How to Pass a Number from a TextBox to a Function?

Here's my code:

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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }


        private void button1_Click(object sender, EventArgs e)
        {
            NumWordsWrapper(3001);
        }

        static String NumWordsWrapper(double n)
        {
            string words = "";
            double intPart;
            double decPart = 0;
            if (n == 0)
            return "zero";
            try
        {
            string[] splitter = n.ToString().Split('.');
            intPart = double.Parse(splitter[0]);
            decPart = double.Parse(splitter[1]);
        }
        catch
        {
        intPart = n;
        }
 
            words = NumWords(intPart);
 
            if (decPart > 0)
            {
            if (words != "")
                words += " and ";
            int counter = decPart.ToString().Length;
            switch (counter)
            {
                case 1: words += NumWords(decPart) + " tenths"; break;
                case 2: words += NumWords(decPart) + " hundredths"; break;
                case 3: words += NumWords(decPart) + " thousandths"; break;
                case 4: words += NumWords(decPart) + " ten-thousandths"; break;
                case 5: words += NumWords(decPart) + " hundred-thousandths"; break;
                case 6: words += NumWords(decPart) + " millionths"; break;
                case 7: words += NumWords(decPart) + " ten-millionths"; break;
            }
        }
        return words;
        }

   }

}

Sorry to waste people's time; this should be a simple thing. 

When I click the button, I get the function converting numbers to words, which is great, but it's hard-coded.  So, I'm trying to use a TextBox to pass a value to the function.

I think it should be something like this:

        private void button1_Click(object sender, EventArgs e)
        {
            NumWordsWrapper(This.textBox1.Text);

        }

That doesn't work!!  I've Googled for a solution; haven't come up with anything yet.  Any ideas, anyone?  this must be a simple thing; I'm just not seeing the solution here.

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