private string ToUnicodeString(string s)
{
    StringBuilder sb = new StringBuilder();
    byte[] bytes = Encoding.Unicode.GetBytes(s);
    for (int i = 0; i < bytes.Length; )
    {
        sb.Append("\\u");
        sb.Append(bytes[i + 1].ToString("x2"));
        sb.Append(bytes[i].ToString("x2"));

        i += 2;
    }

    return sb.ToString();
}

private string ToNormalString(string input)
{
    string pattern = "\\\\u([0-9A-Fa-f]{2})([0-9A-Fa-f]{2})";
    Match match = System.Text.RegularExpressions.Regex.Match(input,pattern);
    System.Collections.ArrayList numlist = new System.Collections.ArrayList();

    while(match.Success)
    {
        string s = match.Groups[2].Value;
        byte num = byte.Parse(s, System.Globalization.NumberStyles.AllowHexSpecifier);
        numlist.Add(num);
        s = match.Groups[1].Value;
        num = byte.Parse(s, System.Globalization.NumberStyles.AllowHexSpecifier);
        numlist.Add(num);
        match = match.NextMatch();
    }

    byte[] bytes = new byte[numlist.Count];
    numlist.CopyTo(bytes);
    string result = Encoding.Unicode.GetString(bytes);
    return result;
}

------解决方案--------------------
需要先知道对方的编码方式，然后才能使用Encoding的GetString方式解码：

C# code
string s=Encoding.Default.GetString(buffer);