Checking user input for invalid characters

i have a console application that asks user for their sales figures over a month. I have made the program reject entries below zero and ask the user to enter their sales figures again. But now I want the same thing to happen when the user enters a letter or any other character that is not a number The code i currently have is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace FIRST_ACTUAL_PROJECT
{
    class Program
    {
        static void Main(string[] args)
        {
            FileStream fin; // this is declaring that you are using a filestream.
            String s;
            int LineNum = 0;
            double seventy_percent_value;
            double thirty_percent_value;
            const i开发者_如何学编程nt max_num_of_items = 20; // this means that there will always be a maximum of 20 sales figures because there is a maximum of 20 customers
            double[] sales_figures = new double[max_num_of_items]; // this is the array for the sales figures
            string[] customer = new string[max_num_of_items]; // this is the array for the customers
            double[] licence_fee_in_percent = new double[max_num_of_items]; // this is the array for the licence fee 
            double[] fee_payable = new double[max_num_of_items]; // array for the fees payable in pounds.
            const double MIN_SALES_FIGURE = 0;
            try
            {
                fin = new FileStream("customer list.txt", FileMode.Open);// this is opening the file.
            }
            catch (IOException exc)
            {
                Console.WriteLine(exc.Message + "cannot find file!"); // error message if it does'nt find the file or something went wrong.
                Console.ReadLine();
                return;
            }
            StreamReader fstr_in = new StreamReader(fin); // this is telling the streamreader which file to read.
            try
            {
                while ((s = fstr_in.ReadLine()) != null) // this is reading the file until the end.
                {
                    Console.WriteLine(s);
                    customer[LineNum] = s.Split(',')[0];
                    licence_fee_in_percent[LineNum] = double.Parse(s.Split(',')[1]);
                    LineNum = LineNum + 1;
                }
            }
            catch (IOException exc)
            {
                Console.WriteLine(exc.Message);
            }
            for (int CustPos = 0; CustPos < LineNum; CustPos = CustPos + 1) // this determines what the loop does.
            {
                Console.Write("enter sales figures for" + customer[CustPos] + "  "); // this asks the user to enter the sales figures
                sales_figures[CustPos] = Double.Parse(Console.ReadLine()); // this is user's input is read in and stored.

                while (sales_figures[CustPos] < MIN_SALES_FIGURE)   // this is if the user enters a number below zero.
                {
                    Console.WriteLine("");
                    Console.WriteLine("entry invalid");
                    Console.WriteLine("");
                    Console.WriteLine("enter sales figures for" + customer[CustPos] + "  ");
                    sales_figures[CustPos] = Double.Parse(Console.ReadLine());
                }

                    Console.WriteLine(" ");
                    fee_payable[CustPos] = (sales_figures[CustPos] / 100.0) * licence_fee_in_percent[CustPos];
                    Console.WriteLine(customer[CustPos] + " ----------- " + fee_payable[CustPos]);
                    Console.WriteLine("Licence fee to be paid in GBP is :" + fee_payable[CustPos]);         //this section displays the cust name, sales figure 70/30.
                    seventy_percent_value = ((fee_payable[CustPos] / 10.0) * 7);
                    Console.WriteLine("70 percent of this fee is" + seventy_percent_value);
                    thirty_percent_value = ((fee_payable[CustPos] / 10.0) * 3);
                    Console.WriteLine("30 percent of this fee is" + thirty_percent_value);
                    Console.WriteLine(" ");
                }

            }
            Console.WriteLine("Customer name" + "\t" + "sales" + "\t" + "fee paid" + "\t" + "70% value" + "\t" + "30% value" + "\t");
            for (int DisplayPos = 0; DisplayPos < LineNum; DisplayPos = DisplayPos + 1)
            {
                seventy_percent_value = ((fee_payable[DisplayPos] / 10.0) * 7);
                thirty_percent_value = ((fee_payable[DisplayPos] / 10.0) * 3);
                Console.WriteLine(customer[DisplayPos] + "\t" + sales_figures[DisplayPos] + "\t" + fee_payable[DisplayPos] + "\t\t" + seventy_percent_value + " \t\t" + thirty_percent_value + "\t");
            }
            Console.WriteLine(" ");
            Console.WriteLine("Press enter to finish");
            Console.ReadLine();
        }
    }
}

Instead of using Double.Parse, use Double.TryParse - that will return whether or not the number was parsed successfully.

Better yet, use Decimal.TryParse - you shouldn't be using double for currency values.

Additional recommendations:

• Fix the namespace to one which complies with .NET naming conventions
• You've got one enormous method - break it up into several methods which each performs one small task
• Consider using List<T> instead of an array - that way you don't need to pre-allocate everything
• You don't happend to have any particular naming convention for your variables; it would be good to be consistent
• Generally prefer to declare local variables at the point of first use, rather than declaring everything at the top of the method
• Use using statements to close your resources such as streams and readers. (Currently I don't think you close anything, ever.)

I think would probably use regular expressions to check the input is in the correct form.

use TryParse instead of Parse:

Replace:

 sales_figures[CustPos] = Double.Parse(Console.ReadLine()); // this is user's input is read in and stored.

With:

 bool isValidDouble = Double.TryParse(Console.ReadLine(), out sales_figures[CustPos] );

then check for isValidDouble later.