Bdtjtk

I have two forms here and I want to call the function from one to the other in c# winform. I use events to do this, but they do not work. Every time I call changeTheme (), it throws me a System.NullReferenceException error: 'Object reference not set to an instance of an object.'

I tried to call her through the instance of the Main_form form, but it did not work either. I also tried the events, but when I added a function to make it so it was set to null again. But I'd like to do it with an event.

public delegate void statusChange();



public partial class Settings_form : Form

{

    public event statusChange changeTheme;



    //Here is some function, variables declaration and code



    private void UseDarkMode_chk_CheckedChanged(object sender, EventArgs e)

    {

        //Some code

        SettingsClass.UseDarkMode = this.UseDarkMode_chk.Checked;

        //if (changeTheme != null)

            changeTheme();

    }

}







public partial class Main_form : Form

{



    private void Form1_Load(object sender, EventArgs e)

    {



        callChangeTheme();

    }



    private void settingsToolStripMenuItem_Click(object sender, EventArgs e)

    {

        Settings_form settings_Form = new Settings_form();

        settings_Form.Show();

    }





    public void callChangeTheme()

    {

        Settings_form settings_Form = new Settings_form();

        settings_Form.changeTheme += new statusChange(chooseOtherTheme);

    }



    public void chooseOtherTheme()

    {

        if (SettingsClass.UseDarkMode)

            ToDarkMode();

        else ToLightMode();

    }



    public void ToDarkMode()

    {

        this.BackColor = Color.FromArgb(((int)(((byte)(28)))), ((int)(((byte)(28)))), ((int)(((byte)(28)))));

    }



    public void ToLightMode()

    {

         this.BackColor = Color.FromArgb(((int)(((byte)(241)))), ((int)(((byte)(241)))), ((int)(((byte)(241)))));

    }

}

One big issue I notice is here:

public void callChangeTheme()

{

    Settings_form settings_Form = new Settings_form();

    settings_Form.changeTheme += new statusChange(chooseOtherTheme);

}

You're creating a new instance of Settings_form every time you call this method, but you never actually do anything with the instance - the only time you call Show() is on a completely different instance:

private void settingsToolStripMenuItem_Click(object sender, EventArgs e)

{

    Settings_form settings_Form = new Settings_form();

    settings_Form.Show();

}

If you want to act Settings_form from your main form, you should be sure that you're always acting on the same instance.

Also, please do be mindful of subscribing and unsubscribing events in a way that guarantees that you don't accidentally end up with a memory leak. You can see more details on this here: Why and How to avoid Event Handler memory leaks?

The null reference exception is happening because nothing is subscribed to your event.

Generally you'd create an event like this:

public event EventHandler<EventArgs> MyEvent;

It's a lot easier to read than the older event/delegate style but works in exactly the same way.

And fire it like this:

MyEvent?.Invoke(this, EventArgs.Empty);

That way it will only fire if you have subscribers. Again analogous to the if != null fire older style.

With that out of the way, there is a simpler way you can achieve what you want without events.

If you create a derived form and have the change code implemented there you can create the rest of your forms out of that then just call the change code directly:

Your new base form:

public MyDerivedForm : Form

{

    public MyDerivedForm()

    {

        InitializeComponent();

    }



    public void ChangeTheme(bool usedarkmode)

    {

        if (usedarkmode)

            ToDarkMode();

        else

            ToLightMode();

    }



    public void ToDarkMode()

    {

        this.BackColor = Color.FromArgb(28, 28, 28);

    }



    public void ToLightMode()

    {

        this.BackColor = Color.FromArgb(241, 241, 241);

    }

}

I've simplified your FromArgb calls. All your casting was completely superfluous.

Now derive all your other forms from MyDerivedForm:

public MainForm : MyDerivedForm

{

}

Just add a new form to the project and change the : Form to : MyDerivedForm

In your Settings_form you can just loop over all your opened forms and call the ChangeTheme function on each one which will then switch between light and dark mode.

public Settings_form: MyDerivedForm

{

    public Settings_form()

    {

        InitializeComponent();

    }



    private void UseDarkMode_chk_CheckedChanged(object sender, EventArgs e)

    {

        //Some code

        SettingsClass.UseDarkMode = this.UseDarkMode_chk.Checked;



        foreach(MyDerivedForm form in Application.OpenForms.OfType<MyDerivedForm>())

        {

            form.ChangeTheme(SettingsClass.UseDarkMode);

        }

    }

}