public class WebForm1: System.Web.UI.Page{
    protected System.Web.UI.WebControls.RadioButton RadioButton1;
    protected System.Web.UI.WebControls.RadioButton RadioButton2;
    protected System.Web.UI.WebControls.RadioButton RadioButton3;
    protected System.Web.UI.WebControls.Image Image1;
    protected HtmlForm Form1;
    private void Button1_Click(object sender, System.EventArgs e)
    {
        (1)
        {
            if(control is RadioButton)
            {    
            RadioButton rb=control as RadioButton;
                if(rb.Checked)
                {
                    ListBox1.Items.Add(rb.Text);
                    break;
                }
            }
        }
    }
}

• foreach(object control in this.Controls)

• foreach(object control in Page.Controls)

• foreach(object control in Form1.Controls)

• foreach(object control in WebForm1.Controls)