Format

为什么我用Format的时候提示找不到命名空间呢？？？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.IO;

namespace SL13
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            GetDataByWebRequest();//调用request函数
        }

        void GetDataByWebRequest()
        {
            string data = this.txtName.Text.ToString();//源数据
            Uri uri = new Uri(string  Format("http://localhost:1234/MyHandler.ashx?name={0}",data));//定义目标地址
            
            HttpWebRequest request = WebRequest.Create(uri) as HttpWebRequest;

            request.Method = "GET";
            request.BeginGetResponse(new AsyncCallback(ResponseReady),request);
        }
        delegate void ResponseInvoke(string text);
        void ResponseReady(IAsyncResult result)
        {
            HttpWebRequest request = result.AsyncState as HttpWebRequest;

            using (HttpWebResponse response = request.EndGetResponse(result) as HttpWebResponse)
            {
                Stream stream = response.GetResponseStream();

                StreamReader sr = new StreamReader(stream);

                string text = sr.ReadToEnd();
                this.Dispatcher.BeginInvoke((ResponseInvoke)GetResponse,text);
            }
        }
        void GetResponse(string text)
        {
            this.txtBlock.Text = text;
        }
    }
}

这个问题正在解决中……希望高手指教！