在C#中使用ONVIF進行設備發現的步驟如下：

1. 導入ONVIF的相關命名空間：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.Xml;

2. 創建一個ONVIF設備發現類，并實現設備發現的方法：

public class ONVIFDeviceDiscovery
{
    public static List<string> DiscoverDevices()
    {
        List<string> devices = new List<string>();

        // 創建UDP廣播終結點
        EndpointAddress discoveryEndPointAddress = new EndpointAddress("soap.udp://239.255.255.250:3702");

        // 創建消息工廠
        MessageFactory messageFactory = new MessageFactory();

        // 創建消息版本
        AddressingVersion addressingVersion = AddressingVersion.WSAddressing10;

        // 創建消息頭
        MessageHeader unicastHeader = MessageHeader.CreateHeader("Unicast", "", "");
        MessageHeader multicastHeader = MessageHeader.CreateHeader("Multicast", "", "");

        // 創建綁定元素
        BindingElement[] bindingElements = new BindingElement[2];
        bindingElements[0] = new TextMessageEncodingBindingElement();
        bindingElements[1] = new HttpTransportBindingElement();

        // 創建自定義綁定
        CustomBinding customBinding = new CustomBinding(bindingElements);

        // 創建ChannelFactory
        ChannelFactory<IDiscoveryProxy> factory = new ChannelFactory<IDiscoveryProxy>(customBinding, discoveryEndPointAddress);

        // 創建Channel
        IDiscoveryProxy proxy = factory.CreateChannel();
        factory.Open();

        // 發送消息
        Message message = messageFactory.CreateMessage(addressingVersion, "http://schemas.xmlsoap.org/ws/2004/08/addressing/role/anonymous", multicastHeader, new DiscoveryMessage());
        Message reply = proxy.Discover(message);

        // 解析響應
        if (reply != null)
        {
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(reply.GetReaderAtBodyContents());
            XmlNodeList nodeList = xmlDoc.GetElementsByTagName("XAddrs");

            foreach (XmlNode node in nodeList)
            {
                devices.Add(node.InnerText);
            }
        }

        // 關閉通道和工廠
        factory.Close();

        return devices;
    }
}

3. 在主程序中調用設備發現方法并輸出設備信息：

class Program
{
    static void Main(string[] args)
    {
        List<string> devices = ONVIFDeviceDiscovery.DiscoverDevices();

        foreach (string device in devices)
        {
            Console.WriteLine("Discovered device: " + device);
        }

        Console.ReadLine();
    }
}

通過以上步驟，您可以在C#中使用ONVIF進行設備發現，并獲取到設備的地址信息。