C#  

Check whether a number is prime or not C# WebForms

Step 1: Design the WebForm (PrimeNumber.aspx)

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

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Prime Number Checker - Real Time Example</title>
    <style>
        body {
            font-family: Arial;
            background-color: #f0f2f5;
            margin: 50px;
        }
        .container {
            width: 400px;
            margin: auto;
            background: white;
            border-radius: 8px;
            box-shadow: 0px 0px 10px #ccc;
            padding: 20px;
        }
        h2 {
            color: #1A2A80;
            text-align: center;
        }
        .form-control {
            width: 100%;
            padding: 8px;
            margin-top: 10px;
        }
        .btn {
            background-color: #7A85C1;
            color: white;
            border: none;
            padding: 10px 20px;
            margin-top: 10px;
            border-radius: 5px;
        }
        .result {
            font-weight: bold;
            color: #333;
            margin-top: 15px;
            text-align: center;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
        <div class="container">
            <h2>Prime Number Checker</h2>
            <asp:Label ID="lblNumber" runat="server" Text="Enter a Number:"></asp:Label><br />
            <asp:TextBox ID="txtNumber" runat="server" CssClass="form-control"></asp:TextBox><br />
            <asp:Button ID="btnCheck" runat="server" Text="Check Prime" CssClass="btn" OnClick="btnCheck_Click" /><br />
            <asp:Label ID="lblResult" runat="server" CssClass="result"></asp:Label>
        </div>
    </form>
</body>
</html>

Step 2: Backend Logic (PrimeNumber.aspx.cs)

using System;

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

    protected void btnCheck_Click(object sender, EventArgs e)
    {
        int number;
        bool isNumber = int.TryParse(txtNumber.Text.Trim(), out number);

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

        if (IsPrime(number))
        {
            lblResult.Text = $"{number} is a Prime Number.";
            lblResult.ForeColor = System.Drawing.Color.Green;
        }
        else
        {
            lblResult.Text = $"{number} is NOT a Prime Number.";
            lblResult.ForeColor = System.Drawing.Color.Red;
        }
    }

    private bool IsPrime(int num)
    {
        if (num <= 1)
            return false;
        if (num == 2)
            return true;
        if (num % 2 == 0)
            return false;

        for (int i = 3; i <= Math.Sqrt(num); i += 2)
        {
            if (num % i == 0)
                return false;
        }
        return true;
    }
}

Real-Time Example Flow

  1. Open the web page PrimeNumber.aspx.

  2. Enter a number (e.g., 17).

  3. Click the “Check Prime” button.

  4. The result label displays:
    “17 is a Prime Number.”
    or
    “18 is NOT a Prime Number.”

Explanation

  • TextBox (txtNumber) → Used to input the number.

  • Button (btnCheck) → Triggers the backend function.

  • Label (lblResult) → Displays the result dynamically.

  • IsPrime() method → Checks divisibility up to the square root for efficiency.

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