OTP (One Time Password) in Asp.NET

private string generatePassword()
{
    int lenthofpass = 6;
    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 < lenthofpass; i++)
    {
        temp = arr[rand.Next(0, arr.Length)];
        passwordString += temp;
    }
    return passwordString;
}