存儲過程InvalidCastException錯誤

Question

我試圖訪問一個簡單的控制檯應用程序中的存儲過程，出於某種原因，我得到一個InvalidCastException。

任何人都可以幫忙嗎？

class Customer 
    { 
     public int CustomerID { get; set; } 
     public string FirstName { get; set; } 
     public string SecondName { get; set; } 
     public int Age { get; set; } 
    } 

    static void Main(string[] args) 
    { 
     string storedProc = "dogssimple"; 
     List<Customer> custList = new List<Customer>(); 

     SqlConnection conn = new SqlConnection("server=localhost;" + 
            "Trusted_Connection=yes;" + 
            "database=mydatabase; " + 
            "connection timeout=30"); 
     try 
     { 
      conn.Open(); 

      SqlCommand cmd = new SqlCommand(storedProc, conn); 

      cmd.CommandType = CommandType.StoredProcedure; 

      SqlDataReader rdr = cmd.ExecuteReader(); 

      while (rdr.Read()) 
      { 
       Customer addCustomer = new Customer() //..... cast exception here 
       { 
        CustomerID = (int)rdr["CustomerID"], 
        FirstName = (string)rdr["FirstName"], 
        SecondName = (string)rdr["SecondName"], 
        Age = (int)rdr["Age"], 
       }; //.... 

       Console.WriteLine("Customer: {0}, {1}, {2}, {3}", addCustomer.CustomerID, addCustomer.FirstName, addCustomer.Age, addCustomer.Age); 
       custList.Add(addCustomer); 
      } 

      var age = from x in custList 
         select x.Age; 
      var avgAge = age.Average(); 

      Console.WriteLine("The average age of the customers is " + avgAge); 
     } 
     catch (Exception e) 
     { 
      Console.WriteLine(e); ; 
     } 

     Console.ReadLine(); 
} 

Answer 1

ADO.NET將NULL作爲DBNull實例返回（非常奇怪，如果你問我）。因此，請閱讀以下檢查值：

SecondName = rdr["SecondName"] is DBNull ? null : (string)rdr["SecondName"] 

還要確保數字類型匹配，例如。你可能會期望一個int，但你可能會得到一個浮點數或小數點。