Ramdom Password Generation - Asp.net C#

using System;
using System.Data;
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 RandomPassword : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnGenerate_Click(object sender, EventArgs e)
    {
        string allowedChars = "";
        allowedChars = "a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,";
        allowedChars += "A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,";
        allowedChars += "1,2,3,4,5,6,7,8,9,0,!,@,#,$,%,&,?";

        char[] sep ={ ',' };
        string[] arr = allowedChars.Split(sep);
               
        string passwordString = "";

        string temp = "";

        Random rand = new Random();
        for (int i = 0; i < Convert.ToInt32(txtPassLength.Text); i++)
        {
            temp = arr[rand.Next(0, arr.Length)];
            passwordString += temp;
        }

        txtPassword.Text = passwordString;
    }
}