C#

using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using SNMPDll;
namespace tes1c_sharp
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{
MessageBox.Show("welcome bechir");
}

private void button1_Click(object sender, EventArgs e)
{


SNMPAgent snmpAg = new SNMPAgent("10.175.22.234", "read", "public");
SNMPObject snmpob = new SNMPObject("1.3.6.1.4.1.39305.1.3.15");


string result = snmpob.getSimpleValue(snmpAg);

richTextBox1.Text += result;


}
}
}

i receive this error: Null Reference Exception was unhandled.
Thanks for your help.

public static void  Main(System.String[] args)
{
    System.String filename = new System.Text.StringBuilder("message.dat").ToString();