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

Question

When I run my asp.net core 2 projects I get the following error message:

InvalidOperationException: Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContext' while attempting to activate 'ContosoUniversity.Service.Class.StudentService'.

Here is my project structure:

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

IEntityService (related code) :

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

IEntityService (related code) :

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>();
    }
}

Entity :

public abstract class BaseEntity { 

}

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

IStudentService :

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

StudentService :

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; }
}

And finally here is my Startup.cs class :

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?}");
        });
    }
}

What should I do to resolve this problem?

Answer 1

StudentService expects DbContext but the container does not know how to resolve it based on your current startup.

You would need to either explicitly add the context to the service collection

Startup

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

Or update the StudentService constructor to explicitly expect a type the container knows how to resolve.

StudentService

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

Answer 2

I encountered a similar error ie

An unhandled exception occurred while processing the request. InvalidOperationException: Unable to resolve service for type 'MyProjectName.Models.myDatabaseContext' while attempting to activate 'MyProjectName.Controllers.MyUsersController'.

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

What I later figured out was... I was missing the following line ie adding my database context to services:

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

Here goes my ConfigureServices method defined in Startup class:

 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;"));

                }
        ...
        ...
    }

Basically, when you generated model classes from database, all your database tables were mapped into respective Model classes by creating the "New Scaffolded Item" and choosing the appropriate database context during the scaffolding procedure. Now, you need to manually register your database context as a service to the services parameter of ConfigureServices method.

Btw, rather than hard coding your connection string, you'll ideally pick it up from the configuration data. I have attempted to keep things simple here.

Answer 3

if dbcontext inherited from system.data.entity. DbContext then it woud be added like that

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

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

Answer 4

This error is thrown when the options argument is null or cannot be retrieved using GetConnectionString().

I had this error because my appsettings.json file that defines my ConnectionStrings had an extra curly bracket } at the end.

Stupid, but frustrating.