不用System.Web 对 Content进行编码，De编码
string content = "<br/>";
string s1 = WebUtility.HtmlEncode(content);
string s3 = WebUtility.HtmlDecode(s1);
string s4 = WebUtility.UrlEncode(content);
using System.IO;
namespace System.Net
{
//
// 摘要:
// Provides methods for encoding and decoding URLs when processing Web requests.
public static class WebUtility
{
//
// 摘要:
// Converts a string that has been HTML-encoded for HTTP transmission into a decoded
// string.
//
// 参数:
// value:
// The string to decode.
//
// 返回结果:
// A decoded string.
public static string HtmlDecode(string value);
//
// 摘要:
// Converts a string that has been HTML-encoded into a decoded string, and sends
// the decoded string to a System.IO.TextWriter output stream.
//
// 参数:
// value:
// The string to decode.
//
// output:
// A System.IO.TextWriter stream of output.
//
// 异常:
// T:System.ArgumentNullException:
// The output parameter cannot be null if the value parameter is not null.
public static void HtmlDecode(string value, TextWriter output);
//
// 摘要:
// Converts a string to an HTML-encoded string.
//
// 参数:
// value:
// The string to encode.
//
// 返回结果:
// An encoded string.
public static string HtmlEncode(string value);
//
// 摘要:
// Converts a string into an HTML-encoded string, and returns the output as a System.IO.TextWriter
// stream of output.
//
// 参数:
// value:
// The string to encode.
//
// output:
// A System.IO.TextWriter output stream.
//
// 异常:
// T:System.ArgumentNullException:
// The output parameter cannot be null if the value parameter is not null.
public static void HtmlEncode(string value, TextWriter output);
//
// 摘要:
// Converts a string that has been encoded for transmission in a URL into a decoded
// string.
//
// 参数:
// encodedValue:
// A URL-encoded string to decode.
//
// 返回结果:
// Returns System.String. A decoded string.
public static string UrlDecode(string encodedValue);
//
// 摘要:
// Converts an encoded byte array that has been encoded for transmission in a URL
// into a decoded byte array.
//
// 参数:
// encodedValue:
// A URL-encoded System.Byte array to decode.
//
// offset:
// The offset, in bytes, from the start of the System.Byte array to decode.
//
// count:
// The count, in bytes, to decode from the System.Byte array.
//
// 返回结果:
// Returns System.Byte. A decoded System.Byte array.
public static byte[] UrlDecodeToBytes(byte[] encodedValue, int offset, int count);
//
// 摘要:
// Converts a text string into a URL-encoded string.
//
// 参数:
// value:
// The text to URL-encode.
//
// 返回结果:
// Returns System.String. A URL-encoded string.
public static string UrlEncode(string value);
//
// 摘要:
// Converts a byte array into a URL-encoded byte array.
//
// 参数:
// value:
// The System.Byte array to URL-encode.
//
// offset:
// The offset, in bytes, from the start of the System.Byte array to encode.
//
// count:
// The count, in bytes, to encode from the System.Byte array.
//
// 返回结果:
// Returns System.Byte. An encoded System.Byte array.
public static byte[] UrlEncodeToBytes(byte[] value, int offset, int count);
}
}