Financial Loan Calculations using C#

Here is a C# Application I developed forcalculating the monthly payment for a loan, given the loan amount,interest rate and the loan period.

I have used four labels, four text boxes andtwo buttons on a simple Windows form as shown below. In addition, astatus bar along the bottom of the form is used to describe what isexpected in each text field. The Calculate button computes the monthlypayment and displays the result in the fourth text box. Clicking theExit button quits the application. Tool tips are also displayed whenthe mouse hovers over each text box. A timer control is used to displaythe right message in the status bar.

The text boxes preventnon-numeric entry, by filtering out characters other than 0-9 and thedecimal point. The user is not allowed to edit the fourth text box(Payment). See the event handler under the textBox1.KeyPress fordetails of how non-numeric keys are handled. The KeyPressEventHandlertakes care of this. The calculation for monthly payments is performedby the static method calcPayment that is invoked by the click event ofthe Calculate button. The use of both the status bar and tooltips mayappear to be an overkill. However, I left them both in simply toillustrate the use of the tooltips and the status bar messages forhelping the user with data input. Comments are welcome. You may emailme at: smenon@systek.org.

//C# Code for the Financial Loan Calculation Application

using System;
using System.Drawing;
using System.Windows.Forms;

namespace PMTApp
{

public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.Label label1;
private System.Windows.Forms.TextBox textBox1;
private System.Windows.Forms.Label label2;
private System.Windows.Forms.TextBox textBox2;
private System.Windows.Forms.Label label3;
private System.Windows.Forms.TextBox textBox3;
private System.Windows.Forms.Label label4;
private System.Windows.Forms.TextBox textBox4;
private System.Windows.Forms.Button button1;
private System.Windows.Forms.Button button2;
private System.Windows.Forms.StatusBar statusBar1;
private System.Windows.Forms.Timer timer1;
private System.Windows.Forms.ToolTip toolTip1;
private System.ComponentModel.IContainer components;

public Form1()
{

InitializeComponent();

}

protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

private void InitializeComponent()
{
this.components = new System.ComponentModel.Container();
this.textBox4 = new System.Windows.Forms.TextBox();
this.label1 = new System.Windows.Forms.Label();
this.label2 = new System.Windows.Forms.Label();
this.label3 = new System.Windows.Forms.Label();
this.button1 = new System.Windows.Forms.Button();
this.button2 = new System.Windows.Forms.Button();
this.textBox2 = new System.Windows.Forms.TextBox();
this.textBox3 = new System.Windows.Forms.TextBox();
this.label4 = new System.Windows.Forms.Label();
this.textBox1 = new System.Windows.Forms.TextBox();
this.statusBar1 = new System.Windows.Forms.StatusBar();
this.timer1 = new System.Windows.Forms.Timer(this.components);
this.toolTip1 = new System.Windows.Forms.ToolTip(this.components);
this.SuspendLayout();
//
// textBox4
//
this.textBox4.BackColor = System.Drawing.Color.Gainsboro;
this.textBox4.Location = new System.Drawing.Point(136, 144);
this.textBox4.Name = "textBox4";
this.textBox4.ReadOnly = true;
this.textBox4.Size = new System.Drawing.Size(80, 20);
this.textBox4.TabIndex = 4;
this.textBox4.TabStop = false;
this.textBox4.Text = "";
this.toolTip1.SetToolTip(this.textBox4, "Monthly payment");
//
// label1
//
this.label1.Location = new System.Drawing.Point(32, 24);
this.label1.Name = "label1";
this.label1.Size = new System.Drawing.Size(96, 24);
this.label1.TabIndex = 0;
this.label1.Text = "Present value-$";
//
// label2
//
this.label2.Location = new System.Drawing.Point(40, 64);
this.label2.Name = "label2";
this.label2.Size = new System.Drawing.Size(96, 24);
this.label2.TabIndex = 0;
this.label2.Text = "Interest rate-%";
//
// label3
//
this.label3.Location = new System.Drawing.Point(40, 112);
this.label3.Name = "label3";
this.label3.Size = new System.Drawing.Size(96, 24);
this.label3.TabIndex = 0;
this.label3.Text = "Period";
//

// button1
//
this.button1.BackColor = Color.FromArgb(((System.Byte)(0)),
((System.Byte)(192)), ((System.Byte)(192)));
this.button1.Location = new System.Drawing.Point(72, 216);
this.button1.Name = "button1";
this.button1.Size = new System.Drawing.Size(72, 24);
this.button1.TabIndex = 5;
this.button1.Text = "Calculate";
this.toolTip1.SetToolTip(this.button1, "Calculate the monthly payment");
this.button1.Click += new System.EventHandler(this.button1_Click);

