GET random Unique OTP(One Time Password) in C#.NET

Use this Code for random Unique OTP(One Time Password) generation:

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;
    }