Select して、ToListして、ForEach

private void button1_Click(object sender, EventArgs e)
{
    textBox1.Text = "";
    new List<int> { 1, 2, 3, 4, 5 }
        .Select(x => (x * 2).ToString())
        .ToList()
        .ForEach(x => textBox1.Text += x + ",");

}