Updating values in Grid view based on dropdown list

Thank you Amit, I am getting the desired output now :)

Remove below lines:

GridView1.DataSource = null;
GridView1.DataSource = SqlDataSource2;

Hi Amit, I tried your code and amgetting the below error: 

I'd be glad to help you with the code for the Button1_Click event in your ASP.NET application to filter the GridView based on the selected class and mark attendance. Here's the improved code:

attendance.aspx.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;

public partial class Faculty_attendance : System.Web.UI.Page
{
    SqlConnection cn = new SqlConnection("Data Source=DESKTOP-UIGAOQI\\SQLEXPRESS;Initial Catalog=mydb;Integrated Security=True;");

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) // Only load data on initial page load
        {
            BindDropDown();
        }
    }

    private void BindDropDown()
    {
        // Assuming you have a connection string in your Web.config
        string connectionString = ConfigurationManager.ConnectionStrings["mydbConnectionString"].ConnectionString;
        cn.ConnectionString = connectionString;

        string sql = "SELECT DISTINCT [Class] FROM [student]";
        SqlCommand cmd = new SqlCommand(sql, cn);

        try
        {
            cn.Open();
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            adapter.Fill(dt);

            DropDownList1.DataSource = dt;
            DropDownList1.DataTextField = "Class";
            DropDownList1.DataValueField = "Class";
            DropDownList1.DataBind();
            DropDownList1.Items.Insert(0, new ListItem("-- Select Class --", "")); // Add default item
        }
        catch (SqlException ex)
        {
            // Handle database connection or query errors
            Label4.Text = "Error loading classes: " + ex.Message;
        }
        finally
        {
            cn.Close();
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        string selectedClass = DropDownList1.SelectedValue;

        if (selectedClass != "") // Check if a class is selected
        {
            string connectionString = ConfigurationManager.ConnectionStrings["mydbConnectionString"].ConnectionString;
            cn.ConnectionString = connectionString;

            // Update SqlDataSource2 to filter by class
            string sql = "SELECT DISTINCT [RegNo], [Name] FROM [student] WHERE Class = @Class";
            SqlDataSource2.SelectCommand = sql;
            SqlDataSource2.SelectParameters.Clear();
            SqlDataSource2.SelectParameters.Add(new SqlParameter("@Class", selectedClass));

            SqlDataSource2.DataBind();
            GridView1.DataBind(); // Rebind the GridView
        }
        else
        {
            Label4.Text = "Please select a class to filter.";
        }
    }

    protected void Button2_Click(object sender, EventArgs e)
    {
        // Implement your attendance update logic here
        // ...
    }

    // ... other event handlers
}