System.NullReferenceException Handled

Question

Here's code I have written for a simple event based adder, this is the error I constantly get: Object reference not set to an instance of an object.

I am a beginner, so what's my error here? What am I missing? Thanks.

private void txtTwo_TextChanged(object sender, TextChangedEventArgs e)
{
            int numberOne, numberTwo, number3;
            if (int.TryParse(txtOne.Text, out numberOne))
            {
                // DO NOTHING
            }
            else
            {
                MessageBoxButton buttons = MessageBoxButton.OK;
                MessageBoxImage icon = MessageBoxImage.Error;
                MessageBox.Show("Not An Integer! Only Integers Allowed !", "Error : First Number", buttons, icon);
                txtOne.Clear();
            }
            if (int.TryParse(txtTwo.Text, out numberTwo))
            {
                //DO NOTHING
            }
            else
            {
                MessageBoxButton buttons2 = MessageBoxButton.OK;
                MessageBoxImage icon2 = MessageBoxImage.Error;
                MessageBox.Show("Not An Integer! Only Integers Allowed !", "Error : Second Number", buttons2, icon2);
                txtTwo.Clear();
            }

            number3 = numberOne + numberTwo;
            string num3 = number3.ToString();
            txtOut.Text = num3;
}

last line : txtOut.Text = num3; And I am pretty puzzled why it throws the exception. — Karma.Guy, May 01 '13 at 09:58
@Karma.Guy check whether `num3` has any values or not. If the `num3` is null then that is the problem — RajeshKdev, May 01 '13 at 09:59
I think num3 cannot be null since number3 will always have a default non null value. More like txtOut is not instantiated. Is txOut visible on the Windows Form? — Ganesh R., May 01 '13 at 10:00
Yes, It's a WPF Interface and pretty visible all coded normally. — Karma.Guy, May 01 '13 at 10:02
Almost all cases of `NullReferenceException` are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, May 01 '13 at 10:02
@JohnSaunders But in my code everything is pretty initialized. Where is the problem then? — Karma.Guy, May 01 '13 at 10:07
@JohnSaunders So time too learn. Let me recheck the code ! Although i am not sure if I will be able to find the mistake :( — Karma.Guy, May 01 '13 at 10:10
Use the debugger. Set a breakpoint on or before that line and see what the values are. — John Saunders, May 01 '13 at 10:11
@JohnSaunders yup, I noticed num3 is null ! But how do I remove that, I tried to initialize it but no effect ! Tried this :textOutput.Text = number3.ToString(); No effect ! — Karma.Guy, May 01 '13 at 10:18
@JohnSaunders removed num3 totally ! used this: textOutput.Text = number3.ToString(); same exception ! — Karma.Guy, May 01 '13 at 10:21
Try this: `if (textOutput == null) throw new Exception("Guess what?");` — John Saunders, May 01 '13 at 10:25
@JohnSaunders so textOutput is null again ! what do I do now? Tried initializing it to 0 on the WPF form.. still "Guess What" :) — Karma.Guy, May 01 '13 at 10:32

score 3 · Answer 1 · answered May 01 '13 at 10:06

3

In this case it must be txtOut that is null, because num3 has been initialised. Try renaming your txtOut control in the designer or deleting and and recreating it.

answered May 01 '13 at 10:06

Dave Bending

151
2
6

score 0 · Accepted Answer · answered May 01 '13 at 11:20

So finally got it figured.

Well, num3 can't be null because it's a value type, that means that txt3 is null. txtOut is null because the TextChanged event is raised when txtTwo's Text property is set in XAML which likely happens before the txtOut TextBox is created.

So the solution is to remove the TextChanged event from XAML and put it in the constructor, after InitializeComponent:

public MainWindow() {
    InitializeComponent();
    txtTwo.TextChanged += txtTwo_TextChanged;
}

System.NullReferenceException Handled

2 Answers2