using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Text.RegularExpressions;

namespace Example78
{
	/// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class Form1 : System.Windows.Forms.Form
	{
		private System.Windows.Forms.TextBox txtNumber;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;

		public Form1()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after InitializeComponent call
			//
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.txtNumber = new System.Windows.Forms.TextBox();
			this.SuspendLayout();
			// 
			// txtNumber
			// 
			this.txtNumber.Location = new System.Drawing.Point(88, 48);
			this.txtNumber.Name = "txtNumber";
			this.txtNumber.TabIndex = 0;
			this.txtNumber.Text = "";
			this.txtNumber.TextChanged += new System.EventHandler(this.txtNumber_TextChanged);
			// 
			// Form1
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(292, 273);
			this.Controls.Add(this.txtNumber);
			this.Name = "Form1";
			this.Text = "Form1";
			this.ResumeLayout(false);

		}
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.Run(new Form1());
		}

		private bool IsNonNegativeInteger(string s)
		{
			/*foreach (char x in s)
			{
				if (!Char.IsDigit(x))
					return false;
			}
			return true;*/
			return new Regex("^[0-9]*$").IsMatch(s);
		}

		private void txtNumber_TextChanged(object sender, System.EventArgs e)
		{
			if (txtNumber.Text.Length > 0)
			{
				if (!IsNonNegativeInteger(txtNumber.Text))
				{
					MessageBox.Show("Error");
					txtNumber.Text = txtNumber.Text.Substring(0,
						txtNumber.Text.Length - 1);
					txtNumber.SelectionStart = txtNumber.Text.Length;
					txtNumber.Focus();
				}
			}
		}
	}
}