Object reference not set to an instance of an object

I am coding an application that can store information in a listbox and save it to text file.

I am trying to code a right click context menu for the listbox. I have successfully implemented a messagebox that comes up when the user right clicks the listbox and it contains no data....but I noticed when testing...if you click OK on the messageboard and quickly right click the listbox again...the application crashes with an "Object reference not set to an instance of an object" message.

If I could just code it to do nothing when right clicked and also have a working context menu that would be great....here is the code below:



private ContextMenuStrip listboxContextMenu;


private void Form1_Load(object sender, EventArgs e)
{
listboxContextMenu = new ContextMenuStrip();
listboxContextMenu.Opening += new CancelEventHandler(listboxContextMenu_Opening);
listBox1.ContextMenuStrip = listboxContextMenu;

}



private void listboxContextMenu_Opening(object sender, CancelEventArgs e)
{
//Clear the menu and add custom items
listboxContextMenu.Items.Clear();
listboxContextMenu.Items.Add(string.Format("Edit - {0}", listBox1.SelectedItem.ToString()));
}




private void listBox1_MouseDown(object sender, MouseEventArgs e)
{
if (e.Button == MouseButtons.Right)
{
//Select the item under the mouse pointer
listBox1.SelectedIndex = listBox1.IndexFromPoint(e.Location);
if (listBox1.SelectedIndex != -1)
{
listboxContextMenu.Show();
}
else
{
MessageBox.Show("Add an item to the list first.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

}

}

Quote:

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Originally Posted by novamike

listboxContextMenu.Items.Add(string.Format("Edit - {0}",

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Also, it highlights this line upon the crash.

ever heard of CODE tags?

here's the answer to you qiuz ;) probably this is failing

Code:

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listboxContextMenu.Items.Add(string.Format("Edit - {0}", listBox1.SelectedItem.ToString()));

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because the SelectedItem is null and what is null cannot be converted to string, nothing cannot be turned into something

Yeah, you need to check if an item is selected in the first place.

Quote:

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Originally Posted by memeloo

because the SelectedItem is null and what is null cannot be converted to string, nothing cannot be turned into something

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Well, that really just depends on the implementation. For example, the nil object in ruby does inherit from Object and this is perfectly valid code:

Code:

---

puts nil.to_s
=>""

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Quote:

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Originally Posted by BigEd781

Well, that really just depends on the implementation. For example, the nil object in ruby does inherit from Object and this is perfectly valid code:

Code:

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puts nil.to_s
=>""

---

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sure, but this is c# forum so I assume the OP is talking about c# :cool:

ruby hehe, its syntax is so funny

I know, I was commenting on the conceptual nature of your comment and just noting that really it is an implementation detail. What do you want? I'm a stickler for accuracy :P

BTW, I love ruby, it's a really fun language.

So..how do I code around it since it is null?

Do I need an if statement?

Since the only thing the context menu does is allow you to edit the current selection, just don't show it at all if SelectedItem == null. That said... you should probably just select the item under the mouse on a right click. This is better overall, solved the null item problem (unless the list is empty/they did not click an item), and gives the user a better experience.

Quote:

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Originally Posted by BigEd781

Since the only thing the context menu does is allow you to edit the current selection, just don't show it at all if SelectedItem == null. That said... you should probably just select the item under the mouse on a right click. This is better overall, solved the null item problem (unless the list is empty/they did not click an item), and gives the user a better experience.

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Yes but I am having trouble developing the statement to if and else for that...

Quote:

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Originally Posted by novamike

Yes but I am having trouble developing the statement to if and else for that...

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ok, here is a book more helpful then us. you seem not to know the basics :sick:

Quote:

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Originally Posted by memeloo

ok, here is a book more helpful then us. you seem not to know the basics :sick:

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wow, helpful response. why even type that if you aren't trying to help? maybe i study the syntax and learn off that? if you can't help, move on to the next **** topic

Quote:

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Originally Posted by novamike

wow, helpful response. why even type that if you aren't trying to help?

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I already helped you enough but

Quote:

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Originally Posted by novamike

maybe i study the syntax and learn off that?

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you don't even know how to check if a variable is null and I'm not going to teach you that becasue it's in every book and you are just too lazy to read one

Quote:

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Originally Posted by novamike

if you can't help,

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the question is not if I can help but whether I do it

Quote:

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Originally Posted by novamike

move on to the next **** topic

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however after this words I certainly won't

But memeloo, the OP obviously doesn't want to learn, just give him teh codez!

thx douchebags

Right. All I can say is that I hope you never interview for a job where I work.

Teach a man to fish and you feed him for a lifetime. Unless he doesn't like sushi—then you also have to teach him to cook.


:)