Task.Run中的实例变量为null

Question

UPDATE:

public MobileServiceUser CurrentMsUser { get; private set; } 

I have an instance property CurrentMsUser which is populated and I'm trying to reference it within an async method, but for some reason it is null in the method. I've set a breakpoint on the property setter and it never is set to null, so I'm fairly certain the property never becomes null. After the async method returns it's present again. It seems as soon as I'm within the anonymous async method I can't access the property. Here is my method:

public async Task CreateOrRetrieveAppUserAsync()
{
    await Task.Run (async () => {
        try {
            var usersCollection = await _userTable.ToCollectionAsync ();
            var users = usersCollection.
                Where(x =>
                    x.FacebookToken == CurrentMsUser.UserId).
                ToList();
            if (users.Count == 1) {
                CurrentRwUser = users [0];
            } else {
                CurrentRwUser = new User { 
                    FacebookToken = CurrentMsUser.UserId, 
                    GoogleToken = "test",
                    TwitterToken = "test",
                    MicrosoftToken = "test",
                    Email = "test@gmail.com",
                    FacebookId = App.FacebookProvider.GetCurrentUserId (),
                    Name = App.FacebookProvider.GetCurrentUserName ()
                };
                await InsertUserAsync (CurrentRwUser);
                await SyncAsync();
            }
        } catch (Exception ex) {
            Debug.WriteLine ("CreateOrRetrieveAppUser failed: {0}", ex.Message);
        }
    }).ConfigureAwait(continueOnCapturedContext:false);
}

Answer 1

It appears that that variable is thread local, which is why its value is only accessible in your main thread.

You should grab the value of that variable in your main thread before moving to the background thread to avoid this problem.

public async Task CreateOrRetrieveAppUserAsync()
{
    var userId = CurrentMsUser.UserId;
    await Task.Run (async () => {
    //...
    //use userId here
    });
}

Of course, given that you don't appear to have any long running CPU bound work, and all of the work that you have here is already asynchronous task based IO work, you should be able to simply remove the Task.Run call entirely, allowing what little CPU bound work you do have to run in the UI thread, without blocking it at all due to the long running operations all already being asynchronous.

Oh, and since you only need the very first item of usersCollection you should use First , not ToList , to avoid materializing the entire result set into memory. You should also perform that First and your filtering before calling ToCollectionAsync to, if at all possible, do all of this filtering through the query provider rather than materializing the entire collection into your application.