How do I handle Postback from AdaptiveCard in OnTurnAsync-Method

Question

I'm programming a bot that is using adaptive Cards ChoiceSet. Im trying to get the users choice. Now because the result doesnt Show in the Chat, I have to check in the OnTurnAsync-Method, if the Message I get is a postback. How do i do this? This is how I tried -> Null ReferenceException at if (dc.Context.Activity.GetType().GetProperty("ChannelData") != null)

Edit from Botframework Support: Please do not use the code block below. It only works in Emulator. Instead, use:

if (string.IsNullOrWhiteSpace(activity.Text) && activity.Value != null)
{
    activity.Text = JsonConvert.SerializeObject(activity.Value);
}

My OnTurnAsync-Method:

public async Task OnTurnAsync(ITurnContext turnContext, CancellationToken cancellationToken = default(CancellationToken))
    {
        DialogContext dc = null;

        switch (turnContext.Activity.Type)
        {
            case ActivityTypes.Message:
                if (dc.Context.Activity.GetType().GetProperty("ChannelData") != null)
                {
                    var channelData = JObject.Parse(dc.Context.Activity.ChannelData.ToString());
                    if (channelData.ContainsKey("postback"))
                    {
                        var postbackActivit = dc.Context.Activity;
                        postbackActivit.Text = postbackActivit.Value.ToString();
                        await dc.Context.SendActivityAsync(postbackActivit);
                    }
                }

                await ProcessInputAsync(turnContext, cancellationToken);
                break;

Answer 1

It's because of your line: DialogContext dc = null.

It should be: var dc = await Dialogs.CreateContextAsync(turnContext);

Note that Dialogs might need to be replaced with whatever you defined your DialogSet with. There's a few different ways to do it, but here's the upper half of my <myBot>.cs class that I used to test your previous issue:

public class QuickTestBot_CSharpBot : IBot
    {
        private readonly IStatePropertyAccessor<DialogState> _dialogStateAccessor;
        private readonly ConversationState _conversationState;
        public QuickTestBot_CSharpBot(ConversationState conversationState)
        {
            _conversationState = conversationState ?? throw new ArgumentNullException(nameof(conversationState));
            _dialogStateAccessor = _conversationState.CreateProperty<DialogState>(nameof(DialogState));

            Dialogs = new DialogSet(_dialogStateAccessor);
            Dialogs.Add(new QuickDialog());

        }

        private DialogSet Dialogs { get; set; }

        public async Task OnTurnAsync(ITurnContext turnContext, CancellationToken cancellationToken = default(CancellationToken))
        {
            var activity = turnContext.Activity;

            var dc = await Dialogs.CreateContextAsync(turnContext);

            if (string.IsNullOrWhiteSpace(activity.Text))
            {
                activity.Text = JsonConvert.SerializeObject(activity.Value);
            }
    [...]

Here's some links to a few good samples that also use Waterfall Dialogs, so you can see how they set up their <bot>.cs class (note that they don't get input from adaptive cards...this is just to help you set up your waterfall dialog and OnTurnAsync):

1. MultiTurnPromptsBot
2. CardsBot
3. BasicBot