GUI Windows Form C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
public class MainForm : Form {
    private PictureBox happyBox = new PictureBox();
    private int oldX, oldY;
    private bool isDragging;
    private Rectangle dropRect = new Rectangle(100, 100, 140, 170);
    public MainForm() {
        happyBox.SizeMode = PictureBoxSizeMode.StretchImage;
        happyBox.Location = new System.Drawing.Point(64, 32);
        happyBox.Size = new System.Drawing.Size(50, 50);
        happyBox.Cursor = Cursors.Hand;
        happyBox.Image = new Bitmap("happyDude.bmp");
        happyBox.MouseDown += new MouseEventHandler(happyBox_MouseDown);
        happyBox.MouseUp += new MouseEventHandler(happyBox_MouseUp);
        happyBox.MouseMove += new MouseEventHandler(happyBox_MouseMove);
        Controls.Add(happyBox);
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(292, 361);
        this.Text = "Dragging Images";
        this.Paint += new System.Windows.Forms.PaintEventHandler(this.MainForm_Paint);
    }
    void happyBox_MouseMove(object sender, MouseEventArgs e) {
        if (isDragging) {
            happyBox.Top = happyBox.Top + (e.Y - oldY);
            happyBox.Left = happyBox.Left + (e.X - oldX);
        }
    }
    void happyBox_MouseUp(object sender, MouseEventArgs e) {
        isDragging = false;
        if (dropRect.Contains(happyBox.Bounds))
            MessageBox.Show("You win!", "What an amazing test of skill...");
    }
    void happyBox_MouseDown(object sender, MouseEventArgs e) {
        isDragging = true;
        oldX = e.X;
        oldY = e.Y;
    }
    private void MainForm_Paint(object sender, PaintEventArgs e) {
        Graphics g = e.Graphics;
        g.FillRectangle(Brushes.BlueViolet, dropRect);
        g.DrawString("Drag the happy guy in here...",new Font("Times New Roman", 25), Brushes.WhiteSmoke, dropRect);
    }
    [STAThread]
    static void Main() {
        Application.EnableVisualStyles();
        Application.Run(new MainForm());
    }
}