How to keep the global variable not changed when passed to a function? C Sharp

 

How to keep the global variable not changed when passed to a function? C#

Problem

I am developing an add-in for OneNote. In the sinppet below, a global variable AllPageXML is claimed first.
Then the very first time when a window is loaded, AllPageXML is initinized with a value.
After that, the value of AllPageXML is supposed to not be changed.
When the user input some keywords in a textbox, search in AllPageXML will be excuted.
And AllPageXML will be passed to an function FilterXml. After returnning, the value of AllPageXML has been changed.
How can I do to keep the value of AllPageXML not changed? Any comments will be appreciated.

Code

 internal partial class NaviPages : LocalizableForm
    {
        public XElement AllPageXML;
        public NaviPages()
        {
            InitializeComponent();

        }
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            using var one = new OneNote();
            var AllPageXML = one.GetAllPages();
            //...//

        }
        
        private void tBoxKw_TextChanged(object sender, EventArgs e)
        {
            try
            {
                string strKeyWords = tBoxKw.Text.ToLower();
                FilteredPageXml = FilterXml(AllPageXML, strKeyWords);           
            }
            catch
            {
                MessageBox.Show("Failed to filter!");
            }
        }
         public XElement FilterXml(XElement inXML, string strKeyWords)
        {
            string strSecKw, strPageKw;
            XElement root = inXML;
            ///...///
            //delete empty notebook
            foreach (var child1 in root.Elements().Reverse())
            {
                if (!child1.HasElements) child1.Remove();
            }
            return root;
        }

    }

Answer

You should change 2 things:

1. Remove the redeclaration of AllPageXML in the OnLoad method, so it refers to the instance variable instead (remove the var keyword):

protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            using var one = new OneNote();
            AllPageXML = one.GetAllPages();
            //...//

        }

You should change 2 things:
Remove the redeclaration of AllPageXML in the OnLoad method, so it refers to the instance variable instead (remove the var keyword):
protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            using var one = new OneNote();
            AllPageXML = one.GetAllPages();
            //...//

        }
The issue here is that inside the method, a local variable named AllPageXML is declared and assigned a value from one.GetAllPages(). However, this local variable shadows the class-level variable with the same name.
Instead of assigning the value to the class-level variable, the code assigns it to the local variable, which has a limited scope and does not affect the class-level variable. Therefore, when FilterXml is called later in the tBoxKw_TextChanged method, it uses the uninitialized class-level variable, resulting in unexpected behavior.
Add the readonly keyword to the declaration of AllPageXML to indicate that its value should not be modified once initialized. If you do this you can only set the value in the constructor and it can not be set after:
public readonly XElement AllPageXML;