为什么我的 viewModel 对象返回 null？

Question

我有 2 个 类、消息和用户，我创建了一个包含这两个 类 的视图模型：

public class User
{
    [Key]
    public int UserId { get; set; }

    public string Username { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public ICollection<Message>? Messages { get; set; } = new Collection<Message>();

}

public class Message
{
    [Key]
    public int MessageId { get; set; }

    public string MessageContent { get; set; }

    public DateTime DateCreated { get; set; }
}

public class NewMessageViewModel
{
    public Message Message { get; set; }

    public User? Sender { get; set; }

    public User? Receiver { get; set; }
}

然而，当我尝试通过控制器传递 viewModel 并取回一些 json 时，出现 'System.NullReferenceException: 'Object reference not set to an instance of an object.'

错误

我没有正确地通过某些东西，我知道，但为了我的生活，我可以看到什么是错的！我太接近客观性了，所以有人可以指出我犯的愚蠢错误吗？

我的控制器操作：

    [Route("NewMessageViewModel")]
    [HttpGet]
    public NewMessageViewModel GetNewMessageViewModel()
    {

        NewMessageViewModel newmv = new NewMessageViewModel
        {
            Message = { MessageContent = "HelloWord", MessageId = 1, DateCreated = DateTime.Now},
            Receiver = { UserId = 1, Messages = null, FirstName = "Andy", LastName = "Stav", Username = "AndyStav" },
            Sender = { UserId = 2, Messages = null, FirstName = "Test", LastName = "Test", Username = "Test" }

        };

        return newmv;

    }

Answer 1

您忘记初始化消息和用户 class:

        NewMessageViewModel newmv = new NewMessageViewModel
        {
            Message = new Message { MessageContent = "HelloWord", MessageId = 1, DateCreated = DateTime.Now },
            Receiver = new User { UserId = 1, Messages = null, FirstName = "Andy", LastName = "Stav", Username = "AndyStav" },
            Sender = new User { UserId = 2, Messages = null, FirstName = "Test", LastName = "Test", Username = "Test" }

        };

        return newmv;

输出：