使用实体框架时的 InvalidCastException

我正在尝试在对话类中保存用户列表，但在执行 POST 时出现InvalidCastException。

这是我的用户类：

public class User
{
    [Key]
    [DataMember]
    public String Email { get; set; }
    [DataMember]
    public String Password { get; set; }
    [DataMember]
    public Boolean Admin { get; set; }
    public User(String email, String password, Boolean admin)
    {
        Email = email;
        Password = password;
        Admin = admin;
    }
}

这是我的会话课：

[DataContract]
public class Conversation
{
    [Key]
    [DataMember]
    public string Key { get; set; }
    [DataMember]
    public string ConversationName { get; set; }
    [DataMember]
    public string Administrator { get; set; }
    [DataMember]
    [ForeignKey("Email")]
    public List<User> Members { get; set; }
    public Conversation(string key, string name, string admin, List<User> members)
    {
        Key = key;
        ConversationName = name;
        Administrator = admin;
        Members = members;
    }
    public Conversation()
    {
    }
}

这是对话控制器：

发布：

    [HttpPost]
    public async Task<IHttpActionResult> PostConversation(Conversation convo)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }
        foreach (var user in convo.Members)
        {
            db.Entry(user).State = EntityState.Unchanged;
        }
        db.Conversations.Add(convo);
        await db.SaveChangesAsync();
        return CreatedAtRoute("DefaultApi", new { name = convo.Key }, convo);
    }

获取：

[HttpGet]
public IQueryable<Conversation> GetConversations()
{
    return db.Conversations;
}

编辑发表声明：

{
"Key": "123",
"ConversationName": "Test",
"Administrator": "Ken",
"Members": [{"Email": "y@12.com","Password" : "Passw-1", "Admin" : "true"},
            {"Email": "y@123.com","Password" : "Passw-1", "Admin" : "false"}]
}

我得到的错误是：

"ExceptionMessage"： "无法将类型为'System.Collections.Generic.List'1[AcademicAssistant.Models.User]'的对象转换为类型'AcademicAssistant.Models.User'。