How to use Simple Injector in ASP.NET Core MVC

Dependency injection (also known as DI) is a design pattern in which an object receives the objects it depends on rather than creating them directly. Dependency injection facilitates loose coupling and promotes testability and easy maintenance. It allows you to change your implementations without changing the classes or interfaces that leverage those implementations.

Support for dependency injection is included in ASP.NET Core. As a result, you can inject both framework and application services into your classes rather than depend on tightly coupled components.

Simple Injector is a free, fast, and flexible inversion of control library that is easy to use and configure. It supports .NET Core, Xamarin, Mono, and Universal apps and is easily integrated with Web API, MVC, WCF, ASP.NET Core, etc.

This article talks about how we can leverage Simple Injector to implement dependency injection in ASP.NET Core MVC.

To work with the code examples provided in this article, you should have Visual Studio 2019 installed in your system. If you don’t already have a copy, you can download Visual Studio 2019 here.

Create an ASP.NET Core MVC project in Visual Studio 2019

First off, let’s create an ASP.NET Core MVC project in Visual Studio 2019. Following these steps will create a new ASP.NET Core MVC project in Visual Studio 2019 using .NET 5.

1. Launch the Visual Studio IDE.
2. Click on “Create new project.”
3. In the “Create new project” window, select “ASP.NET Core Web App (Model-View-Controller)” from the list of templates displayed.
4. Click Next.
5. In the “Configure your new project” window, specify the name and location for the new project.
6. Optionally check the “Place solution and project in the same directory” check box, depending on your preferences.
7. Click Next.
8. In the “Additional Information” window shown next, select .NET 5.0 as the target framework from the drop-down list at the top. Leave the “Authentication Type” as “None” (default).
9. Ensure that the check boxes “Enable Docker,” “Configure for HTTPS,” and “Enable Razor runtime compilation” are unchecked as we won’t be using any of those features here.
10. Click Create.

We’ll use this project to work with Simple Injector in the subsequent sections of this article. Now follow the steps outlined below to create additional controller(s) in your project:

1. Right-click on the Controllers solution folder.
2. Select Add -> Controller.
3. In the “Add New Scaffolded Item” dialog, select API as the template (by default MVC will be selected).
4. Select the item “API Controller with read/write actions.”
5. Click Add.
6. In the “Add New Item” dialog shown next, specify a name for your new controller
7. Click Add

Install Simple Injector NuGet package

If you have successfully created an ASP.NET Core MVC project, the next thing you should do is add the necessary NuGet packages to your project. To do this, select the project in the Solution Explorer window, right-click and select “Manage NuGet Packages….” In the NuGet Package Manager window, search for the following package and install it.

SimpleInjector.Integration.AspNetCore.Mvc

Alternatively, you can install the package via the NuGet Package Manager Console as shown below.

PM> Install-Package SimpleInjector.Integration.AspNetCore.Mvc

Simple Injector demo in ASP.NET Core MVC

To demonstrate dependency injection using Simple Injector, we’ll first create a service, but only with minimal implementation for the sake of simplicity. Create a class named DemoService and insert the following code:

namespace SimpleInjectorDemo
{
    public class DemoService: IDemoService
    {
        public string GetMessage()
        {
            return "Inside GetMessage method...";
        }
    }
}

The DemoService class implements the IDemoService interface. Here is the IDemoService interface for your reference:

namespace SimpleInjectorDemo
{
    public interface IDemoService
    {
        public string GetMessage();
    }
}

Configure Simple Injector in ASP.NET Core MVC

Next write the following code snippet in the ConfigureServices method of the Startup class. This sets up basic configuration for integrating Simple Injector with ASP.NET Core MVC.

services.AddSimpleInjector(container, options =>
{
    options.AddAspNetCore()
    .AddControllerActivation();
});

You can register your service in the ConfigureServices method of the Startup class using any of the three methods of the LifeStyle class:

• Singleton
• Scoped
• Transient

You can write the following code snippet in the ConfigureServices method to register an instance of DemoService.

container.Register<IDemoService, DemoService>(Lifestyle.Singleton);

Here is the complete code of the ConfigureServices method for your reference:

public void ConfigureServices(IServiceCollection services)
{
   services.AddControllersWithViews();
   services.AddSimpleInjector(container, options =>
   {
        options.AddAspNetCore()
        .AddControllerActivation();
   });
  container.Register<IDemoService, DemoService>(Lifestyle.Singleton);
}

Now add the following code snippet in the Configure method of the Startup class:

app.UseSimpleInjector(container);

Here is the complete code of the Configure method for your reference:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
      app.UseSimpleInjector(container);
      if (env.IsDevelopment()) {
            app.UseDeveloperExceptionPage();
      }
      else {
            app.UseExceptionHandler("/Home/Error");
      }
      app.UseStaticFiles();
      app.UseRouting();
      app.UseAuthorization();
      app.UseEndpoints(endpoints = >{
            endpoints.MapControllerRoute(
            name: "default", pattern:
            "{controller=Home}/{action=Index}/{id?}");
      });
}

Update the Index.cshtml file in ASP.NET Core MVC

Replace the default source code of the Index.cshtml file with the following code:

@{
    ViewData["Title"] = "Home Page";
}
<div class="text-center">
    <h1>"@ViewBag.Message"</h1>
</div>

Use dependency injection in the controller in ASP.NET Core MVC

We’ll now take advantage of constructor injection in the HomeController to retrieve an instance of the DemoService. The following code snippet illustrates how you can achieve this.

public class HomeController : Controller
{
   private IDemoService _demoService;
   public HomeController(IDemoService demoService)
   {
       _demoService = demoService;
   }
   public IActionResult Index()
   {
      ViewBag.Message = _demoService.GetMessage();
      return View();
   }
  //Other action methods
}

Here the Message property of the ViewBag instance is assigned the text message that is retrieved using a call to the GetMessage method of the DemoService class.

When you execute the application, you’ll observe the text message “Inside GetMessage method…” displayed in the web browser as shown in Figure 1 below.

IDG

Simple Injector eliminates the usual complexities of an inversion of control library and simplifies how you can use dependency injection in ASP.NET Core.

Finally, note that optionally you could make a call to the Verify method of the Container class in the Configure method as shown below:

container.Verify();

In this case, the Verify method would throw an exception if there is any error in the configuration you specified.