获取磁盘序列号，出现未将对象引用到设置的实例的问题？

Question

代码如下：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Management;namespace Ex189_读取磁盘序列号
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                string hardDiskID = null;
                ManagementObjectSearcher searcher = new ManagementObjectSearcher("select * from Win32_logicaldisk");
                foreach (ManagementObject id in searcher.Get())
                {
                    hardDiskID = id.GetPropertyValue("VolumeSerialNumber").ToString().Trim();
                    this.label1.Text += string.Format("磁盘序列号为{0}    \t\n", hardDiskID);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }   
    }
}

Answer 1

id.GetPropertyValue("VolumeSerialNumber") is null, tostring 就错了.
 string hardDiskID = null;
                ManagementObjectSearcher searcher = new ManagementObjectSearcher("select * from Win32_logicaldisk");
                if (searcher != null)
                {
                    foreach (ManagementObject id in searcher.Get())
                    {
                        if (id.GetPropertyValue("VolumeSerialNumber") != null)
                        {
                            hardDiskID += id.GetPropertyValue("VolumeSerialNumber").ToString().Trim();
                        }                    }                    Console.WriteLine("磁盘序列号为{0} \t\n", hardDiskID);
                    
                }

Answer 2

    //获取硬盘的物理id号
        public static string getDiskMac()
        {
            try
            {  
                string HDid = null;  
                ManagementClass cimobject = new ManagementClass("Win32_DiskDrive");   
                ManagementObjectCollection moc = cimobject.GetInstances();   
                foreach(ManagementObject mo in moc)   
                {   
                    HDid = (string)mo.Properties["PNPDeviceID"].Value;  
                    break;
                }
                return HDid;
            }
            catch
            {
                return "";
            }
        }

Answer 3

GetPropertyValue估计没得到，返回空置