Add project files.

MainWindow.xaml.cs

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+using System;
+using System.ComponentModel;
+using System.Threading;
+using System.Windows;
+using System.Windows.Controls;
+
+namespace ProgressBar
+{
+    /// <summary>
+    /// Interaction logic for MainWindow.xaml
+    /// </summary>
+    public partial class MainWindow : Window
+    {
+        public int Countdown = 1;
+
+        public MainWindow()
+        {
+            InitializeComponent();
+            Countdown = 1;
+            Slider.Value = 1;
+            ProgressBar.Value = 0;
+            Amount.Text = "1";
+        }
+
+        void Worker_DoWork(object sender, DoWorkEventArgs e)
+        {
+            for (int i = 0; i < Countdown; i++)
+            {
+                (sender as BackgroundWorker).ReportProgress(i);
+                Thread.Sleep(1000);
+            }
+        }
+
+        void Worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
+        {
+            ProgressBar.Value = e.ProgressPercentage;
+            UpdateLayout();
+        }
+
+        void Worker_DoneWorking(object sender, RunWorkerCompletedEventArgs e)
+        {
+            ProgressBar.Visibility = Visibility.Collapsed;
+            ButtonDone.Visibility = Visibility.Visible;
+
+            UpdateLayout();
+
+            ProgressBar.Value = 0;
+            Countdown = 1;
+            Slider.Value = 1;
+            Console.Beep();
+        }
+
+        private void BtnStart_Click(object sender, RoutedEventArgs e)
+        {
+            ProgressBar.Visibility = Visibility.Visible;
+            BtnStart.Visibility = Visibility.Collapsed;
+            Slider.Visibility = Visibility.Collapsed;
+            Amount.Visibility = Visibility.Collapsed;
+            AmountLabel.Visibility = Visibility.Collapsed;
+
+            UpdateLayout();
+
+            BackgroundWorker worker = new BackgroundWorker();
+            worker.WorkerReportsProgress = true;
+            worker.DoWork += Worker_DoWork;
+            worker.ProgressChanged += Worker_ProgressChanged;
+            worker.RunWorkerCompleted += Worker_DoneWorking;
+
+            worker.RunWorkerAsync();
+        }
+
+        private void Slider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
+        {
+            double val = Slider.Value;
+            Countdown = Convert.ToInt32(val);
+            if (ProgressBar != null)
+            {
+                ProgressBar.Maximum = Countdown;
+            }
+            Amount.Text = Countdown.ToString();
+        }
+
+        private void ButtonDone_Click(object sender, RoutedEventArgs e) => Close();
+    }
+}