public IEnumerable<int> GetRandomNumbers(int min, int max, int count)
{
    Random r = new Random();
    for (int i = 0; i < count; i++)
    {
        yield return r.Next(min, max);
    }
}

private void button1_Click(object sender, EventArgs e)
{
    listBox1.Items.Clear();
    foreach (int x in GetRandomNumbers(10, 50, 15))
        listBox1.Items.Add(x);
}