没有System.Web的C#HTMLDecode可能吗？

public static string HtmlDecode(string html)
{
    if (string.IsNullOrEmpty(html))
        return string.Empty;

    var sb = new StringBuilder();
    var i = 0;
    while (i< html.Length)
    {
        if (html[i] == '&')
        {
            var j = html.IndexOf(';', i);
            if (j > i)
            {
                var entity = html.Substring(i, j - i + 1);
                var decoded = DecodeEntity(entity);
                if (decoded != null)
                {
                    sb.Append(decoded);
                    i = j + 1;
                    continue;
                }
            }
        }

        sb.Append(html[i]);
        i++;
    }

    return sb.ToString();
}

private static string DecodeEntity(string entity)
{
    switch (entity)
    {
        case "&lt;": return "<";
        case "&gt;": return ">";
        case "&amp;": return "&";
        case "&quot;": return "\"";
        case "&apos;": return "'";
        default:
            if (entity.StartsWith("&#"))
            {
                var number = entity.Substring(3, entity.Length - 4);
                if (int.TryParse(number, out var value))
                {
                    return ((char)value).ToString();
                }
            }
            else if (entity.StartsWith("&x"))
            {
                var number = entity.Substring(3, entity.Length - 4);
                if (int.TryParse(number, System.Globalization.NumberStyles.HexNumber, null, out var value))
                {
                    return ((char)value).ToString();
                }
            }
            return null;
    }
}