Program to Convert Decimal to Binary in C#

Concept: Decimal to Binary Conversion

A decimal number (base 10) can be converted to binary (base 2) by repeatedly dividing the number by 2 and storing the remainders.

Example

Decimal: 10  
Steps:
10 ÷ 2 = 5 remainder 0  
5 ÷ 2 = 2 remainder 1  
2 ÷ 2 = 1 remainder 0  
1 ÷ 2 = 0 remainder 1  

Binary = 1010 (read remainders in reverse)

Step 1: Design Page – DecimalToBinary.aspx

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

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Decimal to Binary Converter</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>Decimal to Binary Converter</h2>

            <asp:Label ID="lblDecimal" runat="server" Text="Enter Decimal Number:"></asp:Label><br />
            <asp:TextBox ID="txtDecimal" runat="server" CssClass="form-control" placeholder="Example: 10"></asp:TextBox><br />

            <asp:Button ID="btnConvert" runat="server" Text="Convert to Binary" CssClass="btn" OnClick="btnConvert_Click" /><br />

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

Step 2: Backend Logic – DecimalToBinary.aspx.cs

using System;
using System.Text;

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

    protected void btnConvert_Click(object sender, EventArgs e)
    {
        try
        {
            int decimalNumber = Convert.ToInt32(txtDecimal.Text.Trim());

            if (decimalNumber < 0)
            {
                lblResult.Text = "Please enter a positive number.";
                lblResult.ForeColor = System.Drawing.Color.Red;
                return;
            }

            string binary = ConvertToBinary(decimalNumber);
            lblResult.Text = "Binary Equivalent of " + decimalNumber + " is: " + binary;
            lblResult.ForeColor = System.Drawing.Color.Green;
        }
        catch
        {
            lblResult.Text = "Invalid input! Please enter a valid integer.";
            lblResult.ForeColor = System.Drawing.Color.Red;
        }
    }

    // Recursive function to convert decimal to binary
    private string ConvertToBinary(int num)
    {
        if (num == 0)
            return "0";
        else if (num == 1)
            return "1";
        else
            return ConvertToBinary(num / 2) + (num % 2).ToString();
    }
}

Example Outputs

Decimal Input	Binary Output
5	101
10	1010
25	11001
50	110010

Algorithm

Step 1: Read the decimal number
Step 2: If number is 0 → return "0"
Step 3: Divide number by 2
Step 4: Store the remainder
Step 5: Repeat Step 3 until number becomes 0
Step 6: Reverse the remainders to get binary number
Step 7: Display result

Example

Input: 10

Step	num	num/2	num%2	Binary
1	10	5	0	0
2	5	2	1	10
3	2	1	0	010
4	1	0	1	1010

Output → Binary = 1010

Explanation

Uses recursion to repeatedly divide the decimal number by 2.
The remainders form the binary digits when read in reverse.
The recursive function builds the binary string from top to bottom.

Advantages

Simple and clear recursive logic.
Easily adaptable for other conversions (e.g., Octal, Hexadecimal).

Limitations

Works only for non-negative integers.
For very large numbers, recursion depth may increase.

Conclusion

This WebForms example demonstrates how to convert a Decimal number to Binary using recursion in C#.
It’s a great real-time exercise to understand recursion, base conversion, and ASP.NET UI interaction.