AddTransient 不会为每个请求创建一个新实例吗？

Question

I am toying with the dependency injection design pattern in which there exists a transient lifetime which creates a new object on each request. However, when I try to apply it to C#'s startup example, WebApi, it doesn't seem to work as expected. For example, it generates the same random number while the original generates a random integer on each request. I have two questions that

1. Why doesn't it run as I expect?
2. How is the container aware of WeatherForecastController class needs an instance, whose type is of IWeatherForecast , which is used in IEnumerable<IWeatherForecast> Get() method? Otherwise, I get the following error,

System.InvalidOperationException: Unable to resolve service for type 'WebApplication2.IWeatherForecast' while attempting to activate 'WebApplication2.Controllers.WeatherForecastController'.

WeatherForecastController.cs

using Microsoft.AspNetCore.Mvc;

namespace WebApplication2.Controllers;

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    private readonly IWeatherForecast _weatherForecast;
    
    public WeatherForecastController(IWeatherForecast weatherForecast)
    {
        _weatherForecast = weatherForecast;
    }

    [HttpGet(Name = "GetWeatherForecast")]
    public IEnumerable<IWeatherForecast> Get()
    {
        var returnValue= Enumerable.Range(1, 5).Select(index => _weatherForecast).ToArray();

        return returnValue;
        
        /*
         nearly original one, working as expected
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
            {
                Date = DateTime.Now.AddDays(index),
                TemperatureC = Random.Shared.Next(-20, 55),
                Summary = "testing"
            })
            .ToArray();
        */
        
    }
}

WeatherForecast.cs

namespace WebApplication2;

public class WeatherForecast : IWeatherForecast
{
    public WeatherForecast()
    {
        Date = DateTime.Now.AddDays(0);
        TemperatureC = Random.Shared.Next(-20, 55);
        Summary = "testing";
    }

    public DateTime Date { get; set; }
    public int TemperatureC { get; set; }
    public string? Summary { get; set; }
    public int TemperatureF() => 32 + (int)(TemperatureC / 0.5556);

}

public interface IWeatherForecast
{
    public DateTime Date { get; set; }
    public int TemperatureC { get; set; }
    public string? Summary { get; set; } 
    public int TemperatureF();
}

Program.cs, registering only

...
builder.Services.AddTransient<IWeatherForecast, WeatherForecast>();
...

Answer 1

It works as expected indeed.

Two requests give you different results:

Request 1:

[
  {
    "date": "2022-10-09T09:01:36.408982+02:00",
    "temperatureC": 4,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:01:36.408982+02:00",
    "temperatureC": 4,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:01:36.408982+02:00",
    "temperatureC": 4,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:01:36.408982+02:00",
    "temperatureC": 4,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:01:36.408982+02:00",
    "temperatureC": 4,
    "summary": "testing"
  }
]

Request 2:

[
  {
    "date": "2022-10-09T09:02:01.6052197+02:00",
    "temperatureC": 46,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:02:01.6052197+02:00",
    "temperatureC": 46,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:02:01.6052197+02:00",
    "temperatureC": 46,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:02:01.6052197+02:00",
    "temperatureC": 46,
    "summary": "testing"
  },
  {
    "date": "2022-10-09T09:02:01.6052197+02:00",
    "temperatureC": 46,
    "summary": "testing"
  }
]

With transient lifetime a new object is created each time you request it, and this is exactly what is happening. You are requesting it only once in the constructor so only one instance is created each time one controller instance is created. You see the same object repeated 5 times because you use the same instance on the line:

var returnValue = Enumerable.Range(1, 5).Select(index => _weatherForecast).ToArray();

You are not requesting IWeatherForecast on the above line, you are using the instance you requested on the constructor.

If WeatherForecastController has a dependency of IWeatherForecast as stated on its constructor. IWeatherForecast must be registered into the container

builder.Services.AddTransient<IWeatherForecast, WeatherForecast>();

Otherwise you get the error:

System.InvalidOperationException: Unable to resolve service for type 'WebApplication2.IWeatherForecast' while attempting to activate 'WebApplication2.Controllers.WeatherForecastController'

The dependency injection infrastructure knows about the controller because you registered it using:

builder.Services.AddControllers();

And it knows it depends on IWeatherForecast by inspecting the controller constructor using reflection.