How did I get this NullReferenceException error here right after the constructor?

Question

I've had an asp.net website running live on our intranet for a couple of weeks now. I just got an email from my application_error emailer method with an unhandled exception.

Here it is (I've cleaned up some of the paths to make it better displayed)

Exception : Object reference not set to an instance of an object. Stack Trace : at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value) at TimesheetDomain.DataMappers.StaffMemberData.ReadStaff(SqlDataReader reader) in TimesheetDomain\DataMappers\StaffMemberData.cs:line 362

at TimesheetDomain.DataMappers.StaffMemberData.GetStaffMember(String name) in TimesheetDomain\DataMappers\StaffMemberData.cs:line 401

at TimesheetDomain.ServiceLayer.TimesheetManager.GetUserFromName(String name) in TimesheetDomain\ServiceLayer\TimesheetManager.cs:line 199

at UserVerification.GetCurrentUser() in \App_Code\UserVerification.cs:line 29 at WebTimesheets.OnInit(EventArgs e) in \WebTimesheets\WebTimesheets.master.cs:line 159

at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Basically it looks like it's erroring at my ReadStaff method which reads a data reader to build staff member objects. Here is the bit of code:

while (reader != null && reader.Read()) {     StaffMember newMember = null;     string firstName = reader["FirstName"].ToString();     string lastName = reader["LastName"].ToString();     int staffID = (int)reader["StaffID"];     int employSection = (int)reader["EmploySection"];     StaffType employType = (StaffType)employSection;     string emailAddress = reader["EmailInt"].ToString();     int employCode = (int)reader["ibbwid"];      //check if they are an admin staff member      if (IsAdminStaff(employType))     {         newMember = new AdminOfficer(firstName, lastName, employType, staffID, emailAddress, employCode);     }     else     {         //check if they are a supervisor         if (IsASupervisor(staffID))             newMember = new Supervisor(firstName, lastName, employType, staffID, emailAddress, employCode);         else             newMember = new StaffMember(firstName, lastName, employType, staffID, emailAddress, employCode);     }      //add to identity map     if (!_staffMembers.ContainsKey(staffID))         _staffMembers.Add(staffID, newMember); //****THIS IS LINE 362*****     else         _staffMembers[staffID] = newMember; } 

(Line 362 is 3rd last line) I'm using an identity map (just read fowlers book on patterns and thought it was a good idea - may have done it wrong, happy for comments) but that's not overly relevant as later on I use the newMember object elsewhere so if I remove that block the NullReferenceException will occur.

I am struggling to see how on earth newMember is null in the 3rd last line there (which is the line that errored).

Resharper/VS doesn't give me a warning that it could be null - because there's the 3 constructors which I choose from.

Can anyone suggest where I can look to try and fix this error? It's only happened once and that method has been called thousands of times since the site went live.

Thanks

[EDIT] As Requested, here's the IComparer for staff member

/// <summary> /// Comparer for staff members - compares on name /// </summary> public class StaffMemberComparer : IComparer {     public int Compare(object x, object y)     {         //check they are staff members         if (x is StaffMember && y is StaffMember)         {             //do a simple string comparison on names             StaffMember staffX = x as StaffMember;             StaffMember staffY = y as StaffMember;              return String.Compare(staffX.FirstName, staffY.FirstName);         }          throw new Exception("This is for comparing Staff Members");     } } 

and it's used in the IComparable implementation

/// <summary> /// IComparable implementaiton /// </summary> /// <param name="obj">object to compare to</param> /// <returns></returns> public int CompareTo(object obj) {     StaffMemberComparer comparer = new StaffMemberComparer();     return comparer.Compare(this, obj); } 

Answer 1

It is almost certainly a threading issue - see this question and its accepted answer.

Dictionary<>.Insert() will throw a NullReferenceException internally if the dictionary instance is modified from another thread during the insert operation.

Answer 2

As of .NET 4.0 you can use ConcurrentDictionary and avoid the threading issues associated with manipulating the same dictionary from multiple threads simultaneously.