I would take a bit different approach:
public static string randdec(Random r, int l1, int l2)
{
int L1= (int)Math.Pow(10, l1-1);
int L2 = (int)Math.Pow(10, l2-1);
int n1 = r.Next(L1, 10*L1);
int n2 = r.Next(L2, 10*L2);
return n1.ToString() + "." + n2.ToString();
}
that you may use this way
public static void Main()
{
Random r = new Random();
for (int n = 0; n < 10; n++)
Console.WriteLine(randdec(r,8,2));
}