Code for Composite web part(A dropdown will have initial value of list and on selecting value will show List)

using System;
using System.ComponentModel;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace GriddView.CompositeWebPart
{
    [ToolboxItemAttribute(false)]
    public class CompositeWebPart : WebPart
    {
        private DropDownList listdropdown;
        private GridView Itemgridview;
        protected override void CreateChildControls()
        {
            listdropdown = new DropDownList();
            listdropdown.AutoPostBack = true;
            listdropdown.SelectedIndexChanged += new EventHandler(listdropdown_SelectedIndexChanged);
            Controls.Add(listdropdown);
          
            Controls.Add(new LiteralControl("<br/><br/>"));
            Itemgridview = new GridView();
            Controls.Add(Itemgridview);


        }
        protected void listdropdown_SelectedIndexChanged(object sender,EventArgs e)
        {
            var web = SPContext.Current.Web;
            var list = web.Lists[listdropdown.Text];
            var items = list.Items.GetDataTable();
            Itemgridview.DataSource = items;
            Itemgridview.DataBind();
        }
        protected override void OnPreRender(EventArgs e)
        {
            if (Page.IsPostBack) return;
            EnsureChildControls();
            var web = SPContext.Current.Web;
            listdropdown.DataSource = SPContext.Current.Web.Lists;
            listdropdown.DataTextField = "Title";
            listdropdown.DataBind();
        }
    }
}