ASP.NET Core 2 无法解析 Microsoft EntityFrameworkCore DbContext 类型的服务

Question

当我运行我的 asp.net core 2 项目时，我收到以下错误消息：

InvalidOperationException：尝试激活“ContosoUniversity.Service.Class.StudentService”时无法解析“Microsoft.EntityFrameworkCore.DbContext”类型的服务。

这是我的项目结构：

-- solution 'ContosoUniversity'
----- ContosoUniversity
----- ContosoUniversity.Model
----- ContosoUniversity.Service

IEntityService（相关代码）：

public interface IEntityService<T> : IService
 where T : BaseEntity
{
    Task<List<T>> GetAllAsync();      
}

IEntityService（相关代码）：

public abstract class EntityService<T> : IEntityService<T> where T : BaseEntity
{
    protected DbContext _context;
    protected DbSet<T> _dbset;

    public EntityService(DbContext context)
    {
        _context = context;
        _dbset = _context.Set<T>();
    }

    public async virtual Task<List<T>> GetAllAsync()
    {
        return await _dbset.ToListAsync<T>();
    }
}

实体：

public abstract class BaseEntity { 

}

public abstract class Entity<T> : BaseEntity, IEntity<T> 
{
    public virtual T Id { get; set; }
}

学生服务：

public interface IStudentService : IEntityService<Student>
{
    Task<Student> GetById(int Id);
}

学生服务：

public class StudentService : EntityService<Student>, IStudentService
{
    DbContext _context;

    public StudentService(DbContext context)
        : base(context)
    {
        _context = context;
        _dbset = _context.Set<Student>();
    }

    public async Task<Student> GetById(int Id)
    {
        return await _dbset.FirstOrDefaultAsync(x => x.Id == Id);
    }
}

SchoolContext：

public class SchoolContext : DbContext
{
    public SchoolContext(DbContextOptions<SchoolContext> options) : base(options)
    {
    }

    public DbSet<Course> Courses { get; set; }
    public DbSet<Enrollment> Enrollments { get; set; }
    public DbSet<Student> Students { get; set; }
}

最后是我的 Startup.cs 类：

public class Startup
{
    public Startup(IConfiguration configuration, IHostingEnvironment env, IServiceProvider serviceProvider)
    {
        Configuration = configuration;

        var builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true);


        Configuration = builder.Build();

    }

    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.AddDbContext<SchoolContext>(option =>
            option.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));


        services.AddScoped<IStudentService, StudentService>();

        services.AddMvc();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
            app.UseBrowserLink();
        }
        else
        {
            app.UseExceptionHandler("/Home/Error");
        }

        app.UseStaticFiles();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}

我应该怎么做才能解决这个问题？

Answer 1

StudentService 期望 DbContext 但容器不知道如何根据您当前的启动来解决它。

您需要将上下文显式添加到服务集合中

启动

services.AddScoped<DbContext, SchoolContext>();
services.AddScoped<IStudentService, StudentService>();

或者更新StudentService构造函数以明确期望容器知道如何解析的类型。

学生服务

public StudentService(SchoolContext context)
    : base(context)
{ 
    //...
}

Answer 2

我遇到了类似的错误，即

处理请求时发生未处理的异常。 InvalidOperationException：尝试激活“MyProjectName.Controllers.MyUsersController”时无法解析“MyProjectName.Models.myDatabaseContext”类型的服务。

Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, bool isDefaultParameterRequired)

后来我发现...我错过了以下行，即将我的数据库上下文添加到服务：

services.AddDbContext<yourDbContext>(option => option.UseSqlServer("Server=Your-Server-Name\\SQLExpress;Database=yourDatabaseName;Trusted_Connection=True;"));

下面是我在 Startup 类中定义的 ConfigureServices 方法：

 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.Configure<CookiePolicyOptions>(options =>
            {
                // This lambda determines whether user consent for non-essential 
                //cookies is needed for a given request.
                options.CheckConsentNeeded = context => true;
                options.MinimumSameSitePolicy = SameSiteMode.None;
            });

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
            services.AddDbContext<yourDbContext>(option => 
            option.UseSqlServer("Server=Your-Server-Name\\SQLExpress;Database=yourDatabaseName;Trusted_Connection=True;"));

                }
        ...
        ...
    }

基本上，当您从数据库生成模型类时，通过创建“新脚手架项目”并在脚手架过程中选择适当的数据库上下文，将所有数据库表映射到相应的模型类。 现在，您需要手动将您的数据库上下文注册为services 方法的services 参数的服务。

顺便说一句，您最好从配置数据中获取连接字符串，而不是硬编码连接字符串。我试图在这里保持简单。

Answer 3

如果 dbcontext 继承自 system.data.entity.DbContext 那么它会像这样添加

    services.AddScoped(provider => new CDRContext());

    services.AddTransient<IUnitOfWork, UnitOfWorker>();
    services.AddTransient<ICallService, CallService>();

Answer 4

当 options 参数为 null 或无法使用 GetConnectionString() 检索时，将引发此错误。

我遇到此错误是因为定义 ConnectionStrings 的 appsettings.json 文件末尾有一个额外的大括号 }。

愚蠢，但令人沮丧。