Unity Container: Property Injection

In the previous chapter, we learned about constructor injection. Here, we will learn about property injection using Unity Container.

Property injection is a type of dependency injection where dependencies are provided through properties.

Let's understand how we can perform property injection using Unity container. Consider the following example classes.

public interface ICar
{
    int Run();
}

public class BMW : ICar
{
    private int _miles = 0;

    public int Run()
    {
        return ++_miles;
    }
}

public class Ford : ICar
{
    private int _miles = 0;

    public int Run()
    {
        return ++_miles;
    }
}

public class Audi : ICar
{
    private int _miles = 0;

    public int Run()
    {
        return ++_miles;
    }

}
public class Driver
{
    public Driver()
    {
    }

    [Dependency]
    public ICar Car { get; set; }

    public void RunCar()
    {
        Console.WriteLine("Running {0} - {1} mile ", 
                            this.Car.GetType().Name, this.Car.Run());
    }
}

As you can see in the above sample classes, the Driver class is dependent on a property of type ICar. So, we need to set an object of a class that implements ICar to the Car property using Unity container.

Property injection in Unity container can be implemented in two ways:

1. Using the [Dependency] attribute
2. Using run-time configuration

[Dependency] Attribute

For the property injection, we first tell the Unity container which property to inject. So, we need to decorate the dependent properties with the [Dependency] attribute, as shown in the following Driver class.

Example: Apply [Dependency] Attribute

public class Driver
{

    public Driver() 
    {
    }

    [Dependency]
    public ICar Car { get; set; }

    public void RunCar()
    {
        Console.WriteLine("Running {0} - {1} mile ", this.Car.GetType().Name, this.Car.Run());
    }
}

Now, we can register the ICar type and resolve it as shown below.

Example: Property Injection using Unity Container

var container = new UnityContainer();
container.RegisterType<ICar, BMW>();
            
var driver = container.Resolve<Driver>();
driver.RunCar();

Output:

Running BMW - 1 mile

Named Mapping

We can specify a name in the [Dependency("name")] attribute, which can then be used to set the property value.

public class Driver
{
    public Driver() 
    {
    }

    [Dependency("LuxuryCar")]
    public ICar Car { get; set; }

    public void RunCar()
    {
        Console.WriteLine("Running {0} - {1} mile ", this.Car.GetType().Name, this.Car.Run());
    }
}

So now, we can resolve it as below.

Example: Property Injection using Unity Container

var container = new UnityContainer();
container.RegisterType<ICar, BMW>();
container.RegisterType<ICar, Audi>("LuxuryCar");
            
var driver = container.Resolve<Driver>();
driver.RunCar();

Output:

Running Audi - 1 mile

Run-time Configuration

Unity container allows us to configure a property injection with the RegisterType() method if a method is not marked with the [Dependency] attribute. You can pass an object of the InjectionProperty class in the RegisterType() method to specify a property name and a parameter value.

Note : InjectionProperty is derived from the InjectionMember Class. The InjectionMember is an abstract class which can be used to configure injection type. There are three subclasses of InjectionMembers: InjectionConstruction to configure construction injection, InjectionProperty to configure property injection and InjectionMethod to configure method injection.

var container = new UnityContainer();
            
//run-time configuration
container.RegisterType<Driver>(new InjectionProperty("Car", new BMW()));

var driver = container.Resolve<Driver>();
driver.RunCar();

Output:

Running BMW - 1 Mile

As you can see in the above example, container.RegisterType<driver>(new InjectionProperty("Car", new BMW())) registers the Driverclass by passing an object of InjectionProperty that specifies the property name "Car" and the BMWobject as a value. Therefore, Unity container will set an object of BMW to the Car property when we resolve it using container.Resolve<Driver>().