Make an optional service in .NET DI

OptionalServiceInDotnetDI.md

Make an optional service in .NET DI Container

Services in DI container is by default required. To make it optional, give it a defualt value.

Assuming ConsumerService rely on AnotherService while AnotherService is not registered in DI container.

Solution

Having a default value for the optional service.

public class ConsumerService
{
    private readonly AnotherService? _dep;

    public ConsumerService(AnotherService? dep = null) // Works. Optional service that **has a default value** of null.
    {
        _dep = dep;
    }
}

These won't work, and will cause crash upon resolving the service:

Mistake #1

public class ConsumerService
{
    private readonly AnotherService _dep;

    public ConsumerService(AnotherService dep)  // Any service by default is required.
    {
        _dep = dep;
    }
}

Mistake #2

public class ConsumerService2
{
    private readonly AnotherService? _dep;

    public ConsumerService2(AnotherService? dep) // Only have null annotation is not enough, still cause exception.
    {
        _dep = dep;
    }
}