C# String的扩展方法ReplaceFirst

        /// <summary>
        /// 替换第一个符合条件的字符串
        /// </summary>
        /// <param name="value"></param>
        /// <param name="oldValue">所要替换掉的值</param>
        /// <param name="newValue">所要替换的值</param>
        /// <returns>返回替换后的值 所要替换掉的值为空或Null，返回原值</returns>
        public static string ReplaceFirst(this string value, string oldValue, string newValue)
        {
            if (string.IsNullOrEmpty(oldValue))
                return value;

            int idx = value.IndexOf(oldValue);
            if (idx == -1)
                return value;
            value = value.Remove(idx, oldValue.Length);
            return value.Insert(idx, newValue);
        }