需要using System.Management;

/// <summary>
/// Get LocalHost MAC Address
/// </summary>
/// <returns></returns>
public string GetHostMac()
{
ManagementClass mc;
string mac = string.Empty;
mc = new ManagementClass("Win32_NetworkAdapterConfiguration");
ManagementObjectCollection moc = mc.GetInstances();
foreach (ManagementObject mo in moc)
{
if (mo["IPEnabled"].ToString() == "True")
{
mac = mo["MacAddress"].ToString();
}
}
return mac;
}

注意：
工程裡面要新增引用System.Management，不是光是程式碼裡面，方法如下：

右擊工程，新增引用，.net標籤，System.Management，確定。