C#

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
 
namespace WindowsApplication5
{
    /// 
    /// Summary description for Form1.
    /// 
    public class Form1 : System.Windows.Forms.Form
    {
        Single m_Labour = 0.0;
        Single m_Material = 0.0;
        private System.Windows.Forms.TextBox textBox3;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.TextBox textBox2;
        /// 
        /// Required designer variable.
        /// 
        private System.ComponentModel.Container components = null;
 
        public Form1()
        {
            //
            // Required for Windows Form Designer support
            //
            InitializeComponent();

            //
            // TODO: Add any constructor code after InitializeComponent call
            //
        }
 
        /// 
        /// Clean up any resources being used.
        /// 
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null) 
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }
 
#region Windows Form Designer generated code
        /// 
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// 
        private void InitializeComponent()
        {
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.textBox2 = new System.Windows.Forms.TextBox();
            this.textBox3 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(64, 56);
            this.textBox1.Name = "textBox1";
            this.textBox1.TabIndex = 0;
            this.textBox1.Text = "";
            this.textBox1.Leave += new System.EventHandler(this.textBox1_Leave);
            // 
            // textBox2
            // 
            this.textBox2.Location = new System.Drawing.Point(184, 56);
            this.textBox2.Name = "textBox2";
            this.textBox2.TabIndex = 1;
            this.textBox2.Text = "";
            this.textBox2.Leave += new System.EventHandler(this.textBox2_Leave);
            // 
            // textBox3
            // 
            this.textBox3.Location = new System.Drawing.Point(112, 104);
            this.textBox3.Name = "textBox3";
            this.textBox3.TabIndex = 2;
            this.textBox3.Text = "";
            // 
            // Form1
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(292, 266);
            this.Controls.Add(this.textBox3);
            this.Controls.Add(this.textBox2);
            this.Controls.Add(this.textBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
        }
#endregion
 
        /// 
        /// The main entry point for the application.
        /// 
        [STAThread]
        static void Main() 
        {
            Application.Run(new Form1());
        }
 
        private void textBox1_Leave(object sender, System.EventArgs e)
        {
            m_Labour = ValidateInput(textbox1);
            textbox1.Text = m_Labour.ToString("##,###.00");
            UpdateTotalTextBox();
        }
 
        private void textBox2_TextChanged(object sender, System.EventArgs e)
        {
            m_Material = ValidateInput(textbox2);
            textbox2.Text = m_Material.ToString("##,###.00");
            UpdateTotalTextBox();
        }
 
        private Single ValidateInput(ref TextBox textboxToValidate)
        {
            Single returnValue;
 
            try
            {
                returnValue = Single.Parse(textboxToValidate.Text);
            }
            catch (Exception e)
            {
                returnValue = 0;
            }
            return returnValue;
        }
 
        private void UpdateTotalTextbox()
        {
            Single jobTotal = m_Labour + m_Material;
            textBox3.Text = jobTotal.ToString("##,###.00");
        }
    }
}

private const int BufferSize = 4096;