❔ Need help with Event Handlers to update a field.
I am making a unit conversion program in C# wpf as my first C# application. I have created a method that calculates the conversion from for example mm to m. It has 3 inputs, Input Unit - Output Unit - Input Value. Calculates the Output Value.
I want to update the Output Value on 3 different events, InputWaarde_TextChanged or InputEenheid_SelectionChanged or OutputEenheid_SelectionChanged.
But when I have the method in those event handlers I get the following error when I start the program and select a unit, both input or output or when I fill in a value.
Anyone has an idea on how to solve it? And is anyone willing to look at the rest of the code to give feedback? I think I can use some haha!
110 Replies
the error is pretty self-explanatory
SelectedItem.get returned null
I.E. there is no selected item
what do you expect your program to do when there is no selected item in those selectors?
decide that, then write the code to do it
Was this issue resolved? If so, run
/close - otherwise I will mark this as stale and this post will be archived until there is new activity.Thanks for your response! Sorry for my late answer I got a kindof hectic week.
By default nothing is selected, that is true now I understand that now. But how can I set a default value in a combobox? I am searching on google, but I tried a few ways that did not work.
Then It should be solved, right? If I make that it can never return null anymore
if you want a "default" value selected, then set one
you're the one in charge of creating the combobox, you have full control over its state upon creation
all of that is really irrelevant to the error
what do you expect your app to do when nothing is selected?
Just nothing, because it doesn't know what to do
But it's ok if there is always a default state selected.
then that's what you need to code for
that's not what "default state" means
setting a default selection of not nothing doesn't stop the user from selecting nothing
It is, because there is no reason and possablity to select nothing
It only starts as "null" untill you choose an option and then you can never get a "null" again
I would say this isn't a combobox, then
this is the WPF ComboBox class?
Yes
I guess I'm not familiar enough with it
if it's legitimately impossible for the user to select nothing within the control, then yeah, setting a default value should work
Indeed, but I am now watching the next youtube video maybe I get it working now. I hope
I have the default selected index now! But now I get the same error that SelectedItem.get returned null.
So I set it in a try catch and with an if( .... != null) then ..........
it's either being called before you set the index, or it doesn't work that way, and you should set the item directly
But still error, and when I press F5 again, I get my UI screen and it works fine
absolutely not
?
do not try/catch the exception
fix the problem so that the exception never occurs
I set the index directly to index [0] at the ComboBox properties
That is indeed a good idea haha
huh?
If you place a combobox you get it's properties on the right side, there I set the default index to 0 instead of -1
stop using the visual designer immediately
and I'm going to guess what's happening is that setting accomplishes nothing because it happens on construction
when that property is set, there IS no item at index 0. The items get added later on, when your business logic populates them, at which point the index setting of 0 either has already been wiped out, because it was invalid to begin with, or it just doesn't trigger a re-calculation of the SelectedItem property
It does actually select a default value now
But I understand what you mean. Uuh oke but how would you take on this problem then? I really have no idea what I can try anymore
Or what I need to change
stop beating around the bush
if you want SelectedItem to not be null, then SET IT.
I know that I need to set it, but where or how
I found a way of setting it by changing the selectedIndex to 0
did you not say specifically that that does NOT set SelectedItem?
how about the same place and way that you're setting Items?
It does set it
disagree
In the MainWindow
Eenheden is the array of values
sweet
so, if DataContext is the method by which you're passing data down to child controls
`
This is part of the XAML code for the combobox
the data that you need to pass down now consists of both an array of values and an initial selection
Yea indeed, so the array of values is done. But now the initial selection
indeed
This?
This works^, but now in the InputEenheid_SelectionChanged it get's stuck
Found the problem! I have a method that requires 3 inputs as string. The InputUnit, the OutputUnit and the Value.
Now I placed:
and it works.
But when I change that "m" to OutputEenheid.SelectedItem.ToString() and keep the rest the same it breaks. So it only get's stuck on this:
But it is exactly the same as InputEenheid.SelectedItem.ToString() so idk why it doesn't understand it
And that same method is also called in another section in InputWaarde_TextChanged and there it works fine
Wait, can a method be called 2 times in different moments with the same parameters?
uhmmm..... yes?
Can't I "run" the method:
in an if statement, placed in the MainWindow that is only run if InputWaarde_TextChanged or InputEenheid_SelectionChanged or OutputEenheid_SelectionChanged is true?
I really haven't the slightest clue where we've gone here
what does any of this have to do with setting SelectedItem?
That block of code above is the only thing I want to execute on the change of one of 3 events.
