自定义控件中的脚本注册

今日写自定义控件的时候发现。RegisterClientScriptBlock注册JS无效。代码如下：

            // 写入JS
            if (_enableLoadingState)
            {
                // 
                System.Text.StringBuilder sb = new System.Text.StringBuilder();
                //sb.Append("<script type=\"javascript\">");
                sb.Append("function showLoading(){");
                sb.Append("var eleC = document.getElementById(\"" + ID_CLIENT_PREFIX + "Content\");");
                sb.Append("alert('OK');");
                sb.Append("eleC.style.display = \"none\";");
                sb.Append("document.getElementById(\"" + ID_CLIENT_PREFIX + "Loading\").style.display = \"block\";");
                sb.Append("}");
                //sb.Append("</script>");

                this.Page.ClientScript.RegisterStartupScript(this.GetType(), this.UniqueID, sb.ToString());
                this.Page.ClientScript.RegisterClientScriptBlock(this.GetType(), this.UniqueID + "___", sb.ToString());
            }

RegisterStartupScript 方法可以，但为什么RegisterClientScriptBlock不行呢？