AddDbContext was called with configuration, but the context type 'NewsContext' only declares a parameterless constructor?

问题

An error occurred while starting the application.

ArgumentException: AddDbContext was called with configuration, but the context type 'NewsContext' only declares a parameterless constructor. This means that the configuration passed to AddDbContext will never be used. If configuration is passed to AddDbContext, then 'NewsContext' should declare a constructor that accepts a DbContextOptions<NewsContext> and must pass it to the base constructor for DbContext.
Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.CheckContextConstructors<TContext>()

ArgumentException: AddDbContext was called with configuration, but the context type 'NewsContext' only declares a parameterless constructor. This means that the configuration passed to AddDbContext will never be used. If configuration is passed to AddDbContext, then 'NewsContext' should declare a constructor that accepts a DbContextOptions<NewsContext> and must pass it to the base constructor for DbContext.
Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.CheckContextConstructors<TContext>()
Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.AddDbContext<TContextService, TContextImplementation>(IServiceCollection serviceCollection, Action<IServiceProvider, DbContextOptionsBuilder> optionsAction, ServiceLifetime contextLifetime, ServiceLifetime optionsLifetime)
Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.AddDbContext<TContextService, TContextImplementation>(IServiceCollection serviceCollection, Action<DbContextOptionsBuilder> optionsAction, ServiceLifetime contextLifetime, ServiceLifetime optionsLifetime)
Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.AddDbContext<TContext>(IServiceCollection serviceCollection, Action<DbContextOptionsBuilder> optionsAction, ServiceLifetime contextLifetime, ServiceLifetime optionsLifetime)
News.Startup.ConfigureServices(IServiceCollection services) in Startup.cs
+
            services.AddDbContext<NewsContext>(options =>
Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)
Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()
Microsoft.AspNetCore.Hosting.Internal.WebHost.Initialize()
Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()

原因

NewsContext.cs

using Microsoft.EntityFrameworkCore;

namespace News.Service
{
    public class NewsContext : DbContext
    {
        public DbSet<News.Model.Entity.News> News { get; set; }
        public DbSet<News.Model.Entity.Banner> Banner { get; set; }
        public DbSet<News.Model.Entity.Comment> Comment { get; set; }
        public DbSet<News.Model.Entity.NewsClassify> NewsClassify { get; set; }
    }
}

Startup.cs

 public void ConfigureServices(IServiceCollection services)
        {
            ......

            services.AddDbContext<NewsContext>(options =>
            {
              options.UseSqlServer(Configuration.GetConnectionString("MsSqlConnection"), db => db.UseRowNumberForPaging());
            });

            ......

        }

该错误表示，如果通过AddDbContext配置NewsContext，那么需要添加一个DbContextOptions<NewsContext>类型参数的构造函数到NewsContext类。否则.net core 不能注入时带上AddDbContext添加的配置

解决方法

如上面所说，NewsContext类添加一个DbContextOptions<NewsContext>类型参数的构造函数

using Microsoft.EntityFrameworkCore;

namespace News.Service
{
    public class NewsContext : DbContext
    {
        public NewsContext(DbContextOptions<NewsContext> options) : base(options)
        {
        }

        public DbSet<News.Model.Entity.News> News { get; set; }
        public DbSet<News.Model.Entity.Banner> Banner { get; set; }
        public DbSet<News.Model.Entity.Comment> Comment { get; set; }
        public DbSet<News.Model.Entity.NewsClassify> NewsClassify { get; set; }
    }
}