Format

为什么我用Format的时候提示找不到命名空间呢？？？

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 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Net;
 5 using System.Windows;
 6 using System.Windows.Controls;
 7 using System.Windows.Documents;
 8 using System.Windows.Input;
 9 using System.Windows.Media;
10 using System.Windows.Media.Animation;
11 using System.Windows.Shapes;
12 using System.IO;
13 
14 namespace SL13
15 {
16     public partial class MainPage : UserControl
17     {
18         public MainPage()
19         {
20             InitializeComponent();
21         }
22 
23         private void Button_Click(object sender, RoutedEventArgs e)
24         {
25             GetDataByWebRequest();//调用request函数
26         }
27 
28         void GetDataByWebRequest()
29         {
30             string data = this.txtName.Text.ToString();//源数据
31             Uri uri = new Uri(string  Format("http://localhost:1234/MyHandler.ashx?name={0}",data));//定义目标地址
32             
33             HttpWebRequest request = WebRequest.Create(uri) as HttpWebRequest;
34 
35             request.Method = "GET";
36             request.BeginGetResponse(new AsyncCallback(ResponseReady),request);
37         }
38         delegate void ResponseInvoke(string text);
39         void ResponseReady(IAsyncResult result)
40         {
41             HttpWebRequest request = result.AsyncState as HttpWebRequest;
42 
43             using (HttpWebResponse response = request.EndGetResponse(result) as HttpWebResponse)
44             {
45                 Stream stream = response.GetResponseStream();
46 
47                 StreamReader sr = new StreamReader(stream);
48 
49                 string text = sr.ReadToEnd();
50                 this.Dispatcher.BeginInvoke((ResponseInvoke)GetResponse,text);
51             }
52         }
53         void GetResponse(string text)
54         {
55             this.txtBlock.Text = text;
56         }
57     }
58 }

这个问题正在解决中……希望高手指教！