MVC5 / C# - Cannot perform runtime binding on a null reference

Question

I'm trying to figure out what's causing a Cannot perform runtime binding on a null reference error from this code:

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++;
    }
}

The n++ increment seems to be the cause of this error and I don't understand why.

Updated code to reflect possible solutions. However, the error still remains.

Tried this with the same result:

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

There are two issues in your code:

Consider this:

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

This simplified piece of code throws Cannot perform runtime binding on a null reference since speakers is not defined (null reference).

You can fix it by defining speakers in the dynamic ViewBag before the loop:

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

The second issue:

ViewBag.speakers[n] = speakers;

speakers in your code is a List, you might want to define ViewBag.speakers as a List<List<string>> and call .Add(speakers) instead of accessing using an index (you might get index was out of range)

Answer 2

Calling the .ToString() method on a null column value can result in the Cannot perform runtime binding on a null reference. Use Convert.ToString() instead; it will return an empty string if you attempt to convert a null value and won't require additional code for null checking.