How to Generate Random Secret key with Number and Alphabet in c#

How to Generate Random Secret key with Number and Alphabet in c# public void randomnumber()
    {
        String guid = "";
        String allowchars = " ";
        allowchars = "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";
        allowchars += "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";
        allowchars += "1,2,3,4,5,6,7,8,9,0";
        char[] sep = { ',' };
        String[] arr = allowchars.Split(sep);
        String temp = "";
        Random rnd = new Random();
        for (int i = 0; i < 8; i++)
        {
            temp = arr[rnd.Next(0, arr.Length)];
            guid += temp;
        }
        number1.Text = guid.ToString();
    }

o/p: Number with Apphabet: 2HjiKG4