Question

How do I combine constructor injection with “manual” constructor parameters? ie.

public class SomeObject
{
    public SomeObject(IService service, float someValue)
    {
    }
}

Where IService should be resolved/injected by my DI container, and someValue should be specified. How do I mix the two?

Answer 1

Such constructs should be avoided whenever possible. Therefore, ask yourself: is this parameter really required as constructor argument? Or can SomeObject be replaced by a stateless one which is reused by everyone that depends on it by passing the parameter to the method you execute on the object?

e.g. Instead of

public class SomeObject
{
    private float someValue
    public SomeObject(IService service, float someValue)
    {
        this.someValue = someValue
    }

    public float Do(float x)
    {
        return this.Service.Get(this.someValue) * x;
    }
}

use

public class SomeObject
{
    public SomeObject(IService service)
    {
    }

    public float Do(float x, float someValue)
    {
        return this.Service.Get(someValue) * x;
    }
}

---

If it is required go for a factory:

public interface ISomeObjectFactory
{
    ISomeObject CreateSomeObject(float someValue);
}

public class SomeObjectFactory : ISomeObjectFactory
{
    private IKernel kernel;
    public SomeObjectFactory(IKernel kernel) 
    {
        this.Kernel = kernel;
    }

    public ISomeObject Create(float someValue)
    {
        return this.kernel.Get<ISomeObject>(WithConstructorArgument("someValue", someValue);
    }
}

Preview:
Ninject 2.4 won’t require the implementation anymore but allow

kernel.Bind<ISomeObjectFactory>().ToFactory();  // or maybe .AsFactory();