Get All Windows Handle

using System.Runtime.InteropServices;

// The signature for the callback method.
public delegate bool CallBack(IntPtr hwnd, int lParam);

[DllImport("user32.dll")]
public static extern int EnumWindows(CallBack callback, int param);
[DllImport("user32.dll")]
public static extern int GetWindowText(IntPtr hWnd,
    StringBuilder lpString, int nMaxCount);

static void Main(string[] args)
{
    // Request that the operating system enumerate all windows,
    // and trigger your callback with the handle of each one.
    EnumWindows(DisplayWindowInfo, 0);
    Console.ReadLine();
}

// The method that will receive the callback. The second
// parameter is not used, but is needed to match the
// callback's signature.
public static bool DisplayWindowInfo(IntPtr hWnd, int lParam)
{
    int chars = 100;
    StringBuilder buf = new StringBuilder(chars);
    if (GetWindowText(hWnd, buf, chars) != 0)
    {
        Console.WriteLine(buf);
    }
    return true;
}

Form Shadow

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private const int CS_DROPSHADOW = 0x00020000;

    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams p = base.CreateParams;
            p.ClassStyle |= CS_DROPSHADOW;
            return p;
        }
    }
}

Form Opacity

private void button1_Click(object sender, EventArgs e)
{
    this.Opacity = 1;
    for (int i = 0; i < 100; i++)
    {
        this.Opacity -= 0.01;
        Application.DoEvents();
    }
}

Follower

bool b = false;
        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            b = true;
            a.Clear();
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            b = false;

        }
        ArrayList a = new ArrayList();
        Point[] p = new Point[50];
        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (b == true)
            {
                a.Add(new Point(e.X, e.Y));
                if (a.Count >= 50)
                {
                    for (int i = 0; i < a.Count - 50; i++)
                    {
                        a.RemoveAt(i);
                    }

                    a.CopyTo(p);
                }
                else
                {
                    a.CopyTo(p);
                }
                Bitmap bp = new Bitmap(this.Width, this.Height);
                Graphics g = Graphics.FromImage(bp);
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                Pen pn = new Pen(Brushes.Black);
                pn.Width = 2;
                pn.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;
                g.DrawLines(pn, p);
                this.BackgroundImage = bp;
            }

Filled Squere

private void Form1_Paint(object sender, PaintEventArgs e)
{
    Graphics g = e.Graphics;
    // Create pen.
    Pen blackPen = new Pen(Color.Black, 3);
    // Create points that define polygon.
    Point point1 = new Point(30, 50);
    Point point2 = new Point(100, 25);
    Point point3 = new Point(200, 5);
    Point point4 = new Point(250, 50);
    Point point5 = new Point(270, 100);
    Point point6 = new Point(250, 250);
    Point[] curvePoints = {point1, point2, point3, point4, point5, point6};
    // Draw polygon to screen.
    g.DrawPolygon(blackPen, curvePoints);
    // Fill polygon
    g.FillPolygon(Brushes.Red, curvePoints);
}