NullReferenceException in System.Web calling ServiceStack service with GetAsync from async ServiceStack service

Question

I have a ServiceStack service that uses async from top to bottom. Below is a simplified example with all layers collapsed down to the Service Definition.

public async Task<ReadItemResponse> Get(Dto.ReadItem request)
{
    using (JsonServiceClient client = new JsonServiceClient("http://someserver/itemservice"))
    {
        ReadItemResponse response = await client.GetAsync(new ReadItem
        {
            ItemNumber = request.ItemNumber
        });

        return new Dto.ReadItemResponse
        {
            Item = new Dto.Item
            {
                ItemNumber = response.Item.ItemNumber,
                ItemName = response.Item.ItemName
            }
        };
    }
}

When I make numerous concurrent calls to the service, the above code causes a NullReferenceException in System.Web within 1 - 30 seconds.

System.Web.dll!System.Web.ThreadContext.AssociateWithCurrentThread(bool setImpersonationContext)
System.Web.dll!System.Web.HttpApplication.OnThreadEnterPrivate(bool setImpersonationContext)
System.Web.dll!System.Web.HttpApplication.System.Web.Util.ISyncContext.Enter()
System.Web.dll!System.Web.Util.SynchronizationHelper.SafeWrapCallback(System.Action action = {Method = {System.Reflection.RuntimeMethodInfo}})
System.Web.dll!System.Web.Util.SynchronizationHelper.QueueAsynchronous.AnonymousMethod__0(System.Threading.Tasks.Task _)
mscorlib.dll!System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke()
mscorlib.dll!System.Threading.Tasks.Task.Execute()
mscorlib.dll!System.Threading.Tasks.Task.ExecutionContextCallback(object obj)
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)
mscorlib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot = Id = 1131, Status = Running, Method = "Void <QueueAsynchronous>b__0(System.Threading.Tasks.Task)")
mscorlib.dll!System.Threading.Tasks.Task.ExecuteEntry(bool bPreventDoubleExecution)
mscorlib.dll!System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()
mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()
[Native to Managed Transition]

If I replace the JsonServiceClient with an async call to something like System.Net.WebRequest (to retrieve and desalinize a JSON response), I never see the exception.

I have tried the JsvServiceClient with the same results. I have tried to use the ConfigureAwait(false) with no success. I have ensured that the web.config file is calling out:

<compilation targetFramework="4.5.2" debug="true" />
<httpRuntime targetFramework="4.5" />

I have wrapped the body of the Get method with:

await Task.Factory.StartNew(() =>

which fixes the problem but drastically reduces the performance of the service. The throughput goes from being able to process 100 concurrent requests in 2-3 seconds to something closer to 20-40 seconds.

I have also been able to make async database calls without encountering the NullReferenceException.

This definitely feels like an issue with the ServiceClient. Any help would be greatly appreciated. Thanks!

Answer 1

使用JsonHttpClient代替JsonServiceClient修复了该问题。