ClientScriptManager csm = Page.ClientScript;
        //Script标记靠近<form>标签
        //csm.RegisterClientScriptBlock(this.GetType(), "alert", js,false);
        //Script标记靠近</form>标签
        csm.RegisterStartupScript(this.GetType(), "alert", js, false);

动态方式
RegisterClientScriptBlock    生成的脚本位于<form>标签之后
RegisterStartupScript    生成的脚本位于</form>标签之前  需要页面元素
Response.Write()    生成的脚本位于Html代码的顶部即<!DOCTYPE>之前
protected void Page_Load(object sender, EventArgs e)
{
  if (!Page.ClientScript.IsStartupScriptRegistered(this.GetType(), "scriptKey"))
  {
      string scriptText = "<script language='javascript'>         
      document.getElementById('txtUserName').focus();</script>";
      Page.ClientScript.RegisterStartupScript(this.GetType(), "scriptKey", scriptText);

　　ScriptManager.RegisterStartupScript(this, typeof(Page), "alertExist", "alert('您选择的商品（编号：" + Vcode + "）已在购物车存在！')", true);
  }