//
// button2
//
this.button2.BackColor = Color.FromArgb(((System.Byte)(0)), ((System.Byte)(192)), ((System.Byte)(192)));
this.button2.Location = new System.Drawing.Point(168, 216);
this.button2.Name = "button2";
this.button2.Size = new System.Drawing.Size(72, 24);
this.button2.TabIndex = 6;
this.button2.Text = "E&xit";
this.button2.Click += new System.EventHandler(this.button2_Click);
//
// textBox2
//
this.textBox2.BackColor = System.Drawing.Color.White;
this.textBox2.Location = new System.Drawing.Point(136, 64);
this.textBox2.Name = "textBox2";
this.textBox2.Size = new System.Drawing.Size(80, 20);
this.textBox2.TabIndex = 2;
this.textBox2.Text = "10.00";
this.toolTip1.SetToolTip(this.textBox2, "Interest rate per year");
this.textBox2.KeyPress += new System.Windows.Forms.KeyPressEventHandler(this.txt_KeyPress);
this.textBox2.TextChanged += new System.EventHandler(this.txt_TextChanged);
//
// textBox3
//
this.textBox3.BackColor = System.Drawing.Color.White;
this.textBox3.Location = new System.Drawing.Point(136, 104);
this.textBox3.Name = "textBox3";
this.textBox3.Size = new System.Drawing.Size(80, 20);
this.textBox3.TabIndex = 3;
this.textBox3.Text = "60";
this.toolTip1.SetToolTip(this.textBox3, "Period in months");
this.textBox3.KeyPress += new System.Windows.Forms.KeyPressEventHandler(this.txt_KeyPress);
this.textBox3.TextChanged += new System.EventHandler(this.txt_TextChanged);
//
// label4
//
this.label4.Location = new System.Drawing.Point(40, 152);
this.label4.Name = "label4";
this.label4.Size = new System.Drawing.Size(96, 24);
this.label4.TabIndex = 0;
this.label4.Text = "Payment PMT – $";
//

// textBox1
//
this.textBox1.BackColor = System.Drawing.Color.White;
this.textBox1.Location = new System.Drawing.Point(136, 24);
this.textBox1.Name = "textBox1";
this.textBox1.Size = new System.Drawing.Size(80, 20);
this.textBox1.TabIndex = 1;
this.textBox1.Text = "250
00";
this.toolTip1.SetToolTip(this.textBox1, "Loan amount");
this.textBox1.KeyPress += new System.Windows.Forms.KeyPressEventHandler(this.txt_KeyPress);
this.textBox1.TextChanged += new System.EventHandler(this.txt_TextChanged);
//
// statusBar1
//
this.statusBar1.Location = new System.Drawing.Point(0, 253);
this.statusBar1.Name = "statusBar1";
this.statusBar1.Size = new System.Drawing.Size(292, 20);
this.statusBar1.TabIndex = 8;
this.statusBar1.Text = "Ready";
//
// timer1
//
this.timer1.Enabled = true;
this.timer1.Tick += new System.EventHandler(this.timer1_Tick);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.BackColor = System.Drawing.Color.FromArgb(((System.Byte)(176)), ((System.Byte)(231)), ((System.Byte)(216)));
this.ClientSize = new System.Drawing.Size(292, 273);

this.Controls.Add(textBox1);
this.Controls.Add(textBox2);
this.Controls.Add(textBox3);
this.Controls.Add(textBox4);

this.Controls.Add(label1 );
this.Controls.Add(label2);
this.Controls.Add(label3);
this.Controls.Add(label4);
this.Controls.Add(button1);
this.Controls.Add(button2);
this.Controls.Add(statusBar1);

this.MaximizeBox = false;
this.MinimizeBox = false;
this.Name = "Form1";
this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
this.Text = "PMT Calculator";
this.ResumeLayout(false);

}

// The main entry point for the application.
static void Main()
{
Application.Run(new Form1());
}

private void button2_Click(object sender, System.EventArgs e)
{
Application.Exit();
}

static bool CheckForNumeric(char ch)
{
//allow only numbers and a decimal point and backspace key
int keyInt = (int)ch ;
if (( keyInt < 48 || keyInt > 57) && keyInt != 46 && keyInt !=

return false;
else
return true ;
}

private void txt_TextChanged(object sender, EventArgs e)
{
textBox4.Text ="" ;

}

private void txt_KeyPress(object sender, KeyPressEventArgs e)
{
//allow only numbers and a decimal point and backspace key
if (CheckForNumeric(e.KeyChar) == false)
e.Handled = true ;

}

private void button1_Click(object sender, System.EventArgs e)
{

double pv = Convert.ToDouble(textBox1.Text);
double intrate = Convert.ToDouble(textBox2.Text) ;
double period = Convert.ToDouble(textBox3.Text) ;
double pmt = calcPayment( pv,period, intrate) ;

string str = pmt.ToString( "f2" );
textBox4.Text = str ;

}

public static double calcPayment(double presentValue, double
financingPeriod,
double interestRatePerYear)
{
double a, b, x;
double monthlyPayment;
a = (1 + interestRatePerYear / 1200);
b = financingPeriod;
x = Math.Pow(a, b);
x = 1 / x;
x = 1 – x;

monthlyPayment = (presentValue ) * (interestRatePerYear / 1200) / x;
return(monthlyPayment);
}

private void timer1_Tick(object sender, System.EventArgs e)
{
if(textBox1.Focused == true)
statusBar1.Text =" Enter the present value of the loan amount – $" ;
else if(textBox2.Focused == true)
statusBar1.Text =" Enter the annual interest rate – %" ;
else if(textBox3.Focused == true)
statusBar1.Text =" Enter the number of months financed" ;
else if(textBox4.Focused == true)
statusBar1.Text =" Calculated Monthly Loan Payment -$" ;

else
statusBar1.Text =" " ;

}
}
}
//End of namespace PMTApp

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