## Formatting a percentage

I have a form that I am working on, and have a quick question:

The form has the user enter a value for Interest Rate, like 12 for 12%. I need it to format anything entered in that box so that it displays as 12.00%. The problem is, if I just use something like txtInterestRate.Text = interestRateYearly.ToString ("p"); and the user enters 12 it displays 1,200.00%.

I am trying to use String.Format instead, and this is what I have so far:
Code:
```using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace FutureValue
{
// This is the starting point for exercise 9-3 from
// "Murach's C# 2010" by Joel Murach
// (c) 2010 by Mike Murach & Associates, Inc.
// www.murach.com

public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void btnCalculate_Click(object sender, System.EventArgs e)
{
try
{
if (DataIsValid())
{
// TODO: use Strip method here, where necessary
decimal monthlyInvestment = Convert.ToDecimal(Strip(txtMonthlyInvestment.Text));
decimal interestRateYearly =
Convert.ToDecimal(Strip(txtInterestRate.Text));
int years =
Convert.ToInt32(Strip(txtYears.Text));

int months = years * 12;
decimal interestRateMonthly = interestRateYearly / 12 / 100;
decimal futureValue = CalculateFutureValue(
monthlyInvestment, interestRateMonthly, months);

txtMonthlyInvestment.Text = monthlyInvestment.ToString("c");
string.Format ("Value: {0:P2}.", txtInterestRate.Text);
txtFutureValue.Text = futureValue.ToString("c");
txtMonthlyInvestment.Focus();
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message + "\n\n" +
ex.GetType().ToString() + "\n" +
ex.StackTrace, "Exception");
}
}

public bool DataIsValid()
{
return
// Validate the Monthly Investment text box
IsPresent(txtMonthlyInvestment, "Monthly Investment") &&
IsDecimal(txtMonthlyInvestment, "Monthly Investment") &&
IsWithinRange(txtMonthlyInvestment, "Monthly Investment", 1, 1000) &&

// Validate the Yearly Interest Rate text box
IsPresent(txtInterestRate, "Yearly Interest Rate") &&
IsDecimal(txtInterestRate, "Yearly Interest Rate") &&
IsWithinRange(txtInterestRate, "Yearly Interest Rate", 1, 20) &&

// Validate the Number of Years text box
IsPresent(txtYears, "Number of Years") &&
IsInt32(txtYears, "Number of Years") &&
IsWithinRange(txtYears, "Number of Years", 1, 40);
}

public bool IsPresent(TextBox textBox, string name)
{
if (textBox.Text == "")
{
MessageBox.Show(name + " is a required field.", "Entry Error");
textBox.Focus();
return false;
}
return true;
}

// TODO: improve this IsDecimal method
public bool IsDecimal(TextBox textBox, string name)
{
string s = textBox.Text;
int decimalCount = 0;
bool validDecimal = true;
foreach (char c in s)
{
if (!(
c == '0' || c == '1' || c == '2' || c == '3' ||
c == '4' || c == '5' || c == '6' || c == '7' ||
c == '8' || c == '9' || c == '.' || c == '\$' ||
c == '%' || c == ',' || c == ' '
))
{
validDecimal = false;
break;
}
if (c == '.')
{
decimalCount++;
}
}
if (validDecimal && decimalCount <= 1)
{
return true;
}
else
{
MessageBox.Show(name + " must be a decimal value.", "Entry Error");
textBox.Focus();
return false;
}
}

public string Strip(string s)
{
foreach (char c in s)
{
if (c == '\$' || c == '%' || c == ',' || c == ' ')
{
int i = s.IndexOf(c);
s = s.Remove(i, 1);
}
}
return s;
}

// TODO: improve this IsInt32 method
public bool IsInt32(TextBox textBox, string name)
{
string s = textBox.Text;
bool validDecimal = true;
foreach (char c in s)
{
if (!(
c == '0' || c == '1' || c == '2' || c == '3' ||
c == '4' || c == '5' || c == '6' || c == '7' ||
c == '8' || c == '9' || c == '\$' || c == '%' ||
c == ',' || c == ' '
))
{
validDecimal = false;
break;
}
}
if (validDecimal == true)
{
return true;
}
else
{
MessageBox.Show(name + " must be an integer.", "Entry Error");
textBox.Focus();
return false;
}
}
public bool IsWithinRange(TextBox textBox, string name,
decimal min, decimal max)
{
// TODO: add Strip method where necessary
string s = textBox.Text;
decimal number = Convert.ToDecimal(Strip(s));
if (number < min || number > max)
{
MessageBox.Show(name + " must be between " + min +
" and " + max + ".", "Entry Error");
textBox.Focus();
return false;
}
return true;
}

private decimal CalculateFutureValue(decimal monthlyInvestment,
decimal interestRateMonthly, int months)
{
decimal futureValue = 0m;
for (int i = 0; i < months; i++)
{
futureValue = (futureValue + monthlyInvestment)
* (1 + interestRateMonthly);
}
return futureValue;
}

private void btnExit_Click(object sender, System.EventArgs e)
{
this.Close();
}

}
}```
This doesn't change the value at all, so I know I am doing something wrong. My book is not helping so far. Any ideas what I am doing wrong? The book example is:

string balance1 = String.Format("{0:P2}", 12);

This won't work for me as the interest rate is a decimal and not a string. I appreciate any help on this.