7 replies to this topic

[Instigator]

For some strange reason I can't remove an item from a list while iterating the LIST. NOTE: lines with, '*' are the key parts causing this error.

foreach (Employee ae in lstEmployees) // *I get an error here. Is this because of 'lstEmployees.Remove(ae);' below??
             {
                if (ae.getName() == txtName.Text)
                {
                    found = true;
                    lstEmployees.Remove(ae);//*seems to work, but as soon as the loop iteration is done it crashes on the above 'for each'
                    // if the name from the Textbox is in the LIST<>, then remove it and clear all data
                
                    if (MessageBox.Show("Delete? \n", "Confirm Delete", MessageBoxButtons.OKCancel) == DialogResult.OK)
                    {
                        lstEmployees.Remove(ae);
                        MessageBox.Show("Item Removed from list. *comment out later", "Confirm Delete", MessageBoxButtons.OK);
                        
                    }
               }

Quote:

An unhandled exception of type 'System.InvalidOperationException'

Any help would be appreciated!

[Moe]

Generally you can't use a foreach list to alter the collection of things that the foreach loop is looping through. If you could, it would result in some pretty funky behaviour. There are a few ways around this. I thought I had started a thread like this a while ago. I'll see if I can dredge up a link.

[Moe]

A yes, here it is: Using a foreach loop to modify things?

[Instigator]

Okay so that explains to me a lot. Except.. if I can't use 'for each' to delete an object in that list.. what should I be using?

For example, if I'm removing the employee in the 'lstEmployees LIST' where 'employee.getName() = txtBox.text' ..

What's the best solution to this? Can I use a for each to create an index of where 'Employee.getName() = txtBox.text' and then just delete that index outside of the for each?

[Moe]

Yeah, something like that. Either that or use a for loop, but being careful to not screw up your indices as you loop through.

[LordShade]

If you think about it, you shouldn't be using a foreach loop if you're going to modify the collection you're iterating through. Honestly I think this was a stupid language "feature" that was more of a nod to the VB developers than to others.

Does a for each construct allow you to iterate through the collection faster than a simple "for" loop? what about a do...while? A while...?

I am working at a company that the main developer made a comment about the do/while and while loop not being a construct in C#. Was fun to pull out the book and point to it...

for each loops should ONLY BE USED WHEN *NOT* MODIFYING a collection of objects. Don't use it if you plan of wacking an item from the collection. The underlying iteration mechanism may change and any "hacks" you perform may fail later.

Happy coding.

[mpipe]

hmm

use foreach and add whatever you want to delete to a temporary list. loop through that list using a for loop and call MasterList.Remove(ToBeDelete[i]) on each iteration.

[benryves]

In C#, there are properties so you shouldn't write get/set methods. Assuming your employee class currently looks like this:

class Employee {
	private string name;
	public string getName() { return this.name; }
	public string setName(string name) { this.name = value; }
}

...you could do this instead:

// C#2:
class Employee {
	private string name;
	public string Name {
		get { return this.name; }
		set { this.name = value; }
	}
}
// C#3: (If you don't do any error checking on Name)
class Employee {
	public string Name { get; set; }
}

To the question in hand, you can do something a little more elegant (hard-coded to delete John here):

// C#2:
Employees.RemoveAll(delegate(Employee E) { return E.Name == "John" && MessageBox.Show("Delete?", "Confirm", MessageBoxButtons.YesNo) == DialogResult.Yes; });
// C#3:
Employees.RemoveAll(E => E.Name == "John" && MessageBox.Show("Delete?", "Confirm", MessageBoxButtons.YesNo) == DialogResult.Yes);