DbSet.Cast <TEntity>()错误:无法从非泛型DbSet为"实体"类型的对象创建DbSet <IEntity>

Question

版本信息:

我正在使用C#4.5,Entity Framework 6.0和MEF.

代码和单元测试

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()单元测试.

问题

我想将非泛型DbSet转换为其通用版本,但我得到以下异常InvalidCastException: Cannot create a DbSet<IUser> from a non-generic DbSet for objects of type 'User':

var nonGeneric = context.Set(typeof(User));
var generic = nonGeneric.Cast<IUser>(); //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代码仅限于具体的类(我希望不是我需要创建一个围绕非泛型DbSet的包装器将它转换为泛型DbSet或找到当前DbSet实现的替代方案).

如果您想知道我为什么使用接口,即使它们目前不受Microsoft支持,我会给您一些解释(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来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<User>(); 

        //Failed Attempt# 2: But the good thing is that we have access to IUser and its exports
        //then lets get a DbSet<IUser> instead
        var failedAttempt2 = dbContext.Set<IUser>();
        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 EntityTypeConfiguration<IUser>at 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<IUser>().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<IUser>();
        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<IUser>();
        }
        catch (InvalidCastException ex)
        {
            //Cannot create a DbSet<IUser> from a non-generic DbSet for objects of type 'User'.
            throw;
        }
    }
}

Answer 1

为此,我实际上建议使用反射.在DbContext的构造函数中,您可以将属性设置为函数指针:

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

然后,您可以使用以下方法调用它:

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

这应该返回一个类型的对象,DbSet<UserImplementation>然后可以调用.Cast <>()方法.