Dependency injection in ASP.NET Core

Overview of dependency injection

A dependency is an object that another object depends on. Examine the following MyDependency class with a WriteMessage method that other classes depend on:

public class MyDependency
{
    public void WriteMessage(string message)
    {
        Console.WriteLine($"MyDependency.WriteMessage called. Message: {message}");
    }
}

A class can create an instance of the MyDependency class to make use of its WriteMessage method. In the following example, the MyDependency class is a dependency of the IndexModel class:

public class IndexModel : PageModel
{
    private readonly MyDependency _dependency = new MyDependency();

    public void OnGet()
    {
        _dependency.WriteMessage("IndexModel.OnGet");
    }
}

The class creates and directly depends on the MyDependency class. Code dependencies, such as in the previous example, are problematic and should be avoided for the following reasons:

To replace MyDependency with a different implementation, the IndexModel class must be modified.
If MyDependency has dependencies, they must also be configured by the IndexModel class. In a large project with multiple classes depending on MyDependency, the configuration code becomes scattered across the app.
This implementation is difficult to unit test.

Dependency injection addresses these problems through:

The use of an interface or base class to abstract the dependency implementation.
Registration of the dependency in a service container. ASP.NET Core provides a built-in service container, IServiceProvider. Services are typically registered in the app’s Program.cs file.
Injection of the service into the constructor of the class where it’s used. The framework takes on the responsibility of creating an instance of the dependency and disposing of it when it’s no longer needed.

In the sample app, the IMyDependency interface defines the WriteMessage method:

public interface IMyDependency
{
    void WriteMessage(string message);
}

This interface is implemented by a concrete type, MyDependency:

public class MyDependency : IMyDependency
{
    public void WriteMessage(string message)
    {
        Console.WriteLine($"MyDependency.WriteMessage Message: {message}");
    }
}

The sample app registers the IMyDependency service with the concrete type MyDependency. The AddScoped method registers the service with a scoped lifetime, the lifetime of a single request. Service lifetimes are described later in this topic.

using DependencyInjectionSample.Interfaces;
using DependencyInjectionSample.Services;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.AddScoped<IMyDependency, MyDependency>();

var app = builder.Build();

In the sample app, the IMyDependency service is requested and used to call the WriteMessage method:

public class Index2Model : PageModel
{
    private readonly IMyDependency _myDependency;

    public Index2Model(IMyDependency myDependency)
    {
        _myDependency = myDependency;            
    }

    public void OnGet()
    {
        _myDependency.WriteMessage("Index2Model.OnGet");
    }
}

By using the DI pattern, the controller or Razor Page:

Doesn’t use the concrete type MyDependency, only the IMyDependency interface it implements. That makes it easy to change the implementation without modifying the controller or Razor Page.
Doesn’t create an instance of MyDependency, it’s created by the DI container.

The implementation of the IMyDependency interface can be improved by using the built-in logging API:

public class MyDependency2 : IMyDependency
{
    private readonly ILogger<MyDependency2> _logger;

    public MyDependency2(ILogger<MyDependency2> logger)
    {
        _logger = logger;
    }

    public void WriteMessage(string message)
    {
        _logger.LogInformation( $"MyDependency2.WriteMessage Message: {message}");
    }
}

The updated Program.cs registers the new IMyDependency implementation:

using DependencyInjectionSample.Interfaces;
using DependencyInjectionSample.Services;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.AddScoped<IMyDependency, MyDependency2>();

var app = builder.Build();

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