dependency injection with multiple class constructor

Question

I have the class as:

public class ServiceUtility
{
    private IHostingEnvironment _environment;
    public ServiceUtility(IHostingEnvironment environment)
    {
        _environment = environment;
    }
    public string GetFilePath(string fileName)
    {
        return _environment.WebRootFileProvider.GetFileInfo(fileName)?.PhysicalPath;
    }
}

Using in Startup.cs

services.AddScoped<ServiceUtility>();

Using ServiceUtility in another class:

public class LevelUp
{
        private readonly ServiceUtility _serviceUtility;
        public LevelUp(ServiceUtility serviceUtility)
        {
            _serviceUtility = serviceUtility;
        }
        public LevelUp(string input)
        {
          //here the _serviceUtility is null
        }
        public int UserId { get; set; }
        public int Level { get; set; }
}

From controller I am using as:

var object = userList.Select(x => new LevelUp(x.Description));

When I am using the _serviceUtility in Levelup(string input) constructor, the value is always null.

I have also added in Startup.cs class:

services.AddScoped<LevelUp>();

What I am doing wrong here, what is the proper way of dependency injection with multiple constructors?

Steven · Accepted Answer · 2020-07-04 06:54:04Z

2

In your controller you are (manually) calling the LevelUp constructor with the string parameter (ctor(string)) and not the ctor(ServiceUtility) constructor. This LevelUp class is not created by the DI Container, you are creating it yourself. If you wish to supply ServiceUtility to LevelUp, you should pass both the string and ServiceUtility through the same constructor:

public class LevelUp
{
    private readonly ServiceUtility _serviceUtility;

    public LevelUp(ServiceUtility serviceUtility, string input)
    {
        _serviceUtility = serviceUtility;
    }

    public int UserId { get; set; }
    public int Level { get; set; }
}

From within your controller you then have to pass ServiceUtility as well to LevelUp's constructor:

public class HomeController : Controller
{
    private readonly ServiceUtility serviceUtility;

    public HomeController(ServiceUtility serviceUtility)
    {
        this.serviceUtility = serviceUtility;
    }

    public object Index(List userList)
    {
        var object =
            userList.Select(x => new LevelUp(this.serviceUtility, x.Description));
    }
}

edited Jul 4, 2020 at 6:54

answered Jul 4, 2020 at 6:48

Steven

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5 Comments

Rasik Over a year ago

So, there isn't way of doing that by the framework itself? Why do we need to supply by ourselves?

Steven Over a year ago

It's one way or the other: either you leave the construction or the object completely to the DI infrastructure (which means a DI Container or a hand-written Composition Root) -or- you create the object using the new statement. A DI Container can't magically change your compiled code.

Steven Over a year ago

Also note that when when it comes to classes that contain application behavior and are injected with dependencies, it is considered to be a bad thing for them to have multiple constructors.

Rasik Over a year ago

Thank you @steven i will certainly note that. and I will try to refactor the multiple constructors.

Steven Over a year ago

On the other hand, your LevelUp seems more a newable than an injectable. For data-centric objects it's okay to have multiple constructors.

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