Hello World Sample With Microsoft Bot Framework

[BotAuthentication]
public class VendaController: ApiController
{
        public async Task < HttpResponseMessage > Post([FromBody] Activity activity)
        {
            await Conversation.SendAsync(activity, () => new TestDialog());
            return new HttpResponseMessage(System.Net.HttpStatusCode.Accepted);
        }
    }
    [Serializable]
public class TestDialog: IDialog < object >
  {
    public async Task StartAsync(IDialogContext context)
  {
        context.Wait(MessageReceivedATask);
    }
    public async Task MessageReceivedATask(IDialogContext context, IAwaitable < IMessageActivity > argument) {
        if (message.Text == "start") {
            await context.PostAsync("Hello world!");
            context.Wait(MessageReceivedATask);
        } else if (message.Text = "change language") {
            PromptDialog.Choice(context, SelectLanguage, new List < string > () {
                "English",
                "Français",
                "Persian"
            }, "please select your language");
        }
    }
}