How can I move some code from a .aspx page to a .cs file?

Question

I have a webpage admin.aspx (with the code in admin.aspx.cs) and want to refactor it by moving all the code to a new class admin.cs so I can call these functions from the aspx file.

How can I do this?

// Contents of admin.aspx.cs

private void onlyID()
{
    connection.Open();
    SqlCommand command = new SqlCommand("select id from appointment");
    SqlDataReader reader = command.ExecuteReader();
    while (reader.Read())
    {
        id.Add(reader.GetInt64(0));
    }
    connection.Close();
    Response.Redirect("update-appointment.aspx");
}
public void fetchData()
{
    StringBuilder html = new StringBuilder();
    using (SqlCommand command = new SqlCommand())
    {
        command.Connection = connection;
        //command.CommandType = CommandType.Text;
        command.CommandText = "select * from appointment";
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();
        html.Append("<table border='2' width='1100'  style='color:white'>");
        html.Append("<tr align='center'>" +
                        "<td style='font-size:25px'>Column 1</td>" +
                        "<td style='font-size:25px'>Column 2</td>" +
                        "<td style='font-size:25px'>Column 3</td>" +
                        "<td style='font-size:25px'>Column 4</td>" +
                        "<td style='font-size:25px'>Column 5</td>" +
                        "<td style='font-size:25px'>Column 6</td>" +
                        "<td style='font-size:25px'>Column 7</td>" +
                        "<td style='font-size:25px'>Column 8</td>" +
        "</tr>");
        if (reader.HasRows)
        {
            while (reader.Read())
            {
                html.Append("<tr align='center'>");
                html.Append("<td>" + reader[0] + "</td>");
                html.Append("<td>" + reader[1] + "</td>");
                html.Append("<td>" + reader[2] + "</td>");
                html.Append("<td>" + reader[3] + "</td>");
                html.Append("<td>" + reader[4] + "</td>");
                html.Append("<td>" + reader[5] + "</td>");
                html.Append("<td>" + reader[6] + "</td>");
                html.Append("<td>" + "<a href='update-appointment.aspx?id=" + reader[0] + "'><img src='../images/edit.png' style='height:25px;' /></a>" + " " + "<a href='delete-appointment.aspx?id="+reader[0]+"'><img src='../images/del.png' style='height:25px;' /></a>");
                html.Append("</tr>");
            }
            html.Append("</table>");
            PlaceHolder1.Controls.Add(new Literal { Text = html.ToString() });
        }
    }
}

Answer 1

Create ASP.NET folder called App_Code (for better structure), create there file Admin.cs. and content of your Admin.cs:

private string ConStr = WebConfigurationManager.ConnectionString["ConnectionStringName"].ConnectionString;
private SqlConnection connection = new SqlConnection(ConStr);
private static void onlyID(ListBox id) //if control with ID "id" is ListBox, or change to another. You didn't post design page, that's why I just think
{
    try{
        connection.Open();
        SqlCommand command = new SqlCommand("select id from appointment");
        SqlDataReader reader = command.ExecuteReader();
        while (reader.Read() && reader.HasRows)
        {
            id.Add(reader.GetInt64(0));
        }
    }
    catch(Exception ex){
        //If some exception will occur
    }
    finally{
        connection.Close();
        Response.Redirect("update-appointment.aspx");
    }
}

public static void fetchData(ContentPlaceHolder PlaceHolder1)
{
    StringBuilder html = new StringBuilder();
    using (SqlCommand command = new SqlCommand())
    {
        command.Connection = connection;
        //command.CommandType = CommandType.Text;
        command.CommandText = "select * from appointment";
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();
        html.Append("<table border='2' width='1100'  style='color:white'>");
        html.Append("<tr align='center'>" +
                    "<td style='font-size:25px'>Column 1</td>" +
                    "<td style='font-size:25px'>Column 2</td>" +
                    "<td style='font-size:25px'>Column 3</td>" +
                    "<td style='font-size:25px'>Column 4</td>" +
                    "<td style='font-size:25px'>Column 5</td>" +
                    "<td style='font-size:25px'>Column 6</td>" +
                    "<td style='font-size:25px'>Column 7</td>" +
                    "<td style='font-size:25px'>Column 8</td>" +
        "</tr>");
        if (reader.HasRows)
        {
            while (reader.Read())
            {
                html.Append("<tr align='center'>");
                html.Append("<td>" + reader[0] + "</td>");
                html.Append("<td>" + reader[1] + "</td>");
                html.Append("<td>" + reader[2] + "</td>");
                html.Append("<td>" + reader[3] + "</td>");
                html.Append("<td>" + reader[4] + "</td>");
                html.Append("<td>" + reader[5] + "</td>");
                html.Append("<td>" + reader[6] + "</td>");
                html.Append("<td>" + "<a href='update-appointment.aspx?id=" + reader[0] + "'><img src='../images/edit.png' style='height:25px;' /></a>" + " " + "<a href='delete-appointment.aspx?id="+reader[0]+"'><img src='../images/del.png' style='height:25px;' /></a>");
                html.Append("</tr>");
            }
            html.Append("</table>");
            PlaceHolder1.Controls.Add(new Literal { Text = html.ToString() });
        }
    }
}

and then just call this functions in your .aspx.cs file:

private void Page_Load(object sender, EventArgs e){
    Admin.onlyID(id);
    Admin.fetchData(PlaceHolder1);
}