This is a simple C# program code to find area of a rectangle, circle and triangle with user interface and example.

Manually we can find the area of a rectangle by using equation:

Also we can find the area of a circle by using equation:

Also we can find the area of a triangle by using equation:
a = length of first side
b = length of second side
c = length of third side
s = (a + b+ c)/2

Area = sqrt(s*(s-a)*(s-b)*(s-c))

-where sqrt stands for square root

User interface with example:

Here ‘Find Area’ is the ‘button_1’. There are three tabs: One is for ‘Rectangle’, another one is for ‘Triangle’ and last is for ‘Circle’.

Code Behind interface:

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

private void button1_Click(object sender, EventArgs e)
{
if (tabControl1.SelectedTab == tabPage1)
{
double lenght = Convert.ToDouble(textBox2.Text);
}
if (tabControl1.SelectedTab == tabPage2)
{
double a = Convert.ToDouble(textBox4.Text);
double b = Convert.ToDouble(textBox5.Text);
double c = Convert.ToDouble(textBox6.Text);
double s = (a + b + c) / 2;
double Area = Math.Sqrt(s * (s - a) * (s - b) * (s - c));
label5.Text = (Area).ToString();
}
if (tabControl1.SelectedTab == tabPage3)
{