c# - MVC5/C# - 无法对空引用执行运行时绑定(bind)

Question

我试图找出导致此代码出现 Cannot Perform Run Run Runtime Binding on a null Reference 错误的原因:

var query = "SELECT Id, UserName, List_Order, LoggedIn " + 
            "FROM AspNetUsers" +
            "WHERE LoggedIn = 1" + 
            "ORDER BY List_Order ASC";

var conn = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString);
var cmd = new SqlCommand(query, conn);   
conn.Open();
var rdr = cmd.ExecuteReader();
var n = 0;
while(rdr.Read())
{
    if (Convert.ToString(rdr["UserName"]) != null)
    {
        ViewBag.speakers[n] = new string[4] {
            Convert.ToString(rdr["Id"]),
            Convert.ToString(rdr["UserName"]),
            Convert.ToString(rdr["List_Order"]),
            Convert.ToString(rdr["LoggedIn"]) 
        };

        //Exception Details: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: Cannot 
        //perform runtime binding on a null reference
        n++;
    }
}

n++ 增量似乎是此错误的原因，我不明白为什么。

更新了代码以反射(reflect)可能的解决方案。但是，错误仍然存​​在。

尝试了同样的结果:

if (!string.IsNullOrWhiteSpace(Convert.ToString(rdr["UserName"]))) {
        List<string> speakers = new List<string>();
        speakers.Add(Convert.ToString(rdr["Id"]));
        speakers.Add(Convert.ToString(rdr["UserName"]));
        speakers.Add(Convert.ToString(rdr["List_Order"]));
        speakers.Add(Convert.ToString(rdr["LoggedIn"]));

        ViewBag.speakers[n] = speakers;
        n++;
}

Answer 1

您的代码中有两个问题:

考虑一下:

public ActionResult Index()
{
   int n = 0;
   ViewBag.speakers[n] = 5;
   return View();
}

这段简化的代码抛出 Cannot perform runtime binding on a null reference因为未定义扬声器(空引用)。

您可以通过定义 speakers 来修复它在循环之前的动态 ViewBag 中:

ViewBag.speakers = new List<string>();

第二期:

ViewBag.speakers[n] = speakers;

代码中的speakers是一个List，你可能需要定义ViewBag.speakers作为List<List<string>>并调用.Add(speakers)而不是使用索引进行访问(您可能会得到索引超出范围)