Events are:
private void InputWaarde_TextChanged(object sender, TextChangedEventArgs e)
private void InputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
private void OutputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
okay
I.E.
you have logic you want to run any time any of the 3 inputs changes
regardless of which one changed
Exactly
Or the Input Unit changed, or the Output unit changed, or the value changed
then update Output value
And those changed events are the ones I sent above
okay
and what's the problem with that?
I can't get it to work
why?
I know how I want it to work, but it doesn't
what's not working about it?
I can't place InputWaarde_TextChanged into an if statement, because it isn't a bool it's an, correct me if I am wrong, object? I think
it is a method
method, indeed
which is, in fact, not a bool
why do you think you want to put that into an if statement?
if( x or y or z)
then execute the method
what are the conditions a, b, and c?
the events
exactly
so, not conditions
an event is something that OCCURS
not something you can CHECK
Ooh, so an event is not like a bool that is true or false?
put differently...
the CHECK for whether an event is occurring or not has already occurred
that decision has already been made
Ooh like this
the TextBox has already determined that the text within itself has in fact changed, and thus it's raising the event
it's not on you to make a decision about whether the event is occurring, you know it's occurring, it's up to you ti write code to DO something in response
that's what your methods are, they're event handlers
If you say it, it's actually prety logical
that get attached to the events on the silource component
So do I need to place my method:
Inside those 3 event handlers?
correct
But that did not work, then I got the errors. That was what I first got
Let me try again
what errors?
what's null?
so, we're back to you needing to set SelectedItem
I found that if I remove
From event handler InputEenheid_SelectionChanged it works
correct, then you are no longer attempting to call
.ToString() upon a value that may be nullBut I do the same thing in the 2 other events
okay
if SelectedItem can be null, then they are subject to the same issue
Hmm
Uuh, so it thinks it can be null or is it null in that event case?
I got it now! I thinK!
the value of SelectedItem can be null
as evidenced by the fact that it is
when the error occurs
what "it" are you asking about whether it "thinks" the value can be null?
So it first updates the default value of InputEenheid, then it calls the event InputEenheid_Selection changed. That calls my method that finds out that OutputEenheid.SelectenItem = null!
that sounds reasonable
How can we solve this?
Wow this is something I didn't think of earlier
particularly if all of that is happening BEFORE you've set items into the control
SelectedItem is (perhaps) null because you've told it to select index 0, but there IS no item at index 0, because Items is empty, because the logic that adds items has yet to run
you have 2 possible solutions
A) account for the possibility that SelectedItem can be null, and decide what you want to do in that scenario, then code for that
B) prevent SelectedItem from being null
This is my code if that helps
not really
How can I do B, prevent SelectedItem from being null?
depends
you can set it
or you can try and understand why it's null now
and code around whatever scenario is causing that
I do understand why it's null now. But I can't get around that. Because for me it's selecting the index of both input and output at the same time. But the code first does
Now triggers the event handler which sees that OutputEenheid is still null because index is at -1. Gives an error for that
And then
what is your understanding of why it's null?
are you referring to my explanation a little bit ago?
I only proposed that as a possibility
No
I don't know if that's accurate, but it does make sense
for clarity then, what is your understanding?
oh, nevermind, I see
It makes sense, because when I remove the OutputEenheid.SelectedItem.ToString() from the first event handler for InputEenheid_SelectionChanged then I don't have any errors because it is able to set both InputEenheid and OutputEenheid to != null
so, yeah, because of the interdependencies between these inputs, and the nature of how ComboBox works, I would say option B) is impossible
So for option A then, act on the possibility that it can happen. Uuuh how
Place an if in my method that only executes code if (OutputEenheid.SelectenItem != null) ?
that would do it
if we assume that the nulls can only occur during construction, and never because the user selected it, then "do nothing" is an acceptable answer for "what do I want the app to do when nothing is selected"
Uh nope, because my method expects a string and I can't convert null to string and then check if != null
uhm
no
just no
you do not "convert" null to string
I do, look:
yes
And OutputEenheid.SelectedItem returns null
you are invoking
.ToString() on SelectedItem when it can be null
don't do that
you just said you want to check for null
do thatSo place the .ToString inside the Method?
place it wherever you like
just do a null check first
But
It can't be here, right?
what can't?
the null check?
yes
why not?
Because the .ToString is connected to a parameter inthe method
no, ToString() is a method
upon all objects
which is why you're using it to convert your SelectedItem values back to string
which is one of many ways you could be doing that
that's mostly unrelated to the fact that the business method you want to call requires 3 string parameters
which, at the moment, should not be null
Got it
No error's anymore, Thanks man!
😉
Short awnser, I start to like C# a bit more again haha
Was this issue resolved? If so, run
/close - otherwise I will mark this as stale and this post will be archived until there is new activity.