asp.net下UTF-7转GB2312编码的代码（中文）

/// UTF-7转换GB2312编码的方法 
/// </summary> 
/// <param name="str7"></param> 
/// <returns></returns> 
public static string utf72Unicode(string str7) 
{ 
string sUtf7 = ""; 
if (str7.StartsWith("&")) 
{ 
sUtf7 = "+" + str7.Substring(1); 
} 
else 
{ 
sUtf7 = str7; 
} 
byte[] bytes = System.Text.Encoding.GetEncoding("gb2312").GetBytes(sUtf7); 
byte[] byteret = System.Text.Encoding.Convert(System.Text.Encoding.UTF7, System.Text.Encoding.Unicode, bytes); 
string sret = System.Text.Encoding.Unicode.GetString(byteret); 
return sret; 
}