DbSet.Cast<TEntity>() Error: Cannot create a DbSet<IEntity> from a non-generic DbSet for objects of type 'Entity'

问题

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;
        }
    }
}

回答1:

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.

回答2:

replace

nonGeneric.Cast<IUser>();

by

Enumerable.Cast<IUser>(nonGeneric);

回答3:

Ok i know nothing about Entity framework but from looking at the docs

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

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

so actually your code would be the same as this:

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

and to get a IUser

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

or maybe

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

来源：https://stackoverflow.com/questions/19609981/dbset-casttentity-error-cannot-create-a-dbsetientity-from-a-non-generic-d