C#的Convert.ToInt32实现

以下是Convert.ToInt32(String str)的个人实现：

    public class ConvertHelper
    {
        private const int POSITIVE_SIGN_NUM = 43;
        private const int PLUS_SIGN_NUM = 45;
        private const int CHAR_0_NUM = 48;
        private const int CHAR_9_NUM = 57;

        /// <summary>
        /// convert to int32
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static int ToInt32(string str)
        {
            if (str == null || str.Trim().Length < 1)
            {
                throw new ArgumentNullException("str");
            }

            str = str.Trim();
            int firstCharNum = str[0];
            if (firstCharNum > CHAR_9_NUM)
            {
                throw new FormatException();
            }

            int number = 0;
            if (firstCharNum < CHAR_0_NUM)
            {
                if (str.Length == 1)
                {
                    throw new FormatException();
                }

                bool isPositive = true;
                switch (firstCharNum)
                {
                    case POSITIVE_SIGN_NUM:
                        break;
                    case PLUS_SIGN_NUM:
                        isPositive = false;
                        break;
                    default:
                        throw new FormatException();
                }

                for (int i = 1; i < str.Length; i++)
                {
                    if (str[i] < CHAR_0_NUM || str[i] > CHAR_9_NUM)
                    {
                        number = isPositive ? number : 0 - number;
                        break;
                    }
                    else
                    {
                        Check(str, i, true);
                        number = number * 10 + str[i] - CHAR_0_NUM;
                    }
                }
            }
            else
            {
                for (int i = 0; i < str.Length; i++)
                {
                    if (str[i] < CHAR_0_NUM || str[i] > CHAR_9_NUM)
                    {
                        break;
                    }
                    else
                    {
                        Check(str, i, true);
                        number = number * 10 + str[i] - CHAR_0_NUM;
                    }
                }
            }

            return number;
        }

        /// <summary>
        /// check number if overflow
        /// </summary>
        /// <param name="number">string number</param>
        /// <param name="index"></param>
        /// <param name="isPositiveData"></param>
        private static void Check(string number, int index, bool isPositiveData)
        {
            string maxOrMinValue = isPositiveData ? int.MaxValue.ToString() : int.MinValue.ToString();

            if (index == maxOrMinValue.Length)
            {
                for (int j = 0; j < index; j++)
                {
                    if (number[j] < maxOrMinValue[j])
                    {
                        break;
                    }

                    if (number[j] > maxOrMinValue[j])
                    {
                        throw new OverflowException();
                    }
                }
            }
            else
            {
                if (index > maxOrMinValue.Length)
                {
                    throw new OverflowException();
                }
            }
        }
    }

 这里，需要注意的几点是：

1. 如果传参为空，需要判断处理；

2. 首位字符可能是：+/-，0~9；

3. 字符串中可能包含非数字字符；

4. 溢出处理；

5. 效率。