C# Logic Issues

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace EE_Q28634325
{
	public partial class Form1 : Form
	{
		private bool fIsClosing;

		public Form1()
		{
			InitializeComponent();
		}

		private void OnLoad(object sender, EventArgs e)
		{
			tbHours.Enabled = string.IsNullOrEmpty(tbMPH.Text);
			btnCalculate.Enabled = false;
		}

		private void OnClick(object sender, EventArgs e)
		{
			if (!fIsClosing)
			{
				decimal mph = Convert.ToDecimal(tbMPH.Text);
				decimal hours = Convert.ToDecimal(tbHours.Text);
				decimal total = mph * hours;
				lbTotals.Items.Add(string.Format("You traveled {0} in {1} at {2} per hour.",
					total != 1 ? string.Format("{0} miles", total) : string.Format("{0} mile", total), 
					hours != 1 ? string.Format("{0} hours", hours) : string.Format("{0} hour", hours), 
					mph != 1 ? string.Format("{0} miles", mph) : string.Format("{0} mile", mph)));
				tbMPH.Clear();
				tbHours.Clear();
				tbHours.Enabled = btnCalculate.Enabled = false;
			}
		}

		private void OnValidating(object sender, CancelEventArgs e)
		{
			if (!fIsClosing)
			{
				if (sender is TextBox)
				{
					var tb = sender as TextBox;
					Decimal tempValue = -1;
					if (!Decimal.TryParse(tb.Text, out tempValue))
					{
						MessageBox.Show(string.Format("You must enter a numeric value in the {0} textbox.", tb.Equals(tbHours) ? "hours" : "miles per hour (MPH)"));
						e.Cancel = true;
					}
					else
					{
						if (tb.Equals(tbMPH))
							tbHours.Enabled = true;

						if (tb.Equals(tbHours))
							btnCalculate.Enabled = true;
					}
				}
			}
		}

		private void OnClosing(object sender, FormClosingEventArgs e)
		{
			fIsClosing = true;
		}
	}
}

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namespace EE_Q28634325
{
	partial class Form1
	{
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.IContainer components = null;

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
		protected override void Dispose(bool disposing)
		{
			if (disposing && (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.lblMPH = new System.Windows.Forms.Label();
			this.tbMPH = new System.Windows.Forms.TextBox();
			this.tbHours = new System.Windows.Forms.TextBox();
			this.lblHours = new System.Windows.Forms.Label();
			this.lbTotals = new System.Windows.Forms.ListBox();
			this.btnCalculate = new System.Windows.Forms.Button();
			this.SuspendLayout();
			// 
			// lblMPH
			// 
			this.lblMPH.AutoSize = true;
			this.lblMPH.Location = new System.Drawing.Point(13, 13);
			this.lblMPH.Name = "lblMPH";
			this.lblMPH.Size = new System.Drawing.Size(158, 13);
			this.lblMPH.TabIndex = 0;
			this.lblMPH.Text = "Enter the Miles Per Hour (MPH):";
			// 
			// tbMPH
			// 
			this.tbMPH.Location = new System.Drawing.Point(177, 10);
			this.tbMPH.Name = "tbMPH";
			this.tbMPH.Size = new System.Drawing.Size(100, 20);
			this.tbMPH.TabIndex = 1;
			this.tbMPH.Validating += new System.ComponentModel.CancelEventHandler(this.OnValidating);
			// 
			// tbHours
			// 
			this.tbHours.Location = new System.Drawing.Point(177, 36);
			this.tbHours.Name = "tbHours";
			this.tbHours.Size = new System.Drawing.Size(100, 20);
			this.tbHours.TabIndex = 3;
			this.tbHours.Validating += new System.ComponentModel.CancelEventHandler(this.OnValidating);
			// 
			// lblHours
			// 
			this.lblHours.AutoSize = true;
			this.lblHours.Location = new System.Drawing.Point(13, 39);
			this.lblHours.Name = "lblHours";
			this.lblHours.Size = new System.Drawing.Size(156, 13);
			this.lblHours.TabIndex = 2;
			this.lblHours.Text = "Enter the time traveled in hours:";
			// 
			// lbTotals
			// 
			this.lbTotals.FormattingEnabled = true;
			this.lbTotals.Location = new System.Drawing.Point(16, 89);
			this.lbTotals.Name = "lbTotals";
			this.lbTotals.Size = new System.Drawing.Size(260, 160);
			this.lbTotals.TabIndex = 4;
			// 
			// btnCalculate
			// 
			this.btnCalculate.Location = new System.Drawing.Point(200, 63);
			this.btnCalculate.Name = "btnCalculate";
			this.btnCalculate.Size = new System.Drawing.Size(75, 23);
			this.btnCalculate.TabIndex = 5;
			this.btnCalculate.Text = "Calculate";
			this.btnCalculate.UseVisualStyleBackColor = true;
			this.btnCalculate.Click += new System.EventHandler(this.OnClick);
			// 
			// Form1
			// 
			this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
			this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
			this.ClientSize = new System.Drawing.Size(288, 264);
			this.Controls.Add(this.btnCalculate);
			this.Controls.Add(this.lbTotals);
			this.Controls.Add(this.tbHours);
			this.Controls.Add(this.lblHours);
			this.Controls.Add(this.tbMPH);
			this.Controls.Add(this.lblMPH);
			this.Name = "Form1";
			this.Text = "Form1";
			this.FormClosing += new System.Windows.Forms.FormClosingEventHandler(this.OnClosing);
			this.Load += new System.EventHandler(this.OnLoad);
			this.ResumeLayout(false);
			this.PerformLayout();

		}

		#endregion

		private System.Windows.Forms.Label lblMPH;
		private System.Windows.Forms.TextBox tbMPH;
		private System.Windows.Forms.TextBox tbHours;
		private System.Windows.Forms.Label lblHours;
		private System.Windows.Forms.ListBox lbTotals;
		private System.Windows.Forms.Button btnCalculate;
	}
}

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