Add Windows Forms TaskDialog API sample

windowsforms/TaskDialogDemo/cs/Program.cs

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+using System;
+using System.Windows.Forms;
+
+namespace TaskDialogDemo
+{
+    static class Program
+    {
+        /// <summary>
+        /// The main entry point for the application.
+        /// </summary>
+        [STAThread]
+        static void Main()
+        {
+            Application.SetHighDpiMode(HighDpiMode.SystemAware);
+            // Note: Calling Application.EnableVisualStyles() is required for the task
+            // dialog to work.
+            Application.EnableVisualStyles();
+            Application.SetCompatibleTextRenderingDefault(false);
+            Application.Run(new Form1());
+        }
+    }
+}

windowsforms/TaskDialogDemo/cs/README.md

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+---
+languages:
+- csharp
+products:
+- dotnet
+- windows
+page_type: sample
+name: "Windows Forms Sample: Task Dialog (C#)"
+urlFragment: "winforms-taskdialog-cs"
+description: ".NET 5.0 Windows Forms application that demonstrates how to use Task Dialog"
+---
+
+# Task Dialog Demo
+
+This sample is a .NET 5.0 Windows Forms application that demonstrates how to use [`Task Dialog`](https://docs.microsoft.com/en-us/windows/win32/controls/task-dialogs-overview).
+
+![Screenshot](../images/screenshot.jpg)
+
+Credits: this sample was authored by @kpreisser, the author of [`Task Dialog` for Windows Forms](https://github.com/dotnet/winforms/pull/1133).
+
+## Sample prerequisites
+
+This sample is written in C# and targets .NET 5.0 running on Windows. It requires the [.NET 5.0 SDK](https://dotnet.microsoft.com/download/dotnet/5.0).
+
+## Building the sample
+
+To download and run the sample, follow these steps:
+
+01. Download and unzip the sample.
+
+01. In Visual Studio 2019 or later:
+
+    01. On the menu bar, choose **File** > **Open** > **Project/Solution**.
+
+    01. Navigate to the folder that holds the unzipped sample code, and open the project (*.csproj*) file.
+
+    01. Choose the F5 key, or Ctrl+F5 keys, to run the project.
+
+01. From the command line:
+
+    01. Navigate to the folder that holds the unzipped sample code.
+
+    01. At the command line, type `dotnet run`.

windowsforms/TaskDialogDemo/cs/TaskDialogDemo.csproj

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+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net5.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <UseWindowsForms>true</UseWindowsForms>
+  </PropertyGroup>
+
+</Project>