Use Follow Code:

The Controls are Treeview,Gridview

using System;
using System.IO;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

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

}

private void FillDirectory(TreeNode parent, string strPath)
{
DirectoryInfo dr = new DirectoryInfo(strPath);
if (dr.Exists)
{
foreach (DirectoryInfo d in dr.GetDirectories())
{
TreeNode node = new TreeNode();
node.Text = d.Name;
node.Value = d.FullName;
node.PopulateOnDemand = true;
node.SelectAction = TreeNodeSelectAction.SelectExpand;
parent.ChildNodes.Add(node);
}
}
}

protected void DirTreeview_TreeNodePopulate(object sender, TreeNodeEventArgs e)
{
string selectedstring = e.Node.Value;
string strFileName = Path.Combine(currentFolderPath, selectedstring);
FillDirectory(e.Node, strFileName);
//BindDirContents(e.Node, gridviewFiles);
}

public String currentFolderPath
{
get
{
if (ViewState["m_currentFolderPath"] != null)
return ViewState["m_currentFolderPath"].ToString().Trim();
else 
return string.Empty;
}
set
{
ViewState["m_currentFolderPath"] = value;
}
}

private static void BindDirContents(TreeNode node, System.Web.UI.WebControls.GridView gridview)
{
// bind the gridview to the datasource
gridview.DataSource = new System.IO.DirectoryInfo(node.Value).GetFiles();
gridview.DataBind();
}

protected void gridviewFiles_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
System.IO.FileSystemInfo item = (System.IO.FileSystemInfo)e.Row.DataItem;
ImageButton imageButton = (ImageButton)e.Row.FindControl("btnItemIcon");

System.IO.FileInfo fileInfo = (System.IO.FileInfo)item;

Label lblSize = (Label)e.Row.FindControl("lblSize");
lblSize.Text = string.Format("{0:N0} KB", fileInfo.Length / 1000);

if ((fileInfo.Name.EndsWith(".rtf") || fileInfo.Name.EndsWith(".doc") || fileInfo.Name.EndsWith(".txt")))
{
imageButton.ImageUrl = @"Image/doc.jpg";
}
else
{
imageButton.ImageUrl = @"Image/voice.png";
}
}
}

protected void DirTreeview_SelectedNodeChanged(object sender, EventArgs e)
{
BindDirContents(this.DirTreeview.SelectedNode, this.gridviewFiles);
}

protected void gridviewFiles_RowCommand(object sender, GridViewCommandEventArgs e)
{
// handle either opening the item or rebinding the grid
if (e.CommandName == "ItemClick")
{
string name = (string)e.CommandArgument;
System.IO.DirectoryInfo dinfo = new System.IO.DirectoryInfo(this.DirTreeview.SelectedNode.Value);

if (System.IO.File.Exists(System.IO.Path.Combine(dinfo.FullName, name)))
{
System.IO.FileInfo fileInfo = new System.IO.FileInfo(System.IO.Path.Combine(dinfo.FullName, name));
// they clicked on a file, download it to there PC
this.Response.Clear();
this.Response.AddHeader("Content-Disposition", "attachment; filename=" + fileInfo.Name);
this.Response.WriteFile(fileInfo.FullName);
this.Response.End();
}
else
{
foreach (TreeNode node in this.DirTreeview.SelectedNode.ChildNodes)
{
if (node.Text == name)
{
node.Selected = true;
node.Expanded = true;

// expand the parents
TreeNode parentNode = node.Parent;
while (parentNode != null)
{
parentNode.Expanded = true;
parentNode = parentNode.Parent;
}

// bind the gridview to the datasource
BindDirContents(node, this.gridviewFiles);
break;
}
}
}
}
} 
}