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Сабака Чабака
Сабака Чабака

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Your first ASP.NET App: Dependency Injection

#webdev #csharp #aspnet #tutorial

Hello! In this tutorial we will build your own ASP.NET Service Dependency Injection!

Requirements

  • IDE like Visual Studio/JetBrains Rider
  • .NET >= 10
  • Base C# skill
  • CPU (manually)

What is DI

DI (Dependency Injection) in C# is a design pattern where a class receives its dependencies from the outside instead of creating them itself.In short: it replaces hardcoded new operators with passing ready-to-use objects through the constructor.

Base DI types

Microsoft.Extensions.DependencyInjection has 3 base lifetime types. This is:

  • Singleton
  • Scoped
  • Transient

A singleton lives for the entire duration of the application's execution. Just - it's creating one for-all. Like AppDbContext or another.
A scoped lives for every HTTP-request as example. Every HTTP-Request has own Scope, scoped lives in scope like IUserRepository, IAuthService.
A transient lives for the shortest time. It is created every time it is requested from the DI container. Like IEmailSender, IValidator, or lightweight helper services.

Let's create our own DI

I wrote simple service like this

namespace ExampleProject;

public interface IMyService
{
    Task<string> GetHello(string name);
}

public class MyService : IMyService
{
    public async Task<string> GetHello(string name)
    {
        await Task.Delay(150);
        return $"Hello, {name}!";
    }
}
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Now let's add this to our controller (MyController) method public async Task<IActionResult> Get(string name)

Let's replace that method for this

[HttpGet]
    public async Task<IActionResult> Get(string name)
    {
        var start = DateTime.Now;
        var hello = await service.GetHello(name);
        var end = DateTime.Now;
        return Ok($"{hello} It took {end - start} to respond.");
    }
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So, we need to replace class declaration to add constructor to this

public class MyController(IMyService service) : ControllerBase
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Now let's add our Service to DI

using ExampleProject;
using Scalar.AspNetCore;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenApi();
builder.Services.AddControllers();

builder.Services.AddScoped<IMyService, MyService>();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();
}

app.MapControllers();

app.Run();
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Testing

Now run, and open in browser localhost:[your port]/scalar

Press [Test Request], and press [Send]

And now listened: Hello, sabaka! It took 00:00:00.1608323 to respond.

In next chapters we learn Databases, Clean Architecture and more

Top comments (2)

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raknaos profile image
Raknaos •

One thing worth adding for readers of this series: with a primary constructor the injected service silently becomes a field, so the moment someone registers a singleton that takes a scoped dependency, the scoped thing gets captured once and lives for the whole process. Nothing complains at startup — the symptom is state leaking between unrelated requests, and it's miserable to debug later. A sentence on mismatched lifetimes would save people hours.

Small nit on the measurement: DateTime.Now is a fragile way to time a handler, because a clock sync or a laptop sleep/resume can make the delta negative or absurd. Stopwatch is free and trustworthy. Otherwise a clean walkthrough, and the Scalar step makes "actually send a request" much less abstract than the usual intro.

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Сабака Чабака •

Thanks, i will make fixes in next chapter of this series. Thanks for attention : )