It seems like there might be an issue with how you're retrieving the RegNo in your SQL query.

Here's what you should do:

1. Check the value of Application["uid"] to ensure it contains the correct registration number of the student. You can print it or debug to verify its value.

2. Ensure that the column index you're using in GetString corresponds to the correct column in your SELECT statement.

3. Since you're already filtering the result based on RegNo, you don't need the if condition inside the while loop. You can directly assign the values to your text boxes.

Here's the modified Page_Load method:

protected void Page_Load(object sender, EventArgs e)
{
    string regNo = System.Convert.ToString(Application["uid"]);
    
    // Open connection
    cn.Open();
    
    // Query to retrieve student details based on RegNo
    string query = "SELECT * FROM student WHERE RegNo=@RegNo";
    
    // Create SqlCommand object with parameters
    SqlCommand cmd = new SqlCommand(query, cn);
    cmd.Parameters.AddWithValue("@RegNo", regNo);
    
    // Execute query
    SqlDataReader rs = cmd.ExecuteReader();
    
    // Check if there are rows returned
    if (rs.HasRows)
    {
        // Read the first row (assuming RegNo is unique)
        rs.Read();
        
        // Assign values to text boxes
        name.Text = rs.GetString(0); // Assuming name is the first column
        std.Text = rs.GetString(1); // Assuming std is the second column
        mob.Text = rs.GetString(2); // Assuming mob is the third column
        dob.Text = rs.GetString(3); // Assuming dob is the fourth column
        gen.Text = rs.GetString(4); // Assuming gen is the fifth column
        add.Text = rs.GetString(5); // Assuming add is the sixth column
        regno.Text = regNo; // Assuming regno is the seventh column (or it's not needed as you already have regNo)
    }
    
    // Close connection
    cn.Close();
}

In this code:

• I've parameterized your SQL query to prevent SQL injection.
• Removed the unnecessary if condition inside the while loop since you're already filtering based on RegNo.
• Assuming your columns are in the order: Name, Class, Contact, DOB, Gender, Address, RegNo, I've adjusted the GetString indices accordingly. Adjust them if your columns are in a different order.