无法获取要反序列化的类列表

我有一个包含这段代码的类

    public static ExpenseItemRoster ReadFromFile(string filename)
    {
        List<ExpenseItemRoster> deserializedExp = null;
        try
        {
            BinaryFormatter formatter = new BinaryFormatter();
            FileStream stream = new FileStream("D:''myExpensesUpdated.bin", FileMode.Open, FileAccess.Read, FileShare.Read);
            deserializedExp = (List<ExpenseItemRoster>)formatter.Deserialize(stream);
            stream.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine("Unable to deserialize Expenses: {0}", ex.Message);
        }
        return null;
    }
}

}

当我尝试在我的主程序中提出它时，我得到这个：

Unable to cast object of type 'System.String' to type 'System.Collections.Generic.List`1[Payroll.ExpenseItemRoster]'.
Object reference not set to an instance of an object.

几乎可以肯定这是因为我没有正确返回反序列化列表，但我不完全确定我是否需要将反序列化列表转换为字符串，或者如何做到这一点。

编辑：这是它被称为...

 using System;
using System.Collections.Generic;
using System.Runtime.Serialization.Formatters.Binary;
using System.Runtime.Serialization;
using System.IO;
using System.Linq;
using System.Text;
namespace Payroll
{
    class PayrollProgram
    {
        static void Main(string[] args)
        {
            //Create new expenses list
            ExpenseItemRoster expenses = new ExpenseItemRoster();
            //Add original expense items to the expenses list 
            expenses.Add(new ExpenseItem("Airline ticket", new DateTime(2010, 3, 7), 365.98m, "myDiscover", "Chicago sales meeting", "Business class"));
            expenses.Add(new ExpenseItem("Dinner", new DateTime(2010, 1, 5), 489m, "cash", "Local sales call", "Client Sam Adams"));
            expenses.Add(new ExpenseItem("Conference fee", new DateTime(2009, 2, 1), 600m, "myDiscover", "Chicago conference", "Prepaid conference fee"));
            expenses.Add(new ExpenseItem("airline ticket", new DateTime(2009, 12, 20), 1350m, "myVisa", "Emergency customer call", "First class fare"));
            expenses.Add(new ExpenseItem("Lunch", new DateTime(2010, 5, 1), 77m, "myVisa", "Chicago sales meeting", ""));
            expenses.Add(new ExpenseItem("Dry cleaning", new DateTime(2010, 5, 2), 88m, "cash", "Chicago sales meeting", ""));
            expenses.Add(new ExpenseItem("Airline Ticket", new DateTime(2010, 11, 12), 8765m, "myVisa", "Australian sales meeting", "first class travel"));
            expenses.Add(new ExpenseItem("Taxi", new DateTime(2011, 3, 13), 101m, "cash", "New York sales manager meeting", "Trip to airport"));
            expenses.Add(new ExpenseItem("Entertainment", new DateTime(2011, 3, 12), 202m, "cash", "New York sales manager meeting", "Airport closed"));
            ExpenseItem e2 = new ExpenseItem("Conference fee", new DateTime(2009, 2, 1), 600m, "myDiscover", "Chicago conference", "Prepaid conference fee");
            try
            {
                expenses.Add(e2);
            }
            catch (Exception ex)
            {
                Console.WriteLine("{0}: {1}", ex.Message, e2.ToString());
            }
            ExpenseItem e1 = new ExpenseItem("Lunch", new DateTime(2010, 5, 1), 77m, "myVisa", "Chicago sales meeting", "");
            expenses.Delete(e1);
            ExpenseItem find = expenses.Find(e1);
            if (find == null)
                Console.WriteLine("Not Found: {0}", e1.ToString());
            else
                Console.WriteLine("Found: {0}", find.ToString());
            try
            {
                expenses.WriteToFile("D:''myExpensesUpdated.bin");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            try
            {
                ExpenseItemRoster expensesUpdated = ExpenseItemRoster.ReadFromFile("D:''myExpensesUpdated.bin");
                foreach (var e in expensesUpdated) Console.WriteLine("Updated: {0}", e.ToString());
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
    }
}

我知道返回 null 没有任何用处，但是我收到错误"无法将类型'字符串'转换为'工资单.费用项目名册'"

我需要以某种方式将反序列化的名单转换为可用的东西。