In the inventory for my game I want to delete an item if you drop it. This only works in a foreach loop like this:
public List<Item> items = new List<Item>();
public void DeleteItem(Item itemToRemove)
{
foreach (var item in items)
{
items.Remove(itemToRemove);
}
}
but then I get this error:
Collection was modified; enumeration operation may not execute.
When I just try this:
public void DeleteItem(Item itemToRemove)
{
items.Remove(itemToRemove);
}
nothing happens. Does anyone know how to fix this?
To remove an item from a list, it is sufficient to do this:
public void DeleteItem(Item itemToRemove)
{
items.Remove(itemToRemove);
}
You don't actually have to loop through the list. If nothing seems to happen, check the return value of Remove. It returns true if the item was removed, or false if the item couldn't be found in the list.
Regarding the error message you're getting: you must not modify a list by adding/removing items while you're enumerating through it. This will change the length of the list, and throws off the enumeration, so it is not allowed.
That error indicates that you are changing a collection at the same time as you are looping over it. This is unsafe because once the collection changes the iterator state may no longer be valid. For example, the iterator might be pointing to index 3 in a list, but if you remove item 0 (shifting everything else back) then you’d end up skipping ahead. Throwing this error avoids this sort of subtle bug by making it obvious.
In this particular case, you shouldn’t need a loop: just remove the item.
If you want to do bulk removal, consider methods like RemoveAll.
Another option is to loop over a copy of the list, which leaves you free to modify it. For example, you could loop over items.ToArray().
There was another script attached which directly added the item back if I dropped the gun, so that was the error.
Related
Suppose we have a List<string> assetIds = GetAllAssetId(assetentities); called assetIds. Now suppose we have a list with 10 AssetIds in it. If you wanted to add the new AssetItems in a list of string and I need to update the list while doing the foreach loop. what would be the best way to do it?
Please have a look of the code here.
I am getting an Error :Collection was modified; enumeration operation may not execute. If I try to go for second time in foreach loop.
Yout change the collection in foreach loop so what you could do - make a copy of the collection in loop
foreach(var element in assetIds.ToList()) {
assetIds.Add(new Item());
)
collection that the foreach loop operates on is different that the one you add to
This is one way to get rid of the error. In your case I assume that there won't be any cases where the newly added item would be used twice.
Edit:
But again think if this is the solution that you want to go for. Maybe rewriting your code is better option
I have a below code
public bool Notify(bool filterStatus)
{
foreach (IFilterStatusListener listener in listeners)
{
listener.Update(filterStatus);
}
return true;
}
Method Update recursively calls Notify for Children
public void Update(bool status)
{
this.isFilteredFixed = false;
this.Notify(status);
this.RaisePropertyChanged("IsFiltered");
}
When I run this code, I get below error
What should I do ?
When you iterate a collection with a foreach statement you can't change the collection you're iterating on, because the iterator gets lost on which position it actually is.
There are several solutions to this:
You iterate with a for loop
You create a copy of the collection you're iterating on and work on that.
foreach expects that collection stays unchanged. You may try a normal for loop instead.
See MSDN:
The foreach statement is used to iterate through the collection to get the information that you want, but can not be used to add or remove items from the source collection to avoid unpredictable side effects. If you need to add or remove items from the source collection, use a for loop.
does foreach correctly iterate over flexible list?
for example
//will iterate over all items in list?
foreach (var obj in list)
{
//list length changes here
//ex:
list.Add(...);
list.Remove(...);
list.Concat(...);
// and so on
}
and if it does ...how?
You can't modify a collection while enumerating it inside a foreach statement.
You should use another pattern to do what you are trying to do because the for each does not allow you to change the enumerator you are looping to.
For Example:
Imagine if you run a foreach on a sorted list from the beginning, you start processing item with key="A" then you go to "B" then you change "C" to "B", what's going to happen? Your list is resorted and you don't know anymore what you are looping and where you are.
In general you "could" do it with a for(int i=dictionary.count-1; i>=0; --i) or something like that but this also depends on your context, I would really try to use another approach.
Internal Working: IEnumerator<t> is designed to enable the iterator pattern for iterating over collections of elements, rather than the length-index. IEnumerator<t> includes two members.
The first is bool MoveNext(). Using this method, we can move from one element within the collection to the next while at the same time detecting when we have enumerated through every item using the Boolean return.
The second member, a read-only property called Current, returns the element currently in process. With these two members on the collection class, it is possible to iterate over the collection simply using a while loop.
The MoveNext() method in this listing returns false when it moves past the end of the collection. This replaces the need to count elements while looping. (The last member on IEnumerator<t> , Reset(), will reset the enumeration.)
Per the documentation, if changes are made inside the loop the behavior is undefined. Undefined means that there are no restrictions on what it can do, there is no "incorrect behavior" when the behavior is undefined...crash, do what you want, send an email to your boss calling him nasty names and quiting, all equally valid. I would hope for a crash in this case, but again, whatever happens, happens and is considered "correct" according to the documentation.
You cannot change the collection inside the for each loop of the same collection.
if you want you can use for loop to change the collection length.
