DbSet.Cast< TEntity>（）错误：无法创建一个DbSet< IEntity>从非通用DbSet为“实体”类型的对象 [英] DbSet.Cast<TEntity>() Error: Cannot create a DbSet<IEntity> from a non-generic DbSet for objects of type 'Entity'
问题描述
版本信息：
我使用C＃4.5，实体框架6.0和MEF
编码和单元测试
我创建了一个测试项目来解释这个问题：
的 https://skydrive.live.com/redir?resid=E3C97EC293A34048!2234
请打开的UnitTest项目并尝试运行TestIfItWorks（）单元测试。
问题
我想一个非通用DbSet转换成它的通用版本，但我得到了以下异常： InvalidCastException的：无法创建DbSet< IUSER>从非普通DbSet型用户的对象：
VAR非通用=上下文。设置（typeof运算（用户））;
无功通用= nonGeneric.Cast<&IUSER GT;（）; //异常这里
User类正在实施IUSER所以你会觉得演员不应该除非DbSet代码的问题仅限于具体类（但愿不是，否则我需要或者创建一个围绕非通用DbSet的包装将其转换为一个通用的DbSet或找到当前DbSet实现替代）。
如果即使他们目前不支持由Microsoft你想知道为什么我使用的接口，我给你解释一下（希望这将筛选出的反应是说：不这样做，而不是提供解决方案）的：
我使用MEF和的EntityFramework创建一个松散耦合的数据层引擎，通过它我可以提供每个项目的基础实体（及其相应的配置） 。我一直在使用广泛的接口来定义引擎。元数据，并在上下文中的具体实现实体使用MEF在运行时发现的。
从代码摘录
[ TestMethod的]
公共无效TestIfItWorks（）
{
// TODO：请打开App.Config中，改变PluginsPath到你的机器相匹配的Plugins文件夹。使用
（VAR的DbContext =新MyContext（））//请忽略这条线现在。这是的UnitOfWork我与上下文改为创建一个简单的单元测试
{
dbContext.Setup（）; //请忽略这条线现在。这是的UnitOfWork的一部分，我搬到这里创建一个简单的单元测试
//所有这些的目的是为了能够读取和/写入用户信息数据库，同时用户定义类在外部装配
//但是我们可以通过使用MEF IUSER接口导入。
//失败的尝试＃1：使用用户级直接！这并不工作，因为用户是在我们没有参考
// VAR外部类failedAttempt1 = dbContext.Set<使用者>（）;
//失败的尝试＃2：但好消息是，我们能够获得IUSER和出口
//然后让获得DbSet<&IUSER GT;而不是
VAR failedAttempt2 = dbContext.Set<&IUSER GT;（）;
试
{
VAR throwsException2 = failedAttempt2.FirstOrDefault（）;
}
赶上（InvalidOperationException异常前）
{
// InvalidOperationException异常：
//的实体类型IUSER是不是该机型为当前上下文的一部分。
//它，当我试图定义从EntityTypeConfiguration<继承的类也没有工作; IUSER>对于TestImplementation
}
// OK然后让做不同的这个时候。让我们得到用户类型（即我们知道我们有良好的配置）
//从我们的集装箱，并要求上下文给我们的非通用版本
VAR userImplementationType = Logic.Instance.GetExportedTypes<&IUSER GT;（）。 FirstOrDefault（）;
Assert.IsNotNull（userImplementationType，我们没有带能够TestImplementation加载到目录中，请确保PluginsPath在App.Config中正确设置。）;
VAR非泛型= dbContext.Set（userImplementationType）;
//
//这是工作，到目前为止，我们可以添加使用
// DbSet的非通用版本从数据库中删除记录。您可以取消以下代码块提供了一个独特的ID
//并测试它自己。
//
VAR NEWUSER = Logic.Instance.New<&IUSER GT;（）;
newUser.Id =99;
newUser.UserName =Aidin Sadighi
nonGeneric.Add（NEWUSER）;
试
{
dbContext.SaveChanges（）;
}
赶上（DbUpdateException前）
{
//这是可以的，因为很可能这是一个重复的用户。只是增加了标识，以使其唯一。
}
//失败的尝试＃3：铸造非通用DbSet仿制
试
{
// TODO：我需要解决这个问题。请帮我
VAR genericSet = nonGeneric.Cast<&IUSER GT;（）;
}
赶上（InvalidCastException的前）
{
//不能创建一个DbSet<&IUSER GT;从非普通DbSet型用户的对象。
抛出;
}
}
}
对于这一点，我真的建议使用反射。在你的DbContext的构造函数，你可以设置一个属性来函数指针：
=方法this.GetType（）GetMethod （设置，新类型[0]）MakeGenericMethod（typeof运算（UserImplementation））。
您可以再调用这个使用：
method.Invoke（这一点，新的对象[0]）;
这应该返回类型的对象 DbSet< UserImplementation> 其中.Cast<>（）方法则可以调用在
Version Info:
I am using C# 4.5, Entity Framework 6.0, and MEF.
Code and Unit Test
I created a Test Project to explain the problem: https://skydrive.live.com/redir?resid=E3C97EC293A34048!2234
Please Open the UnitTest project and try to run TestIfItWorks() unit test.
Problem
I want to convert a non-generic DbSet to its generic version but I am getting the following exception: 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
The User class is implementing IUser so you would think the cast shouldn't be a problem unless DbSet code is restricted to concrete classes (I hope not otherwise I need to either create a wrapper around non-generic DbSet to convert it to a generic DbSet or find an alternative to current DbSet implementation).
If you are wondering why I am using interfaces even though they are not currently supported by Microsoft I give you a little explanation (hopefully this would filter out responses that say "Don't Do That" instead of providing a solution) :
I am using MEF and EntityFramework to create a loosely coupled data layer engine through which I can provide Entities (and their corresponding configurations) per project basis. I have been using Interfaces extensively to define the engine. The meta data and concrete implementation of entities in context are discovered in run time using MEF.
Excerpt from code
[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;
}
}
}
For this, I would actually suggest using reflection. In the constructor of your DbContext, you can set a property to the function pointer:
method = this.GetType().GetMethod("Set", new Type[0]).MakeGenericMethod(typeof(UserImplementation));
You can then invoke this using:
method.Invoke(this, new object[0]);
And this should return an object of type DbSet<UserImplementation> which the .Cast<>() method can then be invoked on.
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