AtoI function - String to Integer in C# .NET

If you ever want to convert a string to integer, you have to go through the following checks

 public static int StringToInt(string s)
        {
            int length = s.Length;
            int i = 0;
            int lastNumber = 0;
            int returnNumber = 0;
            bool numberNegative = false;
            int startPoint = 0;

            if (s[0] == '-')
            {
                numberNegative = true;
                startPoint = 1;
            }
            
            for (i = startPoint; i < length; i++)
            {
                if (s[i] == ' ')
                {
                    continue;
                }
                else
                {
                    if ((s[i] >= '0') && s[i] <= '9')
                    {
                        returnNumber = s[i] - '0';
                        if (i > 0) lastNumber = lastNumber * 10;
                        lastNumber = lastNumber + returnNumber;
                    }
                    else
                    {
                        break;
                    }
                }
            }
            if (numberNegative) 
                lastNumber = -1 * lastNumber;

            return lastNumber;
        }