public static String Cut(this String str, Int32 maxLength, String pad = null)
{
if (String.IsNullOrEmpty(str) || maxLength <= 0 || str.Length < maxLength) return str;
// 计算截取长度
var len = maxLength;
if (!String.IsNullOrEmpty(pad)) len -= pad.Length;
if (len <= 0) return pad;
return str.Substring(0, len) + pad;
}