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;
}