Question

I have the following global List –

 public partial class About : System.Web.UI.Page
 {

    List<string> changes = null;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            List<string> changes = new List<string>();
        }

I am trying to add string values to changes like so –

 protected void CheckBox1_CheckedChanged(Object sender, EventArgs e)
    {

        CheckBox chk = (CheckBox)sender;
        DataListItem item = (DataListItem)chk.NamingContainer;

        TextBox txt = (TextBox)DataList1.Items[item.ItemIndex].FindControl("aliasTextBox");
        string text = txt.Text;


        changes.Add(text);

        ViewState["array"] = changes;

    }

So I am trying to store all changes made in the dataList into changes so that when a user clicks –

protected void Button5_Click(object sender, EventArgs e)
    {
        List<string> changes = (List<string>)ViewState["array"];

        foreach (string text in changes)
        {

            WebService1 ws = new WebService1();
            ws.WebMethod(text);
        }
    }

However when it comes to clicking the button, I get a null reference exception on the – changes.add(text) section. How can I store all the values in changes to be available on button click?

Answer 1

The changes that you are making to a variable will not persist after the postback. You should save the value of this list ‘changes’ to a viewstate, session or application so that you can retain the values of the list even after the postback.

Add this code in the CheckBox1_CheckedChanged event after updating the list

ViewState["Somename"]=changes;

Add this code in the Button5_Click before accessing the list

changes= (List<string> )ViewState["Somename"]