将列表传递给视图 .Net Core

Question

我正在遍历 .Net Core 项目中 SQL 查询的所有结果。这是模型

    public class Mymessagesinfo
{
    public int MyMessagesCount { get; set; }

    public List<int> MessagesIDs { get; set; }
    public List<int> MessagesSendersid { get; set; }
    public List<string> MessagesSenders { get; set; }
    public List<string> MessagesTitles { get; set; }
    public List<string> MessagesInformation { get; set; }

    public List<string> MessagesStatus { get; set; }

}

我在控制器中遍历用户消息，然后将该模型传递给视图

sqlcon.Open();
        int? userid = HttpContext.Session.GetInt32("UserID");
        SqlCommand sqlcom = new SqlCommand("select * from messages where Messagereceiver=" +userid , sqlcon);
        SqlDataReader reader = sqlcom.ExecuteReader();
        if(reader.HasRows)
        {
            
            int index = 0;
            while(reader.Read())
            {
                string s;
                s = reader[0].ToString();

                Mymessages.MessagesIDs.Add(int.Parse(s));
                Mymessages.MessagesSendersid.Add(int.Parse(reader[1].ToString()));
                Mymessages.MessagesTitles.Add(reader[3].ToString());
                Mymessages.MessagesInformation.Add(reader[4].ToString());
                Mymessages.MessagesStatus.Add(reader[5].ToString());
                index++;
            }
            Mymessages.MyMessagesCount = index;

        }

第一行 Mymessages.MessagesIDs.Add(int.Parse(s)); 它抛出异常说 System.NullReferenceException: 'Object reference not set to an instance of an object 我想确保reader 持有结果，所以我添加了int s 并检查了它，它持有它应该的值。 这里出了什么问题？这就是我们应该将类似列表的数据传递给视图的方式吗？

Answer 1

您需要在实体Mymessages 中初始化MessagesIDs，如下所示：

var Mymessages = new Mymessagesinfo()
{
    MessagesIDs = new List<int>()
};

Mymessages.MessagesIDs.Add(id);

或者像这样定义类，

public class Mymessagesinfo
{
    public int MyMessagesCount { get; set; }

    public List<int> MessagesIDs { get; set; } = new List<int>();
    public List<int> MessagesSendersid { get; set; } = new List<int>();
    public List<string> MessagesSenders { get; set; } = new List<string>();
    public List<string> MessagesTitles { get; set; } = new List<string>();
    public List<string> MessagesInformation { get; set; } = new List<string>();

    public List<string> MessagesStatus { get; set; } = new List<string>();

}

Answer 2

以下是我将如何重组您必须使其发挥作用的内容。

首先，你的模型类：

public class Mymessagesinfo
{
    public List<MessageInfo> Messages { get; set; } = new List<MessageInfo>();
}

public class MessageInfo
{
    public int ID { get; set; }
    public int Senderid { get; set; }
    public string Sender { get; set; }
    public string Title { get; set; }
    public string Information { get; set; }
    public string Status { get; set; }
}

使用这种方法，您可以获得一个消息对象列表，而不是一堆包含属性数据的列表。

我建议您从 SQL Server 加载它：

    var data = new Mymessagesinfo();
    int? userid = HttpContext.Session.GetInt32("UserID");

    var messagesTable = new System.Data.DataTable("messages");
    using (var sqlcom = sqlcon.CreateCommand())
    {
        sqlcom.CommandText = $"select * from messages where Messagereceiver='{userid}'";
        using (var adapter = new SqlDataAdapter(sqcom))
        {
            adapter.Fill(messagesTable);
        }
    }
    // we are now done with SQL and have the data in memory...
    foreach(DataRow row in messagesTable.Rows) 
    {
        data.Messages.Add( new MessageInfo {
            ID = row.Field<int>(0), 
            Senderid = row.Field<int>(1), 
            Sender = row.Field<string>(2),
            Title = row.Field<string>(3),
            Information = row.Field<string>(4),
            Status = row.Field<string>(5),
        });
    }
    
    return View(data);

 This is a lot cleaner and by using a DataAdapter and DataTable you minimize the amount of time that the connection to the database is connected.

这是在 MVC 视图中使用此模型的方法：

@Model Mymessagesinfo

<div>
   <!-- This is where you can display the properties of the message. //-->
   <ul>
     @foreach(var message in Model.Messages) 
     {
         <li> @message.Title - @message.Id </li> 
         
     }
   <ul>
<div>