实体框架启用迁移null参考异常

Question

我无法在我的Visual Studio 2013项目中正常进行Code First迁移。 我正在使用EF 6.1和C＃.Net 4.5。 当我尝试运行Enable-Migrations或Enable-Migrations -ContextTypeName DataContext ，出现以下错误：

PM> Enable-Migrations -ContextTypeName DataContext -Force
Checking if the context targets an existing database...
System.NullReferenceException: Object reference not set to an instance of an object.
   at Infrastructure.DataAccess.DataContext..ctor() in c:\Users\mblack\Documents\Visual Studio 2013\Projects\BluemoonApp\Infrastructure\DataAccess\DataContext.cs:line 16
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Data.Entity.Infrastructure.DbContextInfo.CreateInstance()
   at System.Data.Entity.Infrastructure.DbContextInfo..ctor(Type contextType, DbProviderInfo modelProviderInfo, AppConfig config, DbConnectionInfo connectionInfo, Func1 resolver)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, Boolean calledByCreateDatabase)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
   at System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldInitialCreate(String language, String rootNamespace)
   at System.Data.Entity.Migrations.EnableMigrationsCommand.<>c__DisplayClass2.<.ctor>b__0()
   at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
Object reference not set to an instance of an object.

我不知道为什么这行不通。 我尝试通过Nuget软件包管理器控制台重新安装EntityFramework，并删除了Migrations文件夹，然后尝试“重新启用”迁移。

如果您需要其他任何信息，请告诉我，谢谢

编辑：添加DataContext.cs和DataContextInitializer.cs

namespace Infrastructure.DataAccess
{
    public class DataContext : DbContext
    {
        public DataContext()
            : base(ConfigurationManager.ConnectionStrings["BluemoonDev"].ConnectionString)
        {
            Configuration.ProxyCreationEnabled = false;
        }

        public virtual DbSet<Task> Tasks { get; set; }
        public virtual DbSet<Test> Tests { get; set; }
        public virtual DbSet<Core.Models.Configuration> Configurations { get; set; }
        public virtual DbSet<Subscriber> Subscribers { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            //modelBuilder.Entity<Task>().HasKey(t => t.TaskId);


            base.OnModelCreating(modelBuilder);
        }
    }
}

namespace Infrastructure.DataAccess
{
    public class DataContextInitializer : DropCreateDatabaseIfModelChanges<DataContext>
    {
        protected override void Seed(DataContext context)
        {
            // . . . Creating lists for each Entity
            // . . . Adding those items to context and saving changes
        }
    }
}

Answer 1

如果要使用迁移，则需要将初始化程序更改为MigrateDatabaseToLatestVersion或NullDatabaseInitializer。 https://msdn.microsoft.com/zh-CN/data/jj591621.aspx#initializer

Answer 2

问题是您需要在上下文的构造函数中设置连接字符串。

Answer 3

我遇到这种类型的问题，可能是您正在尝试为数据库中的列导入空值。 但是该字段不接受空值。

Answer 4

如果选择了单个启动项目，请签入解决方案属性。