RavenDB LoadAsync。 无法捕捉异常

Question

I have a simple async method that loads an entity asynchronously using LoadAsync. I can't catch exceptions thrown by the async call. Debugging just leads to 'AggregateException was unhandled by user code.' I'm using RavenDB Client 2.0.2375.

How can I catch these exceptions?

Here's the method:

 private async Task<Dummy> GetDummyAsync(string id)
    {
        using (var session = docStore.OpenAsyncSession())
        {
            try
            {
                var dummy = await session.LoadAsync<Dummy>(id);
                return dummy;
            }
            catch (Exception ex)
            {
                return null;
            }
        }
    }

Edit

Here's the exception:

System.AggregateException was unhandled by user code HResult=-2146233088 Message=One or more errors occurred. Source=mscorlib StackTrace: at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions) at System.Threading.Tasks.Task 1.GetResultCore(Boolean waitCompletionNotification) at System.Threading.Tasks.Task 1.get_Result() at Raven.Client.Connection.HttpJsonRequest.<>c_ DisplayClassc.b _9() in c:\\Builds\\RavenDB-Stable\\Raven.Client.Lightweight\\Connection\\HttpJsonRequest.cs:line 128 at Raven.Client.Connection.HttpJsonRequest.ReadJsonInternal(Func 1 getResponse) in c:\\Builds\\RavenDB-Stable\\Raven.Client.Lightweight\\Connection\\HttpJsonRequest.cs:line 351 at Raven.Client.Connection.HttpJsonRequest.<InternalReadResponseStringAsync>b__8(Task 1 task) in c:\\Builds\\RavenDB-Stable\\Raven.Client.Lightweight\\Connection\\HttpJsonRequest.cs:line 128 at System.Threading.Tasks.ContinuationResultTaskFromResultTask 2.InnerInvoke() at System.Threading.Tasks.Task.Execute() InnerException: System.Net.WebException HResult=-2146233079 Message=The remote server returned an error: (404) Not Found. Source=System StackTrace: at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at System.Threading.Tasks.TaskFactory 2.InnerInvoke() at System.Threading.Tasks.Task.Execute() InnerException: System.Net.WebException HResult=-2146233079 Message=The remote server returned an error: (404) Not Found. Source=System StackTrace: at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at System.Threading.Tasks.TaskFactory 2.InnerInvoke() at System.Threading.Tasks.Task.Execute() InnerException: System.Net.WebException HResult=-2146233079 Message=The remote server returned an error: (404) Not Found. Source=System StackTrace: at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at System.Threading.Tasks.TaskFactory 1.FromAsyncCoreLogic(IAsyncResult iar, Func 2 endFunction, Action 1 endAction, Task`1 promise, Boolean requiresSynchronization) InnerException:

Answer 1

The behavior your seeing is valid, and is not because you couldn't catch the exception. It's because the exception was already caught for you.

In RavenDB, if you try to load a document that doesn't exist, the Load or LoadAsync methods will return null . They will not throw an exception.

When you are debugging, you are seeing that under the hood a WebException is thrown when the HTTP response comes back as 404. That exception is handled. You should see it in the output window as a "First Chance Exception".

This is all normal behavior. If you want to check that you can catch exceptions, try something that will actually throw an exception. Perhaps load a document of one type while trying to cast it into another. That will certainly throw an exception.