C#

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foreach (var c in Controls)
            {
                if (c is TextBox)
                {
                    ((TextBox)c).Clear();
                }
            }

// top-level 'using statements ...

public static class Constants
{
   public const double PI = Math.PI;

   public static double E { set; get; } = Math.E;
}

namespace WhatEver
{}

using Prism.Mvvm;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace WpfAsynTaskAwait
{
    public class MainViewModel:BindableBase
    {

        #region "ctor"
        public MainViewModel()
        {
            Text = "1: Main UI thread on: " + Thread.CurrentThread.ManagedThreadId.ToString();
            Task.Run(() => DoSomething(MethodCallback)).Await(CompletedCallback, ErrorCallback);
        }
        #endregion

        async Task DoSomething(Action<string> methodCallback)
        {
            await Task.Delay(1500);
            methodCallback?.Invoke("2: Task starts on thread: " + Thread.CurrentThread.ManagedThreadId.ToString());
            await Task.Delay(1500);
            
            //throw new Exception("Aborted exception on thread " + Thread.CurrentThread.ManagedThreadId.ToString());
            
            methodCallback?.Invoke("3: Task runs on thread: " + Thread.CurrentThread.ManagedThreadId.ToString());
            await Task.Delay(1500);            
        }

        #region "Variables"
        private string _text = "Default value";

        public string Text
        {
            get { return _text; }
            set
            {
                SetProperty(ref _text, value);
            }
        }
        #endregion

        #region "Callback methods"

        private void CompletedCallback()
        {
            Text = "4: Method completed called from thread: " + Thread.CurrentThread.ManagedThreadId.ToString();
        }

        private void MethodCallback(string message)
        {
            Text = message + Environment.NewLine +  "Task callback from thread: " + Thread.CurrentThread.ManagedThreadId.ToString(); 
        }

        private void ErrorCallback(Exception ex)
        {
            Text = ex.Message;
        }
        #endregion
    }
    public static class TaskExtensions
    {
        public async static void Await(this Task task, Action completedCallback, Action<Exception> errorCallback)
        {
            try
            {
                await task;
                completedCallback?.Invoke();
            }
            catch (Exception ex)
            {
                errorCallback?.Invoke(ex);
            }
        }
    }
}