Program to Calculate GCD (Greatest Common Divisor) in C# WebForms

Concept: What is GCD?

The GCD (Greatest Common Divisor) of two numbers is the largest positive integer that divides both numbers without leaving a remainder.

Example

Numbers: 20 and 30  
Divisors of 20: 1, 2, 4, 5, 10, 20  
Divisors of 30: 1, 2, 3, 5, 6, 10, 15, 30  
GCD = 10

Formula (Using Euclid’s Algorithm)

GCD(a, b) = GCD(b, a % b)

When b = 0, then GCD(a, b) = a

Step 1: Design Page – GCDExample.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="GCDExample.aspx.cs" Inherits="GCDExample" %>

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Find GCD using C# WebForms</title>
    <style>
        body {
            font-family: Arial;
            background-color: #f4f7fc;
            margin: 50px;
        }
        .container {
            width: 500px;
            margin: auto;
            background: #fff;
            border-radius: 10px;
            box-shadow: 0 0 10px #ccc;
            padding: 25px;
        }
        h2 {
            text-align: center;
            color: #1A2A80;
        }
        .form-control {
            width: 100%;
            padding: 8px;
            margin-top: 10px;
        }
        .btn {
            background-color: #7A85C1;
            color: white;
            border: none;
            padding: 10px;
            border-radius: 5px;
            cursor: pointer;
            width: 100%;
            margin-top: 15px;
        }
        .btn:hover {
            background-color: #5b68a1;
        }
        .result {
            margin-top: 20px;
            font-weight: bold;
            color: #333;
            text-align: center;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
        <div class="container">
            <h2>Calculate GCD (Greatest Common Divisor)</h2>

            <asp:Label ID="lblNum1" runat="server" Text="Enter First Number:"></asp:Label><br />
            <asp:TextBox ID="txtNum1" runat="server" CssClass="form-control" placeholder="Example: 20"></asp:TextBox><br />

            <asp:Label ID="lblNum2" runat="server" Text="Enter Second Number:"></asp:Label><br />
            <asp:TextBox ID="txtNum2" runat="server" CssClass="form-control" placeholder="Example: 30"></asp:TextBox><br />

            <asp:Button ID="btnCalculate" runat="server" Text="Find GCD" CssClass="btn" OnClick="btnCalculate_Click" /><br />

            <asp:Label ID="lblResult" runat="server" CssClass="result"></asp:Label>
        </div>
    </form>
</body>
</html>

Step 2: Backend Code – GCDExample.aspx.cs

using System;

public partial class GCDExample : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }

    protected void btnCalculate_Click(object sender, EventArgs e)
    {
        try
        {
            int num1 = Convert.ToInt32(txtNum1.Text.Trim());
            int num2 = Convert.ToInt32(txtNum2.Text.Trim());

            if (num1 <= 0 || num2 <= 0)
            {
                lblResult.Text = "Please enter positive numbers only.";
                lblResult.ForeColor = System.Drawing.Color.Red;
                return;
            }

            int gcd = FindGCD(num1, num2);

            lblResult.Text = "GCD of " + num1 + " and " + num2 + " is: " + gcd.ToString();
            lblResult.ForeColor = System.Drawing.Color.Green;
        }
        catch
        {
            lblResult.Text = "Invalid input! Please enter valid integers.";
            lblResult.ForeColor = System.Drawing.Color.Red;
        }
    }

    // Recursive function to find GCD
    private int FindGCD(int a, int b)
    {
        if (b == 0)
            return a; // Base case
        else
            return FindGCD(b, a % b); // Recursive case
    }
}

Example Output

Input	Output
20, 30	GCD of 20 and 30 is: 10
56, 98	GCD of 56 and 98 is: 14
13, 7	GCD of 13 and 7 is: 1

Algorithm

Step 1: Start
Step 2: Input two numbers a and b
Step 3: Repeat while b != 0
          temp = b
          b = a % b
          a = temp
Step 4: GCD = a
Step 5: Display GCD
Step 6: Stop

Example

Input: a = 20, b = 30

Step	a	b	a % b	Recursive Call
1	20	30	20	FindGCD(30, 20)
2	30	20	10	FindGCD(20, 10)
3	20	10	0	FindGCD(10, 0)
4	10	0	-	Return 10

Output → GCD = 10

Explanation

Uses recursion to call the function repeatedly until one number becomes 0.
The other number is the GCD.

Advantages

Very efficient and simple (Euclidean algorithm).
Works even for large numbers.

Limitations

Works only for integers (not decimals).
Both inputs must be positive.

Conclusion

This WebForms example demonstrates how to calculate the Greatest Common Divisor (GCD) using recursion in C#.
It’s a perfect example to understand both recursive functions and Euclid’s Algorithm in real-world web-based applications.