扩展方法：

    public static class Spell
    {
        /// <summary>
        /// 获取汉字字符串的拼音首字母
        /// </summary>
        /// <param name="str">字符串</param>
        /// <returns></returns>
        public static string GetSpell(this string str)
        {
            string result = string.Empty;
            str.ForEach(x => result += GetOne(x.ToString()));
            return result;
        }

        /// <summary>
        ///  获取单个汉字的首拼音
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        private static string GetOne(string str)
        {
            byte[] arrCN = Encoding.Default.GetBytes(str);
            if (arrCN.Length > 1)
            {
                int area = (short)arrCN[0];
                int pos = (short)arrCN[1];
                int code = (area << 8) + pos;
                int[] areacode = { 45217, 45253, 45761, 46318, 46826, 47010, 47297, 47614, 48119, 48119, 49062, 49324, 49896, 50371, 50614, 50622, 50906, 51387, 51446, 52218, 52698, 52698, 52698, 52980, 53689, 54481 };
                for (int i = 0; i < 26; i++)
                {
                    int max = 55290;
                    if (i != 25) max = areacode[i + 1];
                    if (areacode[i] <= code && code < max)
                    {
                        return Encoding.Default.GetString(new byte[] { (byte)(65 + i) });
                    }
                }
                return "_";
            }
            else return str;
        }
    }

调用结果

string value = "五维思考";
Console.WriteLine($"{value}({ value.GetSpell()})");
Console.Read();

//结果：五维思考(WWSK)