截取等宽中英文字符串(比如截取2个，中午截取两个字，英文截取四个单词)

public static class StringExtensions
{
/// <summary>
/// 截取等宽中英文字符串
/// </summary>
/// <param name="str">要截取的字符串</param>
/// <param name="length">要截取的中文字符长度</param>
/// <param name="appendStr">截取后后追加的字符串</param>
/// <returns>截取后的字符串</returns>
public static string CutStr(this string str, int length, string appendStr="")
{
if (str == null) return string.Empty;

var len = length * 2;
int aequilateLength = 0, cutLength = 0;
var encoding = Encoding.GetEncoding("gb2312");

var cutStr = str;
var strLength = cutStr.Length;
byte[] bytes;
for (int i = 0; i < strLength; i++)
{
bytes = encoding.GetBytes(cutStr.Substring(i, 1));
if (bytes.Length >= 2)//不是英文
aequilateLength += 2;
else
aequilateLength++;

if (aequilateLength <= len) cutLength += 1;

if (aequilateLength > len)
return cutStr.Substring(0, cutLength) + appendStr;
}
return cutStr;
}
}