DbSet<T>.Add(T entity) throws System.InvalidOperationException: Sequence contains no matching element

Question

我有一个 EF6 Code First 模型，其实体如下所示：

[Table("Updates")]
public class Update
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Key]
    public Guid Id { get; set; }

    [Required]
    public string CreatedBy { get; set; }

    [Column(TypeName="varchar(max)")]
    public string Comments { get; set; }

    [Required]
    public DateTime CreatedTimestampUtc { get; set; }
}

我的数据库上下文 class 看起来像这样：

public class MyContext : DbContext, IUnitOfWork
{
    private static readonly string ConnectionString =   ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;

    public GlobalizationContext() : base(ConnectionString)
    {
    }

    public DbSet<Update> UpdatesDbSet { get; set; }

    public IQueryable<Update> Updates //IUnitOfWork implementation
    {
        get { return UpdatesDbSet; }
    }

    public async Task AddUpdateTokenAsync(Update updateToken)
    {
        var entity = UpdatesDbSet.Add(updateToken);
        var result = await SaveChangesAsync();
    }
}

当我点击 var entity = UpdatesDbSet.Add(updateToken); 行时，我生成了以下异常：

SetUpdateTokenAsync(Update) System.InvalidOperationException: Sequence contains no matching element
   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
   at System.Data.Entity.Utilities.DbProviderManifestExtensions.GetStoreTypeFromName(DbProviderManifest providerManifest, String name)
   at System.Data.Entity.ModelConfiguration.Configuration.Properties.Primitive.PrimitivePropertyConfiguration.ConfigureColumn(EdmProperty column, EntityType table, DbProviderManifest providerManifest)
   at System.Data.Entity.ModelConfiguration.Configuration.Properties.Primitive.PrimitivePropertyConfiguration.Configure(EdmProperty column, EntityType table, DbProviderManifest providerManifest, Boolean allowOverride, Boolean fillFromExistingConfiguration)
   at System.Data.Entity.ModelConfiguration.Configuration.Properties.Primitive.PrimitivePropertyConfiguration.<>c__DisplayClass4.<Configure>b__3(Tuple`2 pm)
   at System.Data.Entity.Utilities.IEnumerableExtensions.Each[T](IEnumerable`1 ts, Action`1 action)
   at System.Data.Entity.ModelConfiguration.Configuration.Properties.Primitive.PrimitivePropertyConfiguration.Configure(IEnumerable`1 propertyMappings, DbProviderManifest providerManifest, Boolean allowOverride, Boolean fillFromExistingConfiguration)
   at System.Data.Entity.ModelConfiguration.Configuration.Types.StructuralTypeConfiguration.ConfigurePropertyMappings(IList`1 propertyMappings, DbProviderManifest providerManifest, Boolean allowOverride)
   at System.Data.Entity.ModelConfiguration.Configuration.Types.EntityTypeConfiguration.ConfigurePropertyMappings(DbDatabaseMapping databaseMapping, EntityType entityType, DbProviderManifest providerManifest, Boolean allowOverride)
   at System.Data.Entity.ModelConfiguration.Configuration.Types.EntityTypeConfiguration.Configure(EntityType entityType, DbDatabaseMapping databaseMapping, DbProviderManifest providerManifest)
   at System.Data.Entity.ModelConfiguration.Configuration.ModelConfiguration.ConfigureEntityTypes(DbDatabaseMapping databaseMapping, ICollection`1 entitySets, DbProviderManifest providerManifest)
   at System.Data.Entity.ModelConfiguration.Configuration.ModelConfiguration.Configure(DbDatabaseMapping databaseMapping, DbProviderManifest providerManifest)
   at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
   at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
   at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
   at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
   at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
   at System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType)
   at System.Data.Entity.Internal.Linq.InternalSet`1.Initialize()
   at System.Data.Entity.Internal.Linq.InternalSet`1.get_InternalContext()
   at System.Data.Entity.Internal.Linq.InternalSet`1.ActOnSet(Action action, EntityState newState, Object entity, String methodName)
   at System.Data.Entity.Internal.Linq.InternalSet`1.Add(Object entity)
   at System.Data.Entity.DbSet`1.Add(TEntity entity)

看DbSet对象和Update对象，好像都是实例化的，不明白为什么.Add()需要找匹配元素。有人可以向我解释这里的问题是什么，我应该如何纠正它？

更新

根据请求，以下是我的 web.config 文件中的 EF 配置：

<configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</configSections>
. . .
<entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
        <parameters>
            <parameter value="mssqllocaldb" />
        </parameters>
    </defaultConnectionFactory>
    <providers>
        <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
</entityFramework>

...我意识到这是错误的，并将其更改为：

<entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
    <providers>
        <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
</entityFramework>

这并没有改变症状。我还尝试了建议的装饰器更改如下：

[Column("Comments", TypeName="varchar")]
public string Comments { get; set; }

...同样，行为没有区别。

更新 2

所以我想我应该早点提到这一点，我不知道它是否重要（但根据建议的发展方向，它看起来可能是），但我指的是预-现有数据库。有问题的 table 是这样创建的：

CREATE TABLE [dbo].[Updates](
    [Id] [uniqueidentifier] NOT NULL,
    [CreatedTimestampUtc] [datetime] NOT NULL,
    [Comments] [varchar](MAX) NULL,
    [CreatedBy] [nvarchar](260) NOT NULL,
 CONSTRAINT [PK_Updates] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

SET ANSI_PADDING ON
GO

ALTER TABLE [dbo].[Updates] ADD  CONSTRAINT [DF_Updates_Id]  DEFAULT (newid()) FOR [Id]
GO

ALTER TABLE [dbo].[Updates] ADD  CONSTRAINT [DF_Updates_CreatedTimestampUtc]  DEFAULT (getutcdate()) FOR [CreatedTimestampUtc]
GO

ALTER TABLE [dbo].[Updates] ADD  CONSTRAINT [DF_Updates_CreatedBy]  DEFAULT (suser_sname()) FOR [CreatedBy]
GO

Answer 1

尝试改变

[Column(TypeName="varchar(max)")]
public string Comments { get; set; }

到

[Column(TypeName="[varchar](MAX) NULL")]
public string Comments { get; set; }

Answer 2

所以这是几件事的结合。

我的迁移处于一种奇怪的状态；因为我的主要（启动）项目与我的数据访问项目不同，并且有其中包含不同的连接字符串，EF 绝对拒绝即使在包管理器控制台中明确告知要处理哪个项目，也能正确生成迁移代码，同样荒谬错误...我无意中偶然发现了这个，我很生气与问题相比，消息的神秘性质。
我想多了 MAX 值；事实证明 EF6 默认为 NVARCHAR(MAX) 对于所有字符串列，除非明确说明否则（我个人认为这是一个非常糟糕的主意）——或者我猜是通过惯例。只需指定 VARCHAR 然后调整迁移代码，我可以获得 VARCHAR(MAX)... 全部 NVARCHAR(MAX) 不合适的字符串属性，我不得不放入 StringLength 个装饰器。

所以最后，我现在有新的神秘错误...但这是另一个问题。感谢大家的帮助。

DbSet<T>.Add(T entity) throws System.InvalidOperationException: Sequence contains no matching element

DbSet<T>.Add(T entity) throws System.InvalidOperationException: Sequence contains no matching element

c#

entity-framework

ef-code-first