继续之前设定的学习目标前，先来一篇小小的外篇。按照第一篇里的配置方式配置好的工程前两天还能正常工作，昨天却突然无法通过Add-Migration命令进行数据库的升级。错误信息如下：
System.Data.Entity.Core.ProviderIncompatibleException: 从数据库中获取提供程序信息时出错。这可能是 Entity Framework 使用的连接字符串不正确导致的。有关详细信息，请查看内部异常并确保连接字符串正确。 ---> System.Data.Entity.Core.ProviderIncompatibleException: 提供程序未返回 ProviderManifestToken 字符串。 ---> System.Data.SqlClient.SqlException: 在与 SQL Server 建立连接时出现与网络相关的或特定于实例的错误。未找到或无法访问服务器。请验证实例名称是否正确并且 SQL Server 已配置为允许远程连接。 (provider: SQL Network Interfaces, error: 26 - 定位指定的服务器/实例时出错) 在 System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction) 在 System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) 在 System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, Boolean withFailover) 在 System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, SecureString newSecurePassword, Boolean ignoreSniOpenTimeout, TimeoutTimer timeout, Boolean withFailover) 在 System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(ServerInfo serverInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString connectionOptions, SqlCredential credential, TimeoutTimer timeout) 在 System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(TimeoutTimer timeout, SqlConnectionString connectionOptions, SqlCredential credential, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance) 在 System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions) 在 System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions) 在 System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) 在 System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnectionOptions userOptions) 在 System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnectionOptions userOptions) 在 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection) 在 System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) 在 System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) 在 System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions) 在 System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry) 在 System.Data.SqlClient.SqlConnection.Open() 在 System.Data.Entity.SqlServer.SqlProviderServices.<>c__DisplayClass2f.<UsingConnection>b__2d() 在 System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.<>c__DisplayClass1.<Execute>b__0() 在 System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation) 在 System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute(Action operation) 在 System.Data.Entity.SqlServer.SqlProviderServices.UsingConnection(DbConnection sqlConnection, Action`1 act) 在 System.Data.Entity.SqlServer.SqlProviderServices.UsingMasterConnection(DbConnection sqlConnection, Action`1 act) 在 System.Data.Entity.SqlServer.SqlProviderServices.GetDbProviderManifestToken(DbConnection connection) 在 System.Data.Entity.Core.Common.DbProviderServices.GetProviderManifestToken(DbConnection connection) --- 内部异常堆栈跟踪的结尾 --- 在 System.Data.Entity.Core.Common.DbProviderServices.GetProviderManifestToken(DbConnection connection) 在 System.Data.Entity.Utilities.DbProviderServicesExtensions.GetProviderManifestTokenChecked(DbProviderServices providerServices, DbConnection connection) --- 内部异常堆栈跟踪的结尾 --- 在 System.Data.Entity.Utilities.DbProviderServicesExtensions.GetProviderManifestTokenChecked(DbProviderServices providerServices, DbConnection connection) 在 System.Data.Entity.Infrastructure.DefaultManifestTokenResolver.<>c__DisplayClass1.<ResolveManifestToken>b__0(Tuple`3 k) 在 System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory) 在 System.Data.Entity.Infrastructure.DefaultManifestTokenResolver.ResolveManifestToken(DbConnection connection) 在 System.Data.Entity.Utilities.DbConnectionExtensions.GetProviderInfo(DbConnection connection, DbProviderManifest& providerManifest) 在 System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection) 在 System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext) 在 System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input) 在 System.Data.Entity.Internal.LazyInternalContext.InitializeContext() 在 System.Data.Entity.Internal.LazyInternalContext.get_ModelBeingInitialized() 在 System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer) 在 System.Data.Entity.Utilities.DbContextExtensions.<>c__DisplayClass1.<GetModel>b__0(XmlWriter w) 在 System.Data.Entity.Utilities.DbContextExtensions.GetModel(Action`1 writeXml) 在 System.Data.Entity.Utilities.DbContextExtensions.GetModel(DbContext context) 在 System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext) 在 System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration) 在 System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration) 在 System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run() 在 System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate) 在 System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate) 在 System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner) 在 System.Data.Entity.Migrations.Design.ToolingFacade.Scaffold(String migrationName, String language, String rootNamespace, Boolean ignoreChanges) 在 System.Data.Entity.Migrations.AddMigrationCommand.Execute(String name, Boolean force, Boolean ignoreChanges) 在 System.Data.Entity.Migrations.AddMigrationCommand.<>c__DisplayClass2.<.ctor>b__0() 在 System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command) 从数据库中获取提供程序信息时出错。这可能是 Entity Framework 使用的连接字符串不正确导致的。有关详细信息，请查看内部异常并确保连接字符串正确。