.NET 8 Web API依赖注入错误：无法解析ICommanderRepo服务

发送GET请求时触发异常，错误信息如下：

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware
执行请求时发生未处理的异常。
System.InvalidOperationException: 尝试激活'Commander.Controllers.CommandsController'时无法解析'Commander.Data.ICommanderRepo'类型的服务

相关代码 #

CommandsController.cs #

namespace Commander.Controllers
{
    // Api/commands
    [Route("api/commands")]
    [ApiController]
    public class CommandsController : ControllerBase
    {
        private readonly ICommanderRepo _repository; //Dependency injected value

        public CommandsController(ICommanderRepo repository)  //Inject dependency
        {
            _repository = repository;
        } 

        //private readonly MockCommanderRepo _repository = new MockCommanderRepo();
        
        //GET api/commands 
        [HttpGet]
        public ActionResult <IEnumerable<Command>> GetAllCommands()
        {
            var commandItems = _repository.GetAppCommands();

            return Ok(commandItems);
        }

        //GET api/commands/{id}
        [HttpGet("{id}")]
        public ActionResult <Command> GetCommandById(int id)
        {
            var commandItem = _repository.GetCommandById(id);
            return Ok(commandItem);
        }
    }
}

ICommanderRepo.cs #

namespace Commander.Data
{
    public interface ICommanderRepo
    {
        IEnumerable<Command> GetAppCommands();
        Command GetCommandById(int id);
    }
}

MockCommanderRepo.cs #

namespace Commander.Data
{
    public class MockCommanderRepo : ICommanderRepo
    {
        public IEnumerable<Command> GetAppCommands()
        {
            var commands = new List<Command> {
                new Command{Id=0, HowTo="Boil an egg", Line="Boil water", Platform="Kettle & Pan"},
                new Command{Id=1, HowTo="Cut bread", Line="Get a knife", Platform="Knife & chopping board"},
                new Command{Id=2, HowTo="Make cup of tea", Line="Place teabag in cup", Platform="Kettle & cup"}
            };

            return commands;
        }

        public Command GetCommandById(int id)
        {
            return new Command{Id=0, HowTo="Boil an egg", Line="Boil water", Platform="Kettle & Pan"};
        }
    }
}

Command.cs #

namespace Commander.Models
{
    public class Command
    {
        public int Id { get; set; }
        public string HowTo { get; set; } = default!;
        public string Line { get; set; } = default!;
        public string Platform { get; set; }  = default!;
    }
}

Startup.cs #

namespace Commander
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();
            
            services.AddScoped<ICommanderRepo, MockCommanderRepo>(); //Dependency injection
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            app.UseRouting();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

Program.cs #

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

问题原因 #

你使用的是.NET 8，而.NET 6及以后版本默认采用顶级语句的Program.cs作为启动入口，不再自动调用Startup类的ConfigureServices和Configure方法。你在Startup.cs中注册的依赖注入服务根本没有被执行，所以DI容器找不到ICommanderRepo的实现。

解决方法 #

将依赖注入注册代码移到Program.cs的服务配置部分，修改后的Program.cs如下：

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllers();
// 注册ICommanderRepo的实现
builder.Services.AddScoped<Commander.Data.ICommanderRepo, Commander.Data.MockCommanderRepo>();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();
app.UseAuthorization();
app.MapControllers();

app.Run();

或者，如果你想继续使用Startup类，需要在Program.cs中显式调用Startup的配置方法：

var builder = WebApplication.CreateBuilder(args);
var startup = new Startup(builder.Configuration);
startup.ConfigureServices(builder.Services);

var app = builder.Build();

startup.Configure(app, app.Environment);

app.Run();

推荐第一种方案，符合.NET 6+的官方推荐写法。

内容的提问来源于stack exchange，提问作者Clarissa