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!

Icarus · Accepted Answer · 2011-11-04 03:58:55Z

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

answered Nov 4, 2011 at 3:58

Icarus

64k14 gold badges102 silver badges116 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user877852 Over a year ago

Hi @icarus, i removed the ToSrting part, my lb.SelectedValue is null, even though i am clicking on an item. :( Thanks!

Icarus Over a year ago

@user877852: See? that was the issue! mouse_down is not the method you should be handling. At that point nothing has been selected yet. You probably need to handle OnSelectedIndexChanged or something similar. I am not a WPF developer so I can't be 100% sure.

user877852 Over a year ago

Yeah i figured that its empty. Okay no problem, thanks for the help :)

Code Magician · Accepted Answer · 2011-11-04 04:07:27Z

1

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

answered Nov 4, 2011 at 4:07

Code Magician

24.1k7 gold badges63 silver badges77 bronze badges

4 Comments

user877852 Over a year ago

Hi, i dont seem to have an event like that :(

Code Magician Over a year ago

I've always coded it like this <ListBox SelectionChanged="ListBox_SelectionChanged"...>

user877852 Over a year ago

Hi, i put it in the selection changed event, and its working now! o.O thanks!

Code Magician Over a year ago

Awesome! Glad I could assist.

Collectives™ on Stack Overflow

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

2 Answers 2

3 Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related