c# – 如果字段是实例成员,则使用Callback Handler异常

如果CallbackHandler.proxy是静态的,那么一切正常：

using System;
using System.ServiceModel;

namespace ConsoleApplication5
{
    // Define class which implements callback interface of duplex contract
    public class CallbackHandler : ServiceReference1.IStockServiceCallback
    {
        public static InstanceContext site = new InstanceContext(new CallbackHandler());
        public static ServiceReference1.StockServiceClient proxy = new ServiceReference1.StockServiceClient(site);

        //  called from the service
        public void PriceUpdate(string ticker,double price)
        {
        }  
    }

    class Program
    {
        static void Main(string[] args)
        {
            CallbackHandler cbh = new CallbackHandler();
        }
    }
}

但是,如果我将它声明为实例成员,那么我得到System.TypeInitializationException：CallBackHandler的类型初始化程序’引发了异常. —> System.ArgumentNullException.值不能为null异常

using System;
using System.ServiceModel;

namespace ConsoleApplication5
{
    // Define class which implements callback interface of duplex contract
    public class CallbackHandler : ServiceReference1.IStockServiceCallback
    {
        public static InstanceContext site = new InstanceContext(new CallbackHandler());
        public ServiceReference1.StockServiceClient proxy = new ServiceReference1.StockServiceClient(site);

        //  called from the service
        public void PriceUpdate(string ticker,double price)
        {
        }  
    }

    class Program
    {
        static void Main(string[] args)
        {
            CallbackHandler cbh = new CallbackHandler();
        }
    }
}

知道为什么让CallbackHandler.proxy成为实例成员会引发异常吗？

编辑：

In the 2nd case,the instance
constructor in the line marked (*)
runs before the completion of the
static constructor (yes,it’s
possible),but at that point site is
still not assigned.

因此,在第二种情况下,站点应该初始化为null,而在第一种情况下应该为它分配一个非空值？！

但…

起初我认为静态站点为null(无论代理是实例还是静态成员)只是因为它是用CallbackHandler初始化的,如下所述：

So when CLR tries to instantiate an
instance O ( which it would then
assign to site ),it waits for
static field site to get
initialized,while site waits for
O to get created,which in turn
would initialize site field. Since
this could create a deadlock of sort,
site is “the wiser” and thus gets
set to default value null?!

然后我记得如果代理也是静态的,那么该站点不为null,所以我对正在发生的事情的理解改为：

So when CLR tries to instantiate an
instance O ( which it would then
assign to site ),it waits for
static field site to get initialized
( so that it can assign site’s
reference to its instance member
proxy ). while site waits for O
to get created,which in turn would
initialize site field. Since this
could create a deadlock of sort,CLR
detects this potential deadlock and
sets site to null.

但是后来我运行了你的测试代码,由于某种原因,即使代理不是静态的,也会分配站点(因此不为空)：

using System;

namespace ConsoleApplication5
{
    public class InstanceContext
    {
        public InstanceContext(CallbackHandler ch)
        {
            Console.WriteLine("new InstanceContext(" + ch + ")");
        }
    }

    public class StockServiceClient
    {
        public StockServiceClient(InstanceContext ic)
        {
            Console.WriteLine("new StockServiceClient(" + ic + ")");
        }
    }

    // Define class which implements callback interface of duplex contract
    public class CallbackHandler
    {
        public static InstanceContext site = new InstanceContext(new CallbackHandler());
        public StockServiceClient proxy = new StockServiceClient(site);
        public CallbackHandler()
        {
            Console.WriteLine("new CallbackHandler()");
        }
        static CallbackHandler()
        {
            Console.WriteLine("static CallbackHandler()");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(CallbackHandler.site == null); // returns false
        }
    }
}

到底是怎么回事？