c# – LINQ to Entities对带有嵌套对象的Union的null引用

public class User  // DB model
{
    public Guid Id { get; set; }
    public Address Address { get; set; }
    // And other propeties
}

public class Invitation  // DB model
{
    public Guid Id { get; set; }
    // And other propeties
}

public class Address  // DB model
{
    public string Zip { get; set; }
    // And other properties
}

public class ResponseModel
{
    public Guid Id { get; set; }
    public ResponseAddress Address { get; set; }
}

public class ResponseAddress
{
    public string Zip { get; set; }
    // And other properties
}

以下查询分别返回用户和邀请,目的是获得两个查询的并集：

var users = db.Users.Select(x => new ResponseModel() 
{
    Id = x.Id,Address = new ResponseAddress()
    {
        Zip = x.Address.Zip
    }
});
var invitations = db.Invitations.Select(x => new ResponseModel() 
{
    Id = x.Id,Address = new ResponseAddress()
    {
        Zip = String.Empty
    }
});
var union = users.Union(invitations).ToList();

当我尝试使用union DB-side时,我在System.Data.Entity.CoreQuery.PlanCompiler中深入获得了一个null引用异常.如果我单独调用每个部件上的ToList(),它就可以工作;如果我在每个部分上调用ToList()然后将它们联合起来,它就可以了.

users.ToList();
invitations.ToList();
users.ToList().Union(invitations.ToList());

看来如果我在创建ResponseAddress部分之前将它们联合起来,那么在稍后调用Select时创建ResponseAddress部分,它可以工作：

var users = db.Users.Select(x => new  
{
    Id = x.Id,Zip = x.Address.Zip
});
var invitations = db.Invitations.Select(x => new  
{
    Id = x.Id,Zip = String.Empty
});
var union = users.Union(invitations).Select(x=>new ResponseModel() {
    Id = x.Id,Address = new ResponseAddress() {
        Zip = x.Zip
    }
}).ToList();

关于为什么在第一组查询中调用Union会返回空引用异常的任何想法,而最后一个查询中的调用不会？两者都在DB端执行,两者都应该产生类似的查询(理论上几乎相同,除了LINQ查询嵌套的方式.)