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，但我收到以下异常：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（我希望不是否则我需要创建一个围绕非泛型DbSet的包装器来将其转换为通用DbSet或找到当前DbSet实现的替代方法。）

如果您想知道为什么我使用接口，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此方法：

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

这应该返回DbSet<UserImplementation>类型的对象，然后可以调用.Cast＆lt;＆gt;（）方法。

Answer 2

替换

nonGeneric.Cast<IUser>();

通过

Enumerable.Cast<IUser>(nonGeneric);

Answer 3

好的，我对实体框架一无所知，但是从查看文档

http://msdn.microsoft.com/en-us/library/gg696521%28v=vs.103%29.aspx

DbSet<TEntity> item = DbContext.Set<TEntity>;

所以实际上你的代码与此相同：

DbSet<User> nonGeneric = context.Set<User>();

并获得IUser

DbSet<IUser> nonGeneric = context.Set<User>();

或者

var generic = nonGeneric.Cast<DbSet<IUser>>();

Answer 4

由于我无法将通用DbSet转换为我使用的类型化DbSet，因此无法使用类型化IQueryable来完成我需要的DbSet相同的功能。

以下是一个扩展程序，可以帮助您：

    public static IQueryable<T> GetIQueryableByTableName<T>(this DbContext context, string tableName)
    {
        var type = Assembly.GetExecutingAssembly().GetTypes().FirstOrDefault(t => t.Name == tableName);
        if (type == null)
        {
            throw new Exception("GetIQueryableByTableName received an invalid table name.");
        }
        return context.GetType().GetMethod("Set", new Type[0]).MakeGenericMethod(type).Invoke(context, new object[0]) as IQueryable<T>;
    }

DbSet.Cast <tentity>（）错误：无法从非泛型DbSet为“实体”</ityity> </tentity>类型的对象创建DbSet <ientity>

4 个答案: