private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
// ComboBox take data from a BindingSourcethis.TextBox1.Text = this.ComboBox1.SelectedValue.ToString();
}
Why this code generate an exception NullReferenceException when closing the MainForm
if I make this..
this.ComboBox1.SelectedIndexChanged -= new System.EventHandler(this.ComboBox1_SelectedIndexChanged);
the exception is not generated but I haven't the event that I need !!!
Can anyone help me Thank all.
exception on eventhandler SelectedIndexChanged
Harry_Leboeuf
using this code I've this text in the TextBox:
System.Data.DataRowView
I've also changed the property SelectedItem = "ID"; but it's the same... ! why this
thanks bye
Mike Haro
Yes I've see the documentation.
how I can Can you post a little code example
Thanks a lot.
Bit Bucket
OK, so the next thing to do is check what thing is actually causing the exception. From the code snippet you've shown, I can see 4 places that might be causing it, although only 3 of those are possible in practice.
The first is that comboBox1 might be null, in which case your 'if' statement test will throw the exception.
The second is that this.TextBox1 might be null, in which case trying to set its Text property will throw the exception.
The third is that if comboBox1 were null, accessing its SelectedValue property would throw the exception. (It can't be this one though, because you'd already have hit the first case I describe here if comboBox1 is null.)
The fourth is the SelectedValue is null, in which case the call to ToString would throw the exception.
The first thing to do is find out which of these is causing the exception. Use the debugger to inspect these variables and find out which is null.
Once you've done that, we can go about finding out why it is null.
MMDG
..also apply that code the exception is generated
milicica
This works
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e){
TextBox text = new TextBox();text.Text = comboBox1.SelectedItem.ToString();
}
Kinlan
Hi..
Normally, the ComboBox1_SelectedIndexChanged event won't occur after Form closing event.
Did you do anything in the Form closing event or closed event. (e.g. change DataSource property .) If you do this, SelectedIndexChanged event will occur. And If your data source is null or there is no value, ComboBox1.SelectedValue will be null. So the exception is thrown.
Anyway., you can prevent like this.
if
(comboBox1.SelectedIndex != -1)this.textBox1.Text = comboBox1.SelectedValue.ToString();
Carver42
The documentation you say you've read contains an example.
It checks the selected index first. If it's -1 that means nothing is selected.
akin_l
I would expect the reason is that there is no selected item when the form closes.
SelectedValue does two things. First, it retrieves the currently selected item. And then it reads the property specified in ValueMember from the selected item.
So if there is no selected item, SelectedValue essentially means "read a property from an item that isn't there". So a NullReferenceException actually makes perfect sense. There is no selected item, so any attempt to read a property from it has the same effect as you'd get any time you tried to use an object that isn't there.
If you look at the documentation for ComboBox.SelectedValue, the code snippet they show checks the SelectedIndex property first to see if there's anything selected. So in general you should do this - check to see SelectedIndex indicates that there really is a selected item before you use SelectedValue.