How to make a calculator in C#

class Calculator
{
  // To store current number being input
  string input = "0";
  // To store first input number
  string num1 = "0";
  // To store second number
  string num2 = "0";
  // To store the operator
  string function = string.empty;
  // To initialize the program  
  public Form1()
  {
    initializeComponent();
  }
  // Handling numeric input
  public void button_Zero(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "0";
    }
    else {
      input = "0";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_One(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "1";
    }
    else {
      input = "1";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Two(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "2";
    }
    else {
      input = "2";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Three(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "3";
    }
    else {
      input = "3";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Four(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "4";
    }
    else {
      input = "4";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Five(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "5";
    }
    else {
      input = "5";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Six(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "6";
    }
    else {
      input = "6";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Seven(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "7";
    }
    else {
      input = "7";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Eight(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "8";
    }
    else {
      input = "8";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  public void button_Nine(object o, EventArgs e)
  {
    if (input != "0"){
      // Updating input
      input = input + "9";
    }
    else {
      input = "9";
    }
    this.textBox.Text = "";
    this.textBox.Text += input;
  }
  // Handling operator input
  public void button_Plus(object o, EventArgs e)
  {
    num1 = input;
    input = "0";
    this.textBox.Text = "";
    function = "+";
  }
  public void button_Minus(object o, EventArgs e)
  {
    num1 = input;
    input = "0";
    this.textBox.Text = "";
    function = "-";
  }
  public void button_Multiply(object o, EventArgs e)
  {
    num1 = input;
    input = "0";
    this.textBox.Text = "";
    function = "*";
  }
  public void button_Divide(object o, EventArgs e)
  {
    num1 = input;
    input = "0";
    this.textBox.Text = "";
    function = "/";
  }
  // Handling result
  public void button_Equal(object o, EventArgs e)
  {
    num2 = input;
    if (function != string.empty){
      double n1, n2, result;
      try {
        // Convert to double data type
        double.TryParse(num1, n1);
        double.TryParse(num2, n2);
        // Checking operator and calculating output
        switch(function){
          case "+":
            this.textBox.Text = "";
            result = n1 + n2;
            this.textBox.Text = result.ToString();
            break;
          case "-":
            this.textBox.Text = "";
            result = n1 - n2;
            this.textBox.Text = result.ToString();
            break;
          case "*":
            this.textBox.Text = "";
            result = n1 * n2;
            this.textBox.Text = result.ToString();
            break;
          case "/":
            this.textBox.Text = "";
            result = n1 / n2;
            this.textBox.Text = result.ToString();
            break;
        }
      }
      catch { // Handling Math Errors
        this.textBox.Text = "Math Error"
        input = "";
        num1 = "";
        num2 = "";
        function = "";
      }
    }
  }
  // Setting up clear button
  public void button_Clear(object o, EventArgs e)
  {
    input = "";
    num1 = "";
    num2 = "";
    function = "";
    this.textBox.Text = "";
  }
}

This program contains all of the event-handlers for button clicks on the calculator. It is a basic program that uses the globally declared variables input, num1, num2, and function. input contains the number currently being input, num1 contains the first operand, num2 contains the second operand, and function contains the operator.

When the operator is selected, the program transfers the contents of input into num1, inserts the appropriate operator into function, and empties input for the second operand.

When the = button is pressed, the program copies the contents of input into num2 and converts both num1 and num2 into doubles. These converted values are stored in n1 and n2 and used to calculate and display the output. In the case of an error, the catch statement will reset the values and display an error message.

If the C button is pressed, all global variables are reset and the output box is emptied.

Free Resources

How to make a calculator in C#

Initial Setup

UI Setup

Writing the code