Controllare se esiste il file (querystring)

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data.Odbc;
using System.Data;
using System.IO;

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

        if (!IsPostBack)
            popola();

    }

    private void popola()
    { string basePath = "";
            string myPath = Request.QueryString["page"];
        string filePath = string.Concat(basePath, myPath);

        if (!String.IsNullOrEmpty(filePath))
        {

           
        
            if (myPath.IndexOfAny(Path.GetInvalidFileNameChars()) > -1)
                throw new FileNotFoundException("FileName not valid");


            if (!filePath.StartsWith(basePath))
                throw new FileNotFoundException("Path not valid");
            StreamReader sr = new StreamReader(Server.MapPath("/public/pagine/") + filePath + "");
            while (!sr.EndOfStream)
            {
                vipage.InnerHtml += sr.ReadLine().ToString() + '\r' + '\n';

            }
            sr.Close();

        }

        else
        {


            vipage.InnerHtml = "Non c'è niente";
        } 
        
        
     }
}