How do I access the text in a text box from a button click event handler

Im trying to write this simple winform menu and I need to add the contents of the NBox text box to a string so I can display it when a button is pressed, however I keep getting the error that NBox does not exist in the current context. So, how would i got about making the contents of the text box available at the press of a button?

using System;
using System.Windows.Forms;
using System.Drawing;
using System.Diagnostics;
//namespace game{
class MainM : Form{

public MainM(){

    Text = "Adventures Main Menu";
    Size = new Size(400,400);

    //NameBox
    TextBox NBox = new TextBox();
    NBox.Location = new Point(145, 100);
    NBox.Size = new Size(200, 30);

    //Title Label

    Label title = new Label();
    title.Text = "ADVENTURE THE GAME";
    title.Location = new Point(145, 30);
    title.Size =  new Size(200,60);
    title.Font = new Font(defaultFont.FontFamily, defaultFont.Size, FontStyle.Bold);

    //The main menu Buttons and all that jazz
    Button credits = new Button();
    Button start = new Button();

    //Credits Button
    credits.Text = "Credits";
    credits.Size = new Size(75,20);
    credits.Location = new Point(145,275);
    credits.Click += new EventHandler(this.credits_button_click);

    //Start Button
    start.Text = "Start";
    start.Size = new Size(75,20);
    start.Location = new Point(145,200);
    start.Click += new EventHandler(this.start_button_click);


    //Control addition
    this.Controls.Add(title);
    this.Controls.Add(credits);
    this.Controls.Add(start);
    this.Controls.Add(NBox);
}

  public void test(){
            //The Main Window
  }

  private void credits_button_click(object sender, EventArgs e){
    MessageBox.Show("Created by: Me");
  }

 private void start_button_click(object sender, EventArgs e){
    this.Hide();
    string name = NBox.Text;
    MessageBox.Show(name);

    //Process.Start("TextGame.exe");
 }

  public static void Main(){
    Application.Run(new MainM());
  }

} 
//}