Object reference not set to an instance of an object c#

Question

I have a listbox and i'm trying to select an item to display a label. My code is as follows:

private void listBox2_MouseDown(object sender, MouseButtonEventArgs e)
    {

        ListBox lb = (ListBox)sender;

        var selected = lb.SelectedValue.ToString();
        //string selected = listBox2.SelectedItem.ToString();

        label5.Visibility = Visibility.Visible;

        if (selected.ToString() == "Study Date")
        {
            label5.Content = "Format:YYYYMMDD";

        }

        if (selected.ToString() == "Patient's Name") label5.Content = "Enter name in string format.";
     }

But when i click on an item, i get an error as: Object reference not set to instance of an object . I cannot enter the code in the Selection changed event, so please tell me how I can go about this. Thanks!

Answer 1

You have a potential issue here:

var selected = lb.SelectedValue.ToString();

You are calling ToString() even though SelectedValue can be null

Before you call ToString() make sure SelectedValue is not null

Answer 2

IIRC the MouseDown() event fires before the selection is registered. Wouldn't you be better off using the SelectionChanged() event?