default-values.md

Default values

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

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .Bind<ISensor>().To<MotionSensor>()
    .Bind<ISecuritySystem>().To<SecuritySystem>()

    // Composition root
    .Root<ISecuritySystem>("SecuritySystem");

var composition = new Composition();
var securitySystem = composition.SecuritySystem;
securitySystem.Sensor.ShouldBeOfType<MotionSensor>();
securitySystem.SystemName.ShouldBe("Home Guard");

interface ISensor;

class MotionSensor : ISensor;

interface ISecuritySystem
{
    string SystemName { get; }

    ISensor Sensor { get; }
}

// If injection cannot be performed explicitly,
// the default value will be used
class SecuritySystem(string systemName = "Home Guard") : ISecuritySystem
{
    public string SystemName { get; } = systemName;

    // The 'required' modifier ensures that the property is set during initialization.
    // The default value 'new MotionSensor()' provides a fallback
    // if no dependency is injected.
    public required ISensor Sensor { get; init; } = new MotionSensor();
}

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

The key points are:

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

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

• Constructor Default Argument: The SecuritySystem class has a constructor with a default value for the name parameter. If no value is provided, "Home Guard" will be used.
• Required Property with Default: The Sensor 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
{
  public ISecuritySystem SecuritySystem
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new SecuritySystem("Home Guard")
      {
        Sensor = new MotionSensor()
      };
    }
  }
}

Class diagram:

---
 config:
  maxTextSize: 2147483647
  maxEdges: 2147483647
  class:
   hideEmptyMembersBox: true
---
classDiagram
	MotionSensor --|> ISensor
	SecuritySystem --|> ISecuritySystem
	Composition ..> SecuritySystem : ISecuritySystem SecuritySystem
	SecuritySystem *--  MotionSensor : ISensor
	SecuritySystem *--  String : String
	namespace Pure.DI.UsageTests.Basics.DefaultValuesScenario {
		class Composition {
		<<partial>>
		+ISecuritySystem SecuritySystem
		}
		class ISecuritySystem {
			<<interface>>
		}
		class ISensor {
			<<interface>>
		}
		class MotionSensor {
				<<class>>
			+MotionSensor()
		}
		class SecuritySystem {
				<<class>>
			+SecuritySystem(String systemName)
			+ISensor Sensor
		}
	}
	namespace System {
		class String {
			<<class>>
		}
	}

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