用Relector反编译C#程序集

namespace YJC.Toolkit.Web
{
using System;
using System.Text;
using YJC.Toolkit.EasySearch;
using YJC.Toolkit.Sys;
using YJC.Toolkit.SysUtil;

internal class WebTreeDialogPage : WebXmlHttp2Page
{
    public WebTreeDialogPage()
    {
        this.Source = new EasySearch2DataSet();
        base.Style = PageStyle.Custom;
        base.Operation = "TreeExpand";
    }

    protected override void DoPost()
    {
        base.DoPost();
        this.GetXsltFile();
        base.Response.ContentEncoding = Encoding.UTF8;
        string content = this.Transform.TransformContent(base.IsIe);
        if (AppSettings.Current.Debug)
        {
            FileUtil.SaveFile(AppSettings.Current.XmlPath + "EasySearch.html", content);
        }
        base.Response.Write(content);
    }
}

}
各位大神请问一下：
反编译过来的代码 base.Response.ContentEncoding = Encoding.UTF8
为什么Encoding.UTF8出现错误，提示“string”不包含“UTF8”的定义，而且
找不到可接受类型为“string”的第一个参数的扩展方法"UTF8"(是否缺少using指令或程序集引用)

加上
using System.Text.Encoding;