How to destroy elements (or shorten length) of list array in C#

Question

I have the following list:

List<MyOwnList> mylist = new List<MyOwnList>(); mylist[0].Name = "Name0"; mylist[0].Class = "Class0"; mylist[1].Name = "Name1"; mylist[1].Class = "Class1"; mylist[2].Name = "Name2"; mylist[2].Class = "Class2"; mylist[3].Name = "Name3"; mylist[3].Class = "Class3"; mylist[4].Name = "Name4"; mylist[4].Class = "Class4";

I want to shorten the length or let's say destroy elements from position 3 and 4. I used the following code but it still prints "5" when I do mylist.Count

for(int i=0; i<mylist.Count; i++) { if(i>2) { mylist[i] = null; } }

I expect it to print "3" when I do mylist.Count

Zbigniew · Accepted Answer · 2014-01-19 12:13:01Z

7

When you do this:

mylist[i] = null;

you're accually setting ith element to null, so you won't change size of your list. Basicaly you will have null there:

// true bool elementIsNull = mylist[i] == null;

Use RemoveRange method:

// remove 2 elements starting at element with index 3 mylist.RemoveRange(3, 2);

edited Jan 19, 2014 at 12:13

answered Jan 19, 2014 at 11:53

Zbigniew

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4 Comments

Frank Martin Over a year ago

I tried mylist = mylist.RemoveRange(3,2) but it shows error "Cannot implicitly convert type 'void' to System.Collections.List<MyOwnList>

Fabio Marcolini Over a year ago

@FrankMartin this means that RemoveRange doesn't return a list but modify the list it's called on.

Zbigniew Over a year ago

It's because RemoveRange method doesn't return value (void), simply call it like this mylist.RemoveAt(3, 2); and you'r list will be modified.

Fabio Marcolini Over a year ago

You have a typo: in your code you used RemoveAt instead of RemoveRange

Mzf · Accepted Answer · 2014-01-19 11:55:14Z

it's depend how you define "destroy"

if you want to remove the element from the list and "destroy" the cell to reduce the list list you can use the RemoveAt - BUT it would shorten the list

if you want to "destroy" the element , GC will take care of that , as long no one is holding reference to this element

Avi Turner · Accepted Answer · 2014-01-19 12:05:20Z

It really depends what you mean by Destroy.

If MyOwnList implements IDisposable, You would use:

int startIndex; int numberOfItemsToRemove; mylist.GetRange(startIndex, numberOfItemsToRemove).ForEach(m => m.Dispose()); mylist.RemoveRange(startIndex, numberOfItemsToRemove);

Otherwise:

int startIndex; int numberOfItemsToRemove; mylist.RemoveRange(startIndex, numberOfItemsToRemove);

CodeMonkeyNo42 · Accepted Answer · 2014-01-19 12:13:23Z

You have to delete the items in reverse to avoid the indexoutofrange exception and please don't use a property of a list which is going to change in the condition of a loop

try this:

 List<MyOwnList> mylist = new List<MyOwnList>(); mylist.Add(new MyOwnList { Name = "Name0", Class = "Class0" }); mylist.Add(new MyOwnList { Name = "Name1", Class = "Class1" }); mylist.Add(new MyOwnList { Name = "Name2", Class = "Class2" }); mylist.Add(new MyOwnList { Name = "Name3", Class = "Class3" }); mylist.Add(new MyOwnList { Name = "Name4", Class = "Class4" }); mylist[0].Name = "Name0"; mylist[0].Class = "Class0"; mylist[1].Name = "Name1"; mylist[1].Class = "Class1"; mylist[2].Name = "Name2"; mylist[2].Class = "Class2"; mylist[3].Name = "Name3"; mylist[3].Class = "Class3"; mylist[4].Name = "Name4"; mylist[4].Class = "Class4"; int count = mylist.Count; for (int i = count - 1; i >= 0; i--) { if (i > 2) { mylist.RemoveAt(i); //mylist[i] = null; } } Console.WriteLine(mylist.Count);

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