protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        HttpCookie UserInfo = new HttpCookie("UserInfo");
        UserInfo.Value = "bdstjk";
        Response.Cookies.Add(UserInfo);
    }

}

protected void btnRemoveCookie_Click(object sender, EventArgs e)
{
    Response.Cookies.Remove("UserInfo");
    Response.Write("script type=\"text/javascript\">alert(\"刪除Cookie成功！\");/script>");
}

protected void btnCheckCookie_Click(object sender, EventArgs e)
{
    if (Request.Cookies["UserInfo"] != null)
    {
        Response.Write("Cookie存在，"+Request.Cookies["UserInfo"].Value);
    }
    else
    {
        Response.Write("Cookie不存在");
    }
}
 

asp:Button ID="btnRemoveCookie" runat="server" Text="刪除Cookie"
             />
asp:Button ID="btnCheckCookie" runat="server" Text="檢查Cookie"
             />

public void Remove(string name)
{
    if (this._response != null)
    {
        this._response.BeforeCookieCollectionChange();
    }
    this.RemoveCookie(name);
    if (this._response != null)
    {
        this._response.OnCookieCollectionChange();
    }
}

if (Request.Cookies["UserInfo"] != null)
{
    Response.Cookies["UserInfo"].Expires = DateTime.Now.AddDays(-1);
}
Response.Write("script type=\"text/javascript\">alert(\"刪除Cookie成功！\");/script>");