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

i'm trying figure out what's causing cannot perform runtime binding on null reference error code:

var query = "select id, username, list_order, loggedin " +              "from aspnetusers" +             "where loggedin = 1" +              "order 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 null reference         n++;     } } 

the n++ increment seems cause of error , don't understand why.

updated code reflect possible solutions. however, error still remains.

tried 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++; } 

there 2 issues in 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 null reference since speakers not defined (null reference).

you can fix defining speakers in dynamic viewbag before loop:

viewbag.speakers = new list<string>(); 

the second issue:

viewbag.speakers[n] = speakers; 

speakers in code list, might want define viewbag.speakers list<list<string>> , call .add(speakers) instead of accessing using index (you might index out of range)