Question

I want to run a check on a String right before I append it to a StringBuilder to make sure only numeric characters are in the string. What’s a simple way to do that?

Answer 1

Use Integer.TryParse() it will return true if there are only digits in the string. Int32 max value is 2,147,483,647 so if your value is less then that then your fine.
http://msdn.microsoft.com/en-us/library/f02979c7.aspx

You can also use Double.TryParse() which has a max value of 1.7976931348623157E+308 but it will allow a decimal point.

If your looking to get the value that isnt an integer you can always go through the string one at a time

string test = "1112003212g1232";
        int result;
        bool append=true;
        for (int i = 0; i < test.Length-1; i++)
        {
            if(!Int32.TryParse(test.Substring(i,i+1),out result))
            {
                //Not an integer
                append = false;
            }
        }

If append stays true then the string is an integer. Probably a more slick way of doing this but this should work.