<form id="form1" runat="server">
    <div>
    
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
    
    </div>
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
    </form>

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


namespace WebApplication2
{
    public partial class WebForm2 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            // Find control on page.
            Control myControl1 = FindControl("TextBox2");
            if (myControl1 != null)
            {
                // Get control's parent.
                Control myControl2 = myControl1.Parent;
                Response.Write("Parent of the text box is : " + myControl2.ID);
            }
            else
            {
                Response.Write("Control not found");
            }
        }
    }
}