How can I call Web API non-default constructor in new ASP.NET Core

Question

I created a project that uses Web API. I want to have the controller a constructor that accepts one argument for contsructor injection. In startup, I added this:

Startup.cs:

public class Startup
{
    // Other codes ommitted

    public void ConfigureServices(IServiceCollection services)
    {
        // Other codes ommitted

        // Add application services.
        services.AddTransient<IStudentDataAccess, StudentDataAccess>();
    }
}

In controller, I have this constructor:

ValuesController:

[Route("api/[controller]")]
public class ValuesController : Controller
{
    private IStudentDataAccess _studentDataAccess;

    public ValuesController(IStudentDataAccess studentDataAccess)
    {
        _studentDataAccess = studentDataAccess;
    }

    // GET: api/values
    [HttpGet]
    public IEnumerable<string> Get()
    {
        IStudentLogic studentLogic = new StudentLogic(_studentDataAccess);
        return new string[] { "value1", "value2" };
    }
}

But the constructor isn't called. I saw that for the previous ASP.NET versions, you can do this. How do I do this in the new ASP.NET? Or how do I pass a parameter when ASP.NET resolves IStudentDataAccess?

UPDATE:

Ok, I think the problem is my StudentDataAccess class has a non-default constructor that accepts an IDbContext. If I remove the constructor, it works. But I need to be able to pass an IDbContext in StudentDataAccess' constructor. I tried adding this in ConfigureServices:

services.AddTransient<IDbContext, TestDbContext>();

But the constructor isn't getting hit by the breakpoint. Here is the declaration of StudentDataAccess:

public class StudentDataAccess : IStudentDataAccess
{
    private IDbContext _context;

    public StudentDataAccess(IDbContext context)
    {
        _context = context;
    }
}

IDbContext is just an interface with no member.

It should work. Is your controller derived from Microsoft.AspNet.Mvc.Controller? Do you have more than one constructor? Do you have a minimal, complete and verifiable example? — Leonardo Herrera
– Leonardo Herrera, Commented Mar 5, 2016 at 15:42
@Leonardo Herrera: It doesn't unfortunately. I can't copy everything here but what I did is the defaut Web API template, created a new constructor with one parameter, put a breakpoint ont it, and then ran it. It doesn't hit the breakpoint. — g_b
– g_b, Commented Mar 5, 2016 at 16:21
If you put a breakpoint in the 1st line of your get method, does that get hit? — Muqeet Khan
– Muqeet Khan, Commented Mar 5, 2016 at 19:57

Leonardo Herrera · Accepted Answer · 2016-03-05 21:35:16Z

3

Something is not right with what you are showing.

To add your own transient service to your controller:

Define your interface
Define your class implementing your interface
Add services.AddTransient<IYourInterface, YourClass>(); to ConfigureServices in Startup.cs
Add a constructor to your controller that receives your service as a parameter. Make sure you have only one constructor.

My minimal working example:

Startup.cs:

using Microsoft.AspNet.Builder;
using Microsoft.AspNet.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

namespace WebApplication5
{
    public class Startup
    {
        public Startup(IHostingEnvironment env)
        {
        }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.AddTransient<IStudentDataAccess, StudentDataAccess>();
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            app.UseIISPlatformHandler();
            app.UseMvc();
        }
        public static void Main(string[] args) => WebApplication.Run<Startup>(args);
    }
}

ValuesController.cs:

using Microsoft.AspNet.Mvc;
using System.Collections.Generic;

namespace WebApplication5.Controllers
{
    [Route("api/[controller]")]
    public class ValuesController : Controller
    {
        private IStudentDataAccess _studentDataAccess;

        public ValuesController(IStudentDataAccess studentDataAccess)
        {
            _studentDataAccess = studentDataAccess;
        }

        [HttpGet]
        public IEnumerable<string> Get()
        {
            return new string[] { "value", _studentDataAccess.Hello() };
        }
    }
}

StudentDataAccess.cs:

namespace WebApplication5
{
    public interface IStudentDataAccess
    {
        string Hello();
    }

    public class StudentDataAccess : IStudentDataAccess
    {
        public string Hello()
        {
            return "Hello from Service";
        }
    }
}

And the result:

["value","Hello from Service"]

answered Mar 5, 2016 at 21:35

Leonardo Herrera

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6 Comments

g_b Over a year ago

I tried out your example and it does work. In my project, the Interface and the concrete class is in both a different assembly (but StudentDataAccess surely implements IStudentDataAccess). When this is the setup, the constructor isn't hit. But when I move them to the same assembly (just after Startup.cs), it works. Any idea why this is? I have the same code, just different assembly.

g_b Over a year ago

Ok, I think the problem is my StudentDataAccess class has a non-default constructor that accepts an IDbContext. If I remove the constructor, it works. But I need to be able to pass an IDbContext in StudentDataAccess' constructor. Can you tell me how?

g_b Over a year ago

I created a new project and it works even with StudentDataAccess no default constructor. I need to track down what is happening on my original problem, but as you said, it just works.

Leonardo Herrera Over a year ago

I think you need to use configuration options for your service. That's probably another question.

Balanjaneyulu K Over a year ago

Can we have two constructors?

|

Hammer · Accepted Answer · 2017-03-18 12:14:07Z

1

Try putting FromServicesAttribute, for example:

public ValuesController([FromServices]IStudentDataAccess studentDataAccess)
{
    _studentDataAccess = studentDataAccess;
}

this worked for me :)

answered Mar 18, 2017 at 12:14

Hammer

156 bronze badges

1 Comment

Fred Over a year ago

The FromServices attribute is not needed in the constructor. It is used in actions.

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