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;
using System.IO;
public class SimpleEditorForm : Form {
    private string filename = "Untitled";
    public SimpleEditorForm(string filename) {
        InitializeComponent();
        if (filename != null) {
            this.filename = filename;
            OpenFile();
        }
    }
    protected void OpenFile() {
        try {
            textBoxEdit.Clear();
            textBoxEdit.Text = File.ReadAllText(filename);
        } catch (IOException ex) {
            MessageBox.Show(ex.Message, "Simple Editor",
                MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
        }
    }
    private void OnFileNew(object sender, EventArgs e) {
        filename = "Untitled";
        textBoxEdit.Clear();
    }
    private void OnFileOpen(object sender, EventArgs e) {
        if (dlgOpenFile.ShowDialog() == DialogResult.OK) {
            filename = dlgOpenFile.FileName;
            OpenFile();
        }
    }
    private void OnFileSave(object sender, EventArgs e) {
    }
    private void OnFileSaveAs(object sender, EventArgs e) {
    }
    private void InitializeComponent() {
        this.textBoxEdit = new System.Windows.Forms.TextBox();
        this.mainMenu = new System.Windows.Forms.MenuStrip();
        this.miFile = new System.Windows.Forms.ToolStripMenuItem();
        this.miFileNew = new System.Windows.Forms.ToolStripMenuItem();
        this.miFileOpen = new System.Windows.Forms.ToolStripMenuItem();
        this.miFileSave = new System.Windows.Forms.ToolStripMenuItem();
        this.miFileSaveAs = new System.Windows.Forms.ToolStripMenuItem();
        this.dlgOpenFile = new System.Windows.Forms.OpenFileDialog();
        this.mainMenu.SuspendLayout();
        this.SuspendLayout();
        // 
        // textBoxEdit
        // 
        this.textBoxEdit.AcceptsReturn = true;
        this.textBoxEdit.AcceptsTab = true;
        this.textBoxEdit.Dock = System.Windows.Forms.DockStyle.Fill;
        this.textBoxEdit.Location = new System.Drawing.Point(0, 24);
        this.textBoxEdit.Multiline = true;
        this.textBoxEdit.Name = "textBoxEdit";
        this.textBoxEdit.ScrollBars = System.Windows.Forms.ScrollBars.Both;
        this.textBoxEdit.Size = new System.Drawing.Size(562, 219);
        this.textBoxEdit.TabIndex = 0;
        // 
        // mainMenu
        // 
        this.mainMenu.Items.AddRange(new System.Windows.Forms.ToolStripItem[] {
            this.miFile});
        this.mainMenu.Location = new System.Drawing.Point(0, 0);
        this.mainMenu.Name = "mainMenu";
        this.mainMenu.Size = new System.Drawing.Size(562, 24);
        this.mainMenu.TabIndex = 1;
        this.mainMenu.Text = "menuStrip1";
        // 
        // miFile
        // 
        this.miFile.DropDownItems.AddRange(new System.Windows.Forms.ToolStripItem[] {
            this.miFileNew,
            this.miFileOpen,
            this.miFileSave,
            this.miFileSaveAs});
        this.miFile.Name = "miFile";
        this.miFile.Text = "&File";
        // 
        // miFileNew
        // 
        this.miFileNew.Name = "miFileNew";
        this.miFileNew.Text = "&New";
        this.miFileNew.Click += new System.EventHandler(this.OnFileNew);
        // 
        // miFileOpen
        // 
        this.miFileOpen.Name = "miFileOpen";
        this.miFileOpen.Text = "&Open";
        this.miFileOpen.Click += new System.EventHandler(this.OnFileOpen);
        // 
        // miFileSave
        // 
        this.miFileSave.Name = "miFileSave";
        this.miFileSave.Text = "&Save";
        this.miFileSave.Click += new System.EventHandler(this.OnFileSave);
        // 
        // miFileSaveAs
        // 
        this.miFileSaveAs.Name = "miFileSaveAs";
        this.miFileSaveAs.Text = "Save &As";
        this.miFileSaveAs.Click += new System.EventHandler(this.OnFileSaveAs);
        // 
        // dlgOpenFile
        // 
        this.dlgOpenFile.Filter = "Text Documents (*.txt)|*.txt|Wrox Documents (*.wroxtext)|*.wroxtext|All Files|*.*" +
            "";
        this.dlgOpenFile.FilterIndex = 2;
        // 
        // SimpleEditorForm
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(562, 243);
        this.Controls.Add(this.textBoxEdit);
        this.Controls.Add(this.mainMenu);
        this.MainMenuStrip = this.mainMenu;
        this.Name = "SimpleEditorForm";
        this.Text = "Simple Editor";
        this.mainMenu.ResumeLayout(false);
        this.ResumeLayout(false);
        this.PerformLayout();
    }
    private System.Windows.Forms.TextBox textBoxEdit;
    private System.Windows.Forms.MenuStrip mainMenu;
    private System.Windows.Forms.ToolStripMenuItem miFile;
    private System.Windows.Forms.ToolStripMenuItem miFileNew;
    private System.Windows.Forms.ToolStripMenuItem miFileOpen;
    private System.Windows.Forms.ToolStripMenuItem miFileSave;
    private System.Windows.Forms.ToolStripMenuItem miFileSaveAs;
    private System.Windows.Forms.OpenFileDialog dlgOpenFile;
    [STAThread]
    static void Main(string[] args) {
        string filename = null;
        if (args.Length != 0)
            filename = args[0];
        Application.EnableVisualStyles();
        Application.Run(new SimpleEditorForm(filename));
    }
}