Generate Unique Random Numbers in specified Range using C# .Net

The following function returns random number between a range

    private int F(int min, int max)
    {
        Random _r = new Random();
        int n = _r.Next(min, max);
        return n;
    }

You can call it N items using loop

protected void Page_Load(object sender, EventArgs e)
    {
        List<int> ints = new List<int>();
        for (int i = 0; i < 3; i++)
        {
            int no = F(1, 31);
            if (!ints.Exists(n => n == no))
            {
                ints.Add(no);
            }
        }
}