c# - Entity Framework Core - 迁移 - 没有为此对象定义无参数构造函数

Question

我在 Visual Studio 2017 中使用最新的 .Net Core 和 EF Core。我创建了一个模型，它运行良好。此后，我进行了一些修改，并在尝试添加新迁移时遇到以下错误:

Build succeeded.
  0 Warning(s)
  0 Error(s)

Time Elapsed 00:00:09.08
System.MissingMethodException: No parameterless constructor defined for this object.
  at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck)
  at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
  at System.Activator.CreateInstance(Type type, Boolean nonPublic)
  at System.Activator.CreateInstance(Type type)
  at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass12_0.<FindContextTypes>b__3()
  at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
  at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
  at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
  at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
  at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
  at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
No parameterless constructor defined for this object.

我比较了自上次 checkin 以来的代码，并注释掉了某些代码块，以查看错误是否仍然存在，无论我注释掉什么，它仍然失败并出现相同的错误。

问题:有没有办法获得更多关于什么类型没有无参数构造函数的详细信息？或者甚至从 VS 中运行它并可能得到一个断点？

更新:根据一些评论，这里有一些代码。

对于 DbContext 覆盖

public class AlmanacDb : IdentityDbContext<ApplicationUser, ApplicationRole, int> {

  private readonly ILogger logger;

  public AlmanacDb(DbContextOptions<AlmanacDb> options, ILoggerFactory loggerFactory) : base(options) {
    this.logger = loggerFactory.CreateLogger<AlmanacDb>();
  }

  protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) {
    optionsBuilder.UseSqlServer("Server=.\\SQLExpress;Database=Almanac;Trusted_Connection=True;MultipleActiveResultSets=True;");
  }

  ...
}

我确实有一个 IDbContextFactory 但它没有在任何地方被引用并且在没有引用的情况下工作。根据第二条评论中提供的链接，不确定这是否是一个问题。如果没记错的话，只要 IDbContextFactory 在解决方案中，它就应该找到它？

public class AlmanacDbFactory : IDbContextFactory<AlmanacDb> {

  private IConfigurationRoot configuration;
  private readonly ILoggerFactory loggerFactory;

  public AlmanacDbFactory(ILoggerFactory loggerFactory) {
    var builder = new ConfigurationBuilder()
     .SetBasePath(System.AppContext.BaseDirectory)
     .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);

    this.configuration = builder.Build();
    this.loggerFactory = loggerFactory;
  }

  public AlmanacDb Create(DbContextFactoryOptions options) {
    var optionsBuilder = new DbContextOptionsBuilder<AlmanacDb>();
    optionsBuilder.UseSqlServer(
      configuration.GetConnectionString("AlmanacSQL"), m => { m.EnableRetryOnFailure(); }
    );

    return new AlmanacDb(optionsBuilder.Options, loggerFactory);
  }
}

我的Startup.cs类

public void ConfigureServices(IServiceCollection services) {
  try {
    services.AddDbContext<AlmanacDb>(options =>
      options.UseSqlServer(Configuration.GetConnectionString("AlmanacSQL"))
    );
    services.AddScoped(typeof(IRepository<>), typeof(Repository<>));

    services.AddIdentity<ApplicationUser, ApplicationRole>()
      .AddEntityFrameworkStores<AlmanacDb, int>()
      .AddDefaultTokenProviders();

    services.AddScoped<SignInManager<ApplicationUser>, AvantiaSignInManager<ApplicationUser>>();

    services.AddAuthorization(x => {
      x.AddPolicy("EmployeeOnly", p => p.RequireClaim("EmployeeNumber"));
    });

    services.AddMvc();
  } catch (Exception e) {
    Console.WriteLine(e.ToString());
    throw;
  }
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, AlmanacDb context) {
  ... // Nothing actually touches the AlmanacDb within this code so I left it out
}

更新 2:解决方案 看到我正在学习 ASP.Net Core 我正在阅读大量网站(包括 learn.microsoft.com)，我在一个点上输入了 IDbContextFactory 代码，如上所示。我从我的代码中删除了它，错误消失了，一切都构建并创建了迁移。

我打算将@alessalessio 标记为答案，因为我假设(还没有测试过)在 AlmanacDbFactory 构造函数中取出 ILoggerFactory loggerFactory 依赖项就可以了技巧也是如此。

Answer 1

设计时工具会尝试自动查找您的应用程序如何创建 DbContext 类型的实例。如果 EF 找不到合适的方法来初始化您的 DbContext，您可能会遇到此错误。

选项: 1-要么创建一个无参数的构造函数

  public AlmanacDb() { }
  protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
  {           
       optionsBuilder.UseSqlServer(_connString);
  }

  private readonly string _connString = "<your conn string>";

2-

public AlmanacDb Create()
{
     var optionsBuilder = new DbContextOptionsBuilder<AlmanacDb>();
     optionsBuilder.UseSqlServer(connectionString);

     return new AlmanacDb(optionsBuilder.Options);
}

https://learn.microsoft.com/en-us/ef/core/miscellaneous/configuring-dbcontext