GUI Windows Form C#

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
public class Form1 : System.Windows.Forms.Form {
    private System.Windows.Forms.TreeView treeView1;
    ImageList il = new ImageList();
    public Form1() {
        this.treeView1 = new System.Windows.Forms.TreeView();
        this.SuspendLayout();
        this.treeView1.Anchor = (((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Bottom)
           | System.Windows.Forms.AnchorStyles.Left)
           | System.Windows.Forms.AnchorStyles.Right);
        this.treeView1.Font = new System.Drawing.Font("Courier New", 12F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((System.Byte)(0)));
        this.treeView1.HotTracking = true;
        this.treeView1.ImageIndex = -1;
        this.treeView1.Indent = 30;
        this.treeView1.ItemHeight = 30;
        this.treeView1.LabelEdit = true;
        this.treeView1.Location = new System.Drawing.Point(8, 16);
        this.treeView1.SelectedImageIndex = -1;
        this.treeView1.Size = new System.Drawing.Size(360, 272);
        this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
        this.ClientSize = new System.Drawing.Size(376, 309);
        this.Controls.AddRange(new System.Windows.Forms.Control[] {
                                                                      this.treeView1});
        this.Text = "TreeView Control";
        this.Load += new System.EventHandler(this.Form1_Load);
        this.ResumeLayout(false);
    }
    [STAThread]
    static void Main() {
        Application.Run(new Form1());
    }
    private void Form1_Load(object sender, System.EventArgs e) {
        il.Images.Add(new Icon("1.ICO"));
        il.Images.Add(new Icon("2.ICO"));
        il.Images.Add(new Icon("3.ICO"));
        il.Images.Add(new Icon("4.ICO"));
        treeView1.ImageList = il;
        TreeNode rootNode = treeView1.Nodes.Add("USA");
        rootNode.ImageIndex = 0;
        TreeNode states1 = rootNode.Nodes.Add("a");
        states1.ImageIndex = 1;
        TreeNode states2 = rootNode.Nodes.Add("b");
        states2.ImageIndex = 1;
        TreeNode states3 = rootNode.Nodes.Add("c");
        states3.ImageIndex = 1;
        TreeNode states4 = rootNode.Nodes.Add("d");
        states4.ImageIndex = 1;
        TreeNode child = states1.Nodes.Add("A");
        child.ImageIndex = 2;
        child = states1.Nodes.Add("e");
        child.ImageIndex = 2;
        child = states1.Nodes.Add("f");
        child.ImageIndex = 2;
        child = states2.Nodes.Add("g");
        child.ImageIndex = 2;
        child = states2.Nodes.Add("h");
        child.ImageIndex = 2;
        child = states2.Nodes.Add("i");
        child.ImageIndex = 2;
        child = states3.Nodes.Add("j");
        child.ImageIndex = 2;
        child = states3.Nodes.Add("k");
        child.ImageIndex = 2;
        child = states3.Nodes.Add("l");
        child.ImageIndex = 2;
        child = states4.Nodes.Add("m");
        child.ImageIndex = 2;
        child = states4.Nodes.Add("n");
        child.ImageIndex = 2;
        child = states4.Nodes.Add("o");
        child.ImageIndex = 2;
    }
}