Load user input into 10x4 rectangular array

[Inigo Montoya]

Ok, I am working on a form that calculates the future value of an investment. I need to add a 10x4 rectangular array (required) and then display the results when the user clicks exit. I am having trouble figuring out how to load the user input into the array. Here's what I have so far:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace FutureValue
{
    // This is the starting point for exercise 8-2 from
    // "Murach's C# 2010" by Joel Murach
    // (c) 2010 by Mike Murach & Associates, Inc. 
    // www.murach.com

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

        }

        // TODO: Declare the rectangular array and the row index here
        int[,] calculations = new int[10, 4];
        int numberOfRows = calculations.GetLength(0);

        private void btnCalculate_Click(object sender, EventArgs e)
        {
            try
            {
                if (IsValidData())
                {
                    decimal monthlyInvestment =
                        Convert.ToDecimal(txtMonthlyInvestment.Text);
                    decimal interestRateYearly =
                        Convert.ToDecimal(txtInterestRate.Text);
                    int years = Convert.ToInt32(txtYears.Text);

                    int months = years * 12;
                    decimal interestRateMonthly = interestRateYearly / 12 / 100;

                    decimal futureValue = CalculateFutureValue(
                        monthlyInvestment, interestRateMonthly, months);
                    txtFutureValue.Text = futureValue.ToString("c");
                    txtMonthlyInvestment.Focus();

                    // TODO: Add the calculation to the rectangular array here
                    
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message + "\n\n" +
                    ex.GetType().ToString() + "\n" +
                    ex.StackTrace, "Exception");
            }
        }

        public bool IsValidData()
        {
            return
                // Validate the Monthly Investment text box
                IsPresent(txtMonthlyInvestment, "Monthly Investment") &&
                IsDecimal(txtMonthlyInvestment, "Monthly Investment") &&
                IsWithinRange(txtMonthlyInvestment, "Monthly Investment", 1, 1000) &&

                // Validate the Yearly Interest Rate text box
                IsPresent(txtInterestRate, "Yearly Interest Rate") &&
                IsDecimal(txtInterestRate, "Yearly Interest Rate") &&
                IsWithinRange(txtInterestRate, "Yearly Interest Rate", 1, 20) &&

                // Validate the Number of Years text box
                IsPresent(txtYears, "Number of Years") &&
                IsInt32(txtYears, "Number of Years") &&
                IsWithinRange(txtYears, "Number of Years", 1, 40);
        }

        public bool IsPresent(TextBox textBox, string name)
        {
            if (textBox.Text == "")
            {
                MessageBox.Show(name + " is a required field.", "Entry Error");
                textBox.Focus();
                return false;
            }
            return true;
        }

        public bool IsDecimal(TextBox textBox, string name)
        {
            try
            {
                Convert.ToDecimal(textBox.Text);
                return true;
            }
            catch (FormatException)
            {
                MessageBox.Show(name + " must be a decimal value.", "Entry Error");
                textBox.Focus();
                return false;
            }
        }

        public bool IsInt32(TextBox textBox, string name)
        {
            try
            {
                Convert.ToInt32(textBox.Text);
                return true;
            }
            catch (FormatException)
            {
                MessageBox.Show(name + " must be an integer.", "Entry Error");
                textBox.Focus();
                return false;
            }
        }

        public bool IsWithinRange(TextBox textBox, string name,
            decimal min, decimal max)
        {
            decimal number = Convert.ToDecimal(textBox.Text);
            if (number < min || number > max)
            {
                MessageBox.Show(name + " must be between " + min
                    + " and " + max + ".", "Entry Error");
                textBox.Focus();
                return false;
            }
            return true;
        }

        private decimal CalculateFutureValue(decimal monthlyInvestment,
           decimal monthlyInterestRate, int months)
        {
            decimal futureValue = 0m;
            for (int i = 0; i < months; i++)
            {
                futureValue = (futureValue + monthlyInvestment)
                    * (1 + monthlyInterestRate);
            }

            return futureValue;
        }

        private void btnExit_Click(object sender, EventArgs e)
        {
            // TODO: Display the rectangular array in a dialog box here
            string calculationsString = "";
                    for (int i = 0; i < calculations.GetLength(0); i++)
                    {
                        for (int j = 0; j < calculations.GetLength(1); j++
                            calculationsString += calculations[i,j] + " ";

                        calculationsString += "\n";
                    }
            MessageBox.Show(calculationsString, "Future Value Calculations");

            this.Close();
        }

    }
}

int numberOfRows = calculations.GetLength(0); is also giving me an error "A field initializer cannot reference the non-static field, method, or property 'FutureValue.form1.calculations'
Do I need to assign values to the array first?

I already coded the message box at the end, I just mainly need to know how to enter the user input into the array. Thanks for your help.