: The Service class has a constructor with a default value for the name parameter. If no value is provided, “My Service” will be used.
• Required Property with Default: The Dependency property is marked as required but has a default instantiation. This ensures that:
  • The property must be set
  • If no explicit injection occurs, a default value will be used

The following partial class will be generated:

partial class Composition
{
  private readonly Composition _root;

  [OrdinalAttribute(256)]
  public Composition()
  {
    _root = this;
  }

  internal Composition(Composition parentScope)
  {
    _root = (parentScope ?? throw new ArgumentNullException(nameof(parentScope)))._root;
  }

  public IService Root
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new Service()
      {
        Dependency = new Dependency()
      };
    }
  }
}

Class diagram:

---
 config:
  class:
   hideEmptyMembersBox: true
---
classDiagram
	Dependency --|> IDependency
	Service --|> IService
	Composition ..> Service : IService Root
	Service *--  Dependency : IDependency
	namespace Pure.DI.UsageTests.Basics.DefaultValuesScenario {
		class Composition {
		<<partial>>
		+IService Root
		}
		class Dependency {
			+Dependency()
		}
		class IDependency {
			<<interface>>
		}
		class IService {
			<<interface>>
		}
		class Service {
			+Service(String name)
			+IDependency Dependency
		}
	}

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