Static members not initialized - get System.NullReferenceException from array element

Question

hit another problem during my C# practise. This is short explanation of it: In Program.cs I have the following code:

namespace testApp
{
  public class AppSettings
  {
    public static int appState { get; set; }
    public static bool[] stepsCompleted { get; set; }
  }

  public void Settings
  {
    appState = 0;
    bool[] stepsCompleted = new bool[]{false, false, false, false, false};
  }
}

static class MyApp
{
  static void Main()
  {
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.Run(new gameScreen());
    AppSettings appSettings = new AppSettings();
  }
}

And this is in the Form1.Designer.cs:

namespace testApp
{
  private void InitializeComponent() {..}
  private void detectPressedKey(object sender, KeyPressEventArgs e)
  {
    if (e.KeyChar == (char)13) // Enter = code 13
    {
      if (AppSettings.appState == 0)
      {
        if (AppSettings.stepsCompleted[1] == false) // << here we have an EXCEPTION!!!
        {
          this.playSound("warn");
        }
      }
    }
  }
}

The problem is in the commented if where I get NullReferenceException: Object reference not set to an instance of an object. Searched a bit through the net but cannot find where is the problem. AppSettings.stepsCompleted should exist like AppSettings.appState

You are not assigning anything non-null to AppSettings.stepsCompleted, so it will be null. It does exist, but it doesn't have a value, so attempting to access its elements will throw a NRE. — JLRishe
– JLRishe, Commented May 27, 2015 at 4:36

StuartLC · Accepted Answer · 2015-05-27 05:36:46Z

5

You aren't initializing the AppSettings.stepsCompleted anywhere. In fact, testApp.Settings won't compile. Since your AppSettings class has static members, which you access from your form, and assuming you need only a single instance to track state, what you can do is initialize them via a static constructor:

public static class AppSettings // May as well make the class static 
{
  public static int appState { get; set; }
  public static bool[] stepsCompleted { get; set; }

  static AppSettings() // Static constructor
  {
    appState = 0;
    stepsCompleted = new []{false, false, false, false, false};
  } 
}

You then need to remove the line from Main:

AppSettings appSettings = new AppSettings();

Static constructors are guaranteed to be invoked once, prior to the first access

Edit - Full working sample

Program.cs

using System;
using System.Windows.Forms;

namespace testApp
{
    public static class AppSettings // May as well make the class static 
    {
        public static int appState { get; set; }
        public static bool[] stepsCompleted { get; set; }

        static AppSettings() // Static constructor
        {
            appState = 0;
            stepsCompleted = new[] { false, false, false, false, false };
        }
    }

    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new gameScreen());
        }
    }
}

Form1.cs (gameScreen)

using System.Windows.Forms;

namespace testApp
{
    public partial class gameScreen : Form
    {
        public gameScreen()
        {
            InitializeComponent();
        }

        private void gameScreen_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == (char)13) // Enter = code 13
            {
                if (AppSettings.appState == 0)
                {
                    if (AppSettings.stepsCompleted[1] == false)
                    {
                        this.playSound("warn");
                    }
                }
            }
        }

        private void playSound(string someSound)
        {
            MessageBox.Show(string.Format("Sound : {0}", someSound));
        }
    }
}

edited May 27, 2015 at 5:36

answered May 27, 2015 at 4:40

StuartLC

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3 Comments

1000Gbps Over a year ago

Well, did your suggestions and I've got the same exception

1000Gbps Over a year ago

... but realized that I should remove bool[] in front of stepsCompleted in the constructor, in fact I missed it :). Now it works, answer approved! Btw Im'm using embedded resource for playSound().

StuartLC Over a year ago

Apols - I didn't notice that change - new []{false, false, false, false, false} and new bool[]{false, false, false, false, false} should both work - the compiler can infer that its an array of booleans. My Resharper cleanup is set to remove anything it deems redundant.

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