Generic List error

ferahl · October 29th, 2007, 02:43 PM

hello,

I am having trouble using a List object inside a structure:

struct SomeStruct
{
...
public List<Socket> socket;
};

// In a method
SomeStruct somestruct = new SomeStruct();
somestruct.socket = new List<Socket>(); // do I need to new the list?.. still doesn't work

// throws exception "Unhandled Exception: System.NullReferenceException: Object reference not set to
// an instance of an object."
somestruct.socket.Add(asocket);

If I create the list outside of the structure it works fine.

thanks a lot for any help!

ferahl · October 29th, 2007, 04:35 PM

*fixed*

declaring as a class and then initialising the List inside it worked:

class someclass
{
...
public List<Socket> socket = new List<Socket>();
};

joefawcett · October 30th, 2007, 03:28 AM

The following code works for me in a console application:

Code:

using System;
using System.Collections.Generic;
using System.Net.Sockets;

namespace Structs
{
  class Program
  {
    static void Main(string[] args)
    {
      TestStruct1 ts1; //No 'new TestStruct1()' needed.
      ts1.Sockets = new List<Socket>();
      Socket socket1 = new Socket(AddressFamily.InterNetwork, SocketType.Raw, ProtocolType.IPv4);
      ts1.Sockets.Add(socket1);
      Socket socket2 = new Socket(AddressFamily.InterNetwork, SocketType.Raw, ProtocolType.IPv6);
      ts1.Sockets.Add(socket2);
      foreach (Socket socket in ts1.Sockets)
      {
        Console.WriteLine("Address Family: {0}, Socket Type: {1}, Protocol Type: {2}.", socket.AddressFamily, socket.SocketType, socket.ProtocolType);
      }
      Console.ReadLine();
    }
  }

  struct TestStruct1
  {    
    public List<Socket> Sockets;
  }
}

Something you can't do is initialise struct members within the struct itself and notice you don't need to use new when declaring the struct as it's a value type, like Integer32 (int).

--

Joe (Microsoft MVP - XML)