The collection you use in a foreach loop is immutable. As per MSDN
The foreach statement is used to iterate through the collection to get
the information that you want, but can not be used to add or remove
items from the source collection to avoid unpredictable side effects.
If you need to add or remove items from the source collection, use a
for loop.
But as per this link, it looks like this is now possible from .Net 4.0
Why is a foreach loop a read only loop? What reasons are there for this?
I'm not sure exactly what you mean by a "readonly loop" but I'm guessing that you want to know why this doesn't compile:
int[] ints = { 1, 2, 3 };
foreach (int x in ints)
{
x = 4;
}
The above code will give the following compile error:
Cannot assign to 'x' because it is a 'foreach iteration variable'
Why is this disallowed? Trying to assigning to it probably wouldn't do what you want - it wouldn't modify the contents of the original collection. This is because the variable x is not a reference to the elements in the list - it is a copy. To avoid people writing buggy code, the compiler disallows this.
I would assume it's how the iterator travels through the list.
Say you have a sorted list:
Alaska
Nebraska
Ohio
In the middle of
foreach(var s in States)
{
}
You do a States.Add("Missouri")
How do you handle that? Do you then jump to Missouri even if you're already past that index.
If, by this, you mean:
Why shouldn't I modify the collection that's being foreach'd over?
There's no surety that the items that you're getting come out in a given order, and that adding an item, or removing an item won't cause the order of items in the collection to change, or even the Enumerator to become invalid.
Imagine if you ran the following code:
var items = GetListOfTOfSomething(); // Returns 10 items
int i = 0;
foreach(vat item in items)
{
i++;
if (i == 5)
{
items.Remove(item);
}
}
As soon as you hit the loop where i is 6 (i.e. after the item is removed) anything could happen. The Enumerator might have been invalidated due to you removing an item, everything might have "shuffled up by one" in the underlying collection causing an item to take the place of the removed one, meaning you "skip" one.
If you meant "why can't I change the value that is provided on each iteration" then, if the collection you're working with contains value types, any changes you make won't be preserved as it's a value you're working with, rather than a reference.
The foreach command uses the IEnumerable interface to loop throught the collection. The interface only defined methods for stepping through a collection and get the current item, there is no methods for updating the collection.
As the interface only defines the minimal methods required to read the collecton in one direction, the interface can be implemented by a wide range of collections.
As you only access a single item at a time, the entire collection doesn't have to exist at the same time. This is for example used by LINQ expressions, where it creates the result on the fly as you read it, instead of first creating the entire result and then let you loop through it.
Not sure what you mean with read-only but I'm guessing that understanding what the foreach loop is under the hood will help. It's syntactic sugar and could also be written something like this:
IEnumerator enumerator = list.GetEnumerator();
while(enumerator.MoveNext())
{
T element = enumerator.Current;
//body goes here
}
If you change the collection (list) it's getting hard to impossible to figure out how to process the iteration.
Assigning to element (in the foreach version) could be viewed as either trying to assign to enumerator.Current which is read only or trying to change the value of the local holding a ref to enumerator.Current in which case you might as well introduce a local yourself because it no longer has anything to do with the enumerated list anymore.
foreach works with everything implementing the IEnumerable interface. In order to avoid synchronization issues, the enumerable shall never be modified while iterating on it.
The problems arise if you add or remove items in another thread while iterating: depending on where you are you might miss an item or apply your code to an extra item. This is detected by the runtime (in some cases or all???) and throws an exception:
System.InvalidOperationException was unhandled
Message="Collection was modified; enumeration operation may not execute."
foreach tries to get next item on each iteration which can cause trouble if you are modifying it from another thread at the same time.
Let's start with the following snippet:
Foreach(Record item in RecordList){
..
item = UpdateRecord(item, 5);
..
}
The UpdateRecode function changes some field of item and returns the altered object. In this case the compiler throws an exception saying that the item can not be updated in a foreach iteration.
Now the UpdateRecord method is changed so that it returns void and the snippet would look like this:
Foreach(Record item in RecordList){
..
UpdateRecord(item, 5);
..
}
In this case the item would be updated because Record is a reference type. But it makes the code unreadable.
The project I'm working on has lots of foreach-loops with the almost the same code over and over, so I would like to create methods that update parts of the records. Is there a nice way to do this? One that make the code more readable instead of trashing it further more?
If you need to update a collection, don't use an iterator pattern, like you said, its either error prone, or smells bad.
I find that using a for loop with an index a bit clearer in this situation, as its very obvious what you are trying to do that way.
The compiler is complaining that you can't update the collection, not the record. By doing item = UpdateRecord, you are reassigning the iterator variable item.
I disagree that UpdateRecord(item, 5) is in any way unreadable - but if it makes you feel better, an extension method may make it more clear that you are changing the contents of item.
static void Update(this Record item, int value) {
// do logic
}
foreach (Record item in RecordList) {
item.Update(5);
}
Do you need to update the same list? Could you return a new (updated) enumeration instead?
foreach(Record item in RecordList){
..
yield return GetUpdatedRecord(item, 5);
..
}