DbSet.Cast （）错误：无法从非genericsDbSet为“实体”类型的对象创建DbSet

版本信息：

我正在使用C＃4.5，Entity Framework 6.0和MEF。

代码和unit testing

我创建了一个测试项目来解释这个问题： https ： //skydrive.live.com/redir？resid = E3C97EC293A34048！2234

请打开UnitTest项目并尝试运行TestIfItWorks（）unit testing。

问题

我想将非genericsDbSet转换为其通用版本，但是我得到以下exception： InvalidCastException: Cannot create a DbSet from a non-generic DbSet for objects of type 'User' ：

 var nonGeneric = context.Set(typeof(User)); var generic = nonGeneric.Cast(); //Exception in here 

User类正在实现IUser，所以你会认为强制转换不应该是一个问题，除非DbSet代码被限制在具体的类中（我希望不是我需要创建一个围绕非genericsDbSet的包装器将它转换为genericsDbSet或找到当前DbSet实现的替代方案）。

如果你想知道为什么我使用接口，即使它们目前不受微软支持我给你一些解释（希望这会过滤掉“不要那样做”而不是提供解决方案的响应）：

我正在使用MEF和EntityFramework创建一个松散耦合的数据层引擎，通过它我可以为每个项目提供实体（及其相应的配置）。 我一直在广泛使用Interfaces来定义引擎。 使用MEF在运行时发现元数据和上下文中实体的具体实现。

摘自代码

 [TestMethod] public void TestIfItWorks() { //TODO: Please open the App.Config and change the PluginsPath to match the Plugins folder in your machine. using (var dbContext = new MyContext()) //Please ignore this line for now. This was UnitOfWork which I replaced with Context to create a simple unit test { dbContext.Setup(); //Please ignore this line for now. This was part of UnitOfWork which I moved to here to create a simple unit test //The purpose of all these is to be able to read and write user info from/to database while User class is defined in an external assembly //but we can import it by MEF using IUser interface. //Failed Attempt# 1: Use User class directly! This doesnt work because User is in an external class which we dont have reference to //var failedAttempt1 = dbContext.Set(); //Failed Attempt# 2: But the good thing is that we have access to IUser and its exports //then lets get a DbSet instead var failedAttempt2 = dbContext.Set(); try { var throwsException2 = failedAttempt2.FirstOrDefault(); } catch (InvalidOperationException ex) { //InvalidOperationException: // The entity type IUser is not part of the model for the current context. // It also didnt work when I tried to define a class that inherits from EntityTypeConfigurationat TestImplementation } //Ok then lets do it differently this time. Lets get User type (that we know we have good configuration for) //from our Container and ask Context to give us the nonGeneric version var userImplementationType = Logic.Instance.GetExportedTypes().FirstOrDefault(); Assert.IsNotNull(userImplementationType, "We havn't been able to load TestImplementation into catalog. Please ensure the PluginsPath is set correctly at App.Config"); var nonGeneric = dbContext.Set(userImplementationType); // // This is working so far, we can add and remove records from database using // the nonGeneric version of DbSet. You can uncomment the following code block provide a unique ID // and test it yourself. // var newUser = Logic.Instance.New(); newUser.Id = "99"; newUser.UserName = "Aidin Sadighi"; nonGeneric.Add(newUser); try { dbContext.SaveChanges(); } catch (DbUpdateException ex) { //This is OK because most probably this is a duplicate user. Just increase the Id to make it unique. } //Failed Attempt#3: Cast non generic DbSet to generic try { //TODO: I need to fix this. Help me please var genericSet = nonGeneric.Cast(); } catch (InvalidCastException ex) { //Cannot create a DbSet from a non-generic DbSet for objects of type 'User'. throw; } } } 

 method = this.GetType().GetMethod("Set", new Type[0]).MakeGenericMethod(typeof(UserImplementation)); 

 method.Invoke(this, new object[0]); 

 nonGeneric.Cast(); 

 Enumerable.Cast(nonGeneric); 

 DbSet item = DbContext.Set; 

 DbSet nonGeneric = context.Set(); 

 DbSet nonGeneric = context.Set(); 

 var generic = nonGeneric.Cast>();