ASP.NET Core Web API服务无法构造的依赖注入问题排查

错误信息 #

System.AggregateException: 'Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: Middleware.Core.ApplicationService.Services.IProductService Lifetime: Transient ImplementationType: Middleware.Core.ApplicationService.Queries.ProductService': Unable to resolve service for type 'Middleware.Infra.DataAccessLayer.ApplicationDbContextFactory' while attempting to activate 'Middleware.Core.ApplicationService.Queries.ProductService'.)'

错误原因 #

依赖注入容器无法找到你自定义的ApplicationDbContextFactory的注册项。你调用的AddDbContextFactory方法，注册的是Entity Framework内置的IDbContextFactory<ApplicationDbContext>实现，而非你自己编写的ApplicationDbContextFactory类。

解决方案 #

方案一：注册自定义工厂类 #

在依赖注入配置中添加自定义工厂的注册，同时规范DbContext的泛型参数写法：

1. 调整ApplicationDbContext构造函数（使用泛型版本的DbContextOptions，符合EF标准）：

public class ApplicationDbContext : DbContext
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)
    {
    }
    public DbSet<Product> Products { get; set; }
}

2. 同步修改自定义工厂类的参数：

public class ApplicationDbContextFactory : IDbContextFactory<ApplicationDbContext>
{
    private readonly DbContextOptions<ApplicationDbContext> _options;
    public ApplicationDbContextFactory(DbContextOptions<ApplicationDbContext> options)
    {
        _options = options;
    }
    public ApplicationDbContext CreateDbContext()
    {
        return new ApplicationDbContext(_options);
    }
}

3. 更新依赖注入配置：

var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
// 注册泛型DbContextOptions
builder.Services.AddSingleton<DbContextOptions<ApplicationDbContext>>(sp =>
    new DbContextOptionsBuilder<ApplicationDbContext>()
        .UseSqlServer(connectionString)
        .Options);
// 注册自定义工厂
builder.Services.AddTransient<ApplicationDbContextFactory>();
// 保留原有服务注册
builder.Services.AddTransient<IProductService, ProductService>();

方案二：使用EF内置的IDbContextFactory #

如果不需要自定义工厂逻辑，直接依赖EF原生的IDbContextFactory<ApplicationDbContext>即可，无需自己编写工厂类：

1. 修改ProductService的依赖项：

public class ProductService : IProductService
{
    private readonly IDbContextFactory<ApplicationDbContext> _contextFactory;
    public ProductService(IDbContextFactory<ApplicationDbContext> contextFactory)
    {
        _contextFactory = contextFactory;
    }
    public async Task<IEnumerable<Product>> GetAllAsync()
    {
        using (ApplicationDbContext context = _contextFactory.CreateDbContext())
        {
            IEnumerable<Product> products = await context.Products.ToListAsync();
            return products.Select(p => new Product() { Id = p.Id, ProductName = p.ProductName, Price = p.Price, Qty = p.Qty });
        }
    }
}

2. 简化依赖注入配置：

var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
builder.Services.AddDbContextFactory<ApplicationDbContext>(options => options.UseSqlServer(connectionString));
builder.Services.AddTransient<IProductService, ProductService>();

方案三：直接注入ApplicationDbContext（推荐） #

如果你的服务是瞬态或范围生命周期，可直接注入ApplicationDbContext，由DI容器自动管理其生命周期：

1. 修改ProductService：

public class ProductService : IProductService
{
    private readonly ApplicationDbContext _context;
    public ProductService(ApplicationDbContext context)
    {
        _context = context;
    }
    public async Task<IEnumerable<Product>> GetAllAsync()
    {
        IEnumerable<Product> products = await _context.Products.ToListAsync();
        return products.Select(p => new Product() { Id = p.Id, ProductName = p.ProductName, Price = p.Price, Qty = p.Qty });
    }
}

2. 依赖注入配置：

var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
builder.Services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(connectionString));
builder.Services.AddTransient<IProductService, ProductService>();

内容的提问来源于stack exchange，提问作者Amir Monfared