System.StringHelper.Substring C# (CSharp) Method

        public static String Substring(this String str, String after, String before = null, Int32 startIndex = 0, Int32[] positions = null)
        {
            if (String.IsNullOrEmpty(str)) return str;
            if (String.IsNullOrEmpty(after) && String.IsNullOrEmpty(before)) return str;

            /*
             * 1，只有start，从该字符串之后部分
             * 2，只有end，从开头到该字符串之前
             * 3，同时start和end，取中间部分
             */

            var p = -1;
            if (!String.IsNullOrEmpty(after))
            {
                p = str.IndexOf(after, startIndex);
                if (p < 0) return null;
                p += after.Length;

                // 记录位置
                if (positions != null && positions.Length > 0) positions[0] = p;
            }

            if (String.IsNullOrEmpty(before)) return str.Substring(p);

            var f = str.IndexOf(before, p >= 0 ? p : startIndex);
            if (f < 0) return null;

            // 记录位置
            if (positions != null && positions.Length > 1) positions[1] = f;

            if (p >= 0)
                return str.Substring(p, f - p);
            else
                return str.Substring(0, f);
        }