添加UdpAnnouncementEndpoint时ServiceHost抛出NullReferenceException异常

在实现WCF服务公告功能时，ServiceHost抛出NullReferenceException异常，操作完全按照微软官方文档步骤执行。

环境说明 #

• 基于.NET Framework 4.8的控制台应用
• 已引用System.ServiceModel和System.ServiceModel.Discovery程序集
• 单独使用UdpDiscovery功能时可正常运行

可复现代码 #

using System;
using System.ServiceModel;
using System.ServiceModel.Discovery;

[ServiceContract(CallbackContract = typeof(IServerCallback))]
public interface IServer
{
    [OperationContract]
    void Connect();
}

[ServiceContract]
public interface IServerCallback
{
    [OperationContract]
    void KeepAlive();
}

public class MyServiceHost<TContract, TServer> where TServer : TContract where TContract : IServer
{
    private readonly ServiceHost _serviceHost;
    private const string MyIp = "10.82.83.84";

    public MyServiceHost(TServer server)
    {
        var uri = new Uri($"net.tcp://{MyIp}:8586");
        _serviceHost = new ServiceHost(server, uri);
        StartServer();
    }

    private void StartServer()
    {
        //Endpoint.
        var contract = typeof(TContract);
        _serviceHost.AddServiceEndpoint(contract, new NetTcpBinding(), contract.Name);

        //服务公告配置
        ServiceDiscoveryBehavior serviceDiscoveryBehavior = new ServiceDiscoveryBehavior();
        serviceDiscoveryBehavior.AnnouncementEndpoints.Add(new UdpAnnouncementEndpoint());
        _serviceHost.Description.Behaviors.Add(serviceDiscoveryBehavior);

        _serviceHost.Open();
    }
}
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Single)]
public class Server : IServer
{
    public void Connect(){}
}
public static class Test
{
    public static void Main(string[] args)
    {
        var myServiceHost = new MyServiceHost<IServer, Server>(new Server());
        while (Console.ReadLine() != "q"){}
    }
}

问题原因及解决方法 #

这个异常是因为服务终结点未启用发现功能导致的。虽然添加了ServiceDiscoveryBehavior开启服务级别的发现支持，但每个需要参与公告的终结点必须单独配置EndpointDiscoveryBehavior，否则WCF在发送公告时无法获取有效终结点元数据，进而抛出空引用异常。

修改StartServer方法，在添加终结点后注册EndpointDiscoveryBehavior即可解决：

private void StartServer()
{
    //Endpoint.
    var contract = typeof(TContract);
    var endpoint = _serviceHost.AddServiceEndpoint(contract, new NetTcpBinding(), contract.Name);

    // 为终结点添加发现行为
    endpoint.Behaviors.Add(new EndpointDiscoveryBehavior());

    //服务公告配置
    ServiceDiscoveryBehavior serviceDiscoveryBehavior = new ServiceDiscoveryBehavior();
    serviceDiscoveryBehavior.AnnouncementEndpoints.Add(new UdpAnnouncementEndpoint());
    _serviceHost.Description.Behaviors.Add(serviceDiscoveryBehavior);

    _serviceHost.Open();
}

内容的提问来源于stack exchange，提问作者Jossi85