Calling non-static method using DbContext from .NET Core 2.0 Controller

Question

I've created the following helper method which uses a constructor to retrieve the DbContext.

namespace AzureSdkExamples.Helpers
{
    public class ComputeHelpers
    {
        private readonly ApplicationDbContext _dbContext;

        public ComputeHelpers(ApplicationDbContext db)
        {
            _dbContext = db;
        }

        public async Task<List<VirtualMachine>> GetClientVirtualMachines(int customerId)
        {
            var azureClient = _dbContext.CreateAzureClient(customerId);

            var virtualMachines = await azureClient.VirtualMachines.ListAsync();

            List<VirtualMachine> virtualMachineObjects = new List<VirtualMachine>();

            int vmCount = 0;

            foreach (var virtualMachine in virtualMachines)
            {
                virtualMachineObjects.Add(
                    new VirtualMachine
                    {
                        Id = vmCount,
                        VirtualMachineName = virtualMachine.ComputerName,
                        AzureVmId = virtualMachine.VMId
                    }
                );
                vmCount++;
            }


            return virtualMachineObjects;
        }
    }
}

The CreateAzureClient is essentially a db call to retrieve my personal subscriptions and tenant information from Azure. It returns an IAzure type.

I want to them call this non-static method from my Controller. I can't use a constructor on the method as this results in the following error:

InvalidOperationException: Unable to resolve service for type 'AzureSdkExamples.Helpers.ComputeHelpers' while attempting to activate 'AzureSdkExamples.Controllers.VirtualMachinesController'.

This is my controller when calling the helper method:

namespace AzureSdkExamples.Controllers
{
    [Route("api/[controller]")]
    public class VirtualMachinesController : Controller
    {
        private readonly ComputeHelpers _computeHelper;

        public VirtualMachinesController(ComputeHelpers computeHelpers)
        {
            _computeHelper = computeHelpers;
        }

        [HttpGet("{id}", Name = "GetAllVirtualMachines")]
        public async Task<JsonResult> GetAllVirtualMachines(int customerId)
        {
            var virtualMachines = await _computeHelper.GetClientVirtualMachines(customerId);

            return Json(virtualMachines);
        }
    }
}

I'm actually at a bit of a loss on what to do, mainly due to my own misunderstanding of how to appropriately chain these constructors/methods the correct way.

I'm trying to take the best approach to avoid potential memory leaks in a multi-user context.

How do I correctly call this helper method from the controller considering the use of DI in the helper?

Answer 1

You should register ComputeHelpers in the DI container.

You can do that in the Startup class in the ConfigureServices method, eg with AddTransient :

services.AddTransient<ComputeHelpers, ComputeHelpers>();

See the documentation for more details about the Dependency Injection in ASP.NET Core