C# why is int value keep getting reset to 0?

Goal: Keep value of int (professorIndex) reachable when I get to the button click method (btnUpdateAvailability_Click())

Problem: The value gets set correctly initially, then somehow goes to 0

What ive tried: Starting variable at class level. Getting rid of any other references to it, including commenting out where it was set to 0

What am I missing?

C#:

public partial class SiteMaster : MasterPage
{
        private int professorIndex;
        protected void Page_Load(object sender, EventArgs e) {
        //some stuff
        }
        
        protected void cbUpdateAvailability_Click(object sender, EventArgs e)
        {
        CheckBox cbSender = (CheckBox)sender;
        professorIndex = getProfessorIndexCB(cbSender.ClientID);
        //at this point, professorIndex is 1, which is what I want/expect
        }
        
        
        public int getProviderIndexCB(string cbSender)
        {
            //professorIndex = 0;
            switch (cbSender)
            {
                case "chkOnOff1":
                    professorIndex = 0;
                    break;
                case "chkOnOff2":
                    professorIndex = 1;  //This is the one that is triggered
                    break;
            }
            return professorIndex;
        }
        
        
        protected void btnUpdateAvailability_Click(object sender, EventArgs e)
        {
        //at this point, professorIndex is 0, no clue why. It should be one
        }

Answers:

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Method 1

Below a simple demo to store a value you want to persist in a Session and get it back on PostBack.

public int professorIndex = 0;

protected void Page_Load(object sender, EventArgs e)
{
    //check for postback
    if (!IsPostBack)
    {
        //set the index
        professorIndex = 10;

        //store the index in a session
        Session["professorIndex"] = professorIndex;
    }
}


protected void Button1_Click(object sender, EventArgs e)
{
    //check if the session exists
    if (Session["professorIndex"] != null)
    {
        //cast the session back to an int
        professorIndex = (int)Session["professorIndex"];
    }

    //show result
    Label1.Text = $"The professor index is {professorIndex}.";
}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0