How can I show/hide form with checkbox?

Question

Sorry, I am new to C# and am unsure what I am doing wrong.

Here is the code I am using:

private void chkSmallMenu_CheckedChanged(object sender, EventArgs e)
{
    frmSmallMenu sm = null;
    if (chkSmallMenu.Checked) 
    { 
        if (sm is null || sm.IsDisposed)
        { 
            sm = new frmSmallMenu(); 
        } 
        sm.Show(); 
    } 
    else 
    {
        MessageBox.Show("close");
        sm?.Close(); 
    }
}

The window will open but when I uncheck the box nothing happens and I have no idea why. I have tried looking for an answer but nothing has worked for me.

Answer 1

This modification of your code would probably do what you want by first looking whether the other Form is already running or not:

namespace WinFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        /// The following `uselessField ` is a `field`. See also https://stackoverflow.com/questions/295104/what-is-the-difference-between-a-field-and-a-property
        /// `(Since it is `unused`, you would get a CS0169 Warning in the "Error List" window)
        private int uselessField;

        /// <summary>
        /// **Event handler** of the "chkSmallMenu" `CheckBox` control on your `Form`.
        /// (You would probably get an IDE1006 Info in your "Error List" window because
        /// the control name and/or the event handler name respectively, starts with a lower case
        /// https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/naming-rules)
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void chkSmallMenu_CheckedChanged(object sender, EventArgs e)
        {
            // the following `sm` is a `variable`. See also https://stackoverflow.com/questions/295104/what-is-the-difference-between-a-field-and-a-property

            var sm = Application.OpenForms.OfType<frmSmallMenu>().FirstOrDefault();
            // the following `Checked` **property** belongs to the WinForms Checkbox class and `IsDisposed` belongs to the other `Form`
            if (chkSmallMenu.Checked)
            {
                if (sm?.IsDisposed != true)
                {
                    sm = new frmSmallMenu();
                }
                sm.Show();
            }
            else
            {
                MessageBox.Show("close");
                sm?.Close();
            }
        }
    }
}

Answer 2

This fixed my issue:

frmSmallMenu sm = new frmSmallMenu();

private void chkSmallMenu_CheckedChanged(object sender, EventArgs e)
{
    if (chkSmallMenu.Checked == true)
    {
        sm.Show();
    }
    else
    {
        MessageBox.Show("close");
        sm.Hide();
    }
}