Scenario
I have a system that holds races, each race has a unique list of members on that race. (the list is a List< T > )
I want users to be able to remove a member (if they ARE this member) from the list of members on that race.
Problem
I'm trying to get the following code to work:
foreach (string item in hillracing.searchRaces(RaceID).RaceList) // Loop through List with foreach.
{
if (item == SelectedItem)
{
item = null;
}
}
I can't edit the variable because it is in a foreach loop, how would I achieve this another way?
You can just store it and remove it form the collection afterwards.
var toRemove = null;
foreach (string item in hillracing.searchRaces(RaceID).RaceList) // Loop through List with foreach.
{
if (item == SelectedItem)
{
toRemove = item;
break; //Can break here if you're sure there's only one SelectedItem
}
}
hillracing.searchRaces(RaceID).Racelist.Remove(toRemove);
though in this case you could also just use hillracing.searchRaces(RaceID).Racelist.Remove(SelectedItem); and you won't use the foreach loop at all.
You can't modify collection that you are looping using foreach loop. The collection used in foreach is immutable. This is by design.
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.
Using Linq you shouldn't need to loop to find the entry you want to nullify...
// Use of Single() here assumes the object definitely exists.
// Use SingleOrDefaul() if there is a chance it might not exist.
var item = hillracing.searchRaces(RaceID)
.RaceList
.Where(x => x.Item == SelectedItem).Single();
item = null;
Edit: Since you've changed the requirement to remove the item from the list, I think you'd just call the Remove method`with the found item. So the code becomes
// Use of Single() here assumes the object definitely exists.
// Use SingleOrDefaul() if there is a chance it might not exist.
var item = hillracing.searchRaces(RaceID)
.RaceList
.Where(x => x.Item == SelectedItem).Single();
hillracing.searchRaces(RaceID).RaceList.Remove(item);
You can't do that in a foreach loop. If it's an IList/IList<T> which allows random access, like an array or list, you can use a for-loop:
List<string> = hillracing.searchRaces(RaceID).RaceList;
for(int i = 0; i < list.Count; i++)
{
if(list[i] == SelectedItem)
list[i] = null;
}
So you can't add or remove items in the foreach but you also can't replace the reference. The object refers to the original value so you could modify the object(if strings werent immutable) but you can't replace the reference itself in a foreach. This is related.
Use the existing Remove()-method to search and remove the item for you:
hillracing.searchRaces(RaceID).RaceList.Remove(SelectedItem);
Related
foreach(var item in myObservalbleCollection)
{
//samoe coding
}
Above code will loop the items in the observable class from the beginning. I want to loop the this object from the last Item
ex 9,8,7,6,5,4,3,2,1,0 instead of 1,2,3,4,5,6,7,8,9...
Please give me a solution to do this in c#
Instead of using foreach you can use a for loop running from the end of the collection and to the start:
for (var i = myObservableCollection.Count - 1; i >= 0; i -= 1) {
var item = myObservableCollection[i];
// Process item
}
This is possible because ObservableCollection<T> implement IList<T> which provides both the count of the elements in the collection and indexed access to these elements in addition to the forward teration provided by IEnumerable<T> used by foreach. This is the most efficient solution.
You can also use LINQ Reverse which is available for any collection implementing IEnumerable<T>:
foreach (var item in myObservableCollection.Reverse()) {
// Process item
}
This will actually copy all the elements in the collection to a new array before iterating them in reverse order. While less efficient than the first solution it should not matter in most cases.
You could reverse the observablecollection and do the loop
var myObservalbleCollection = new ObservableCollection<YourType>(collection.Reverse());
foreach(var item in myObservalbleCollection)
{
//some coding
}
I am trying to remove object while I am iterating through Collection. But I am getting exception. How can I achieve this?
Here is my code :
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
gems.Remove(gem.Key); // I can't do this here, then How can I do?
OnGemCollected(gem.Value, Player);
}
}
foreach is designed for iterating over a collection without modifing it.
To remove items from a collection while iterating over it use a for loop from the end to the start of it.
for(int i = gems.Count - 1; i >=0 ; i--)
{
gems[i].Value.Update(gameTime);
if (gems[i].Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
Gem gem = gems[i];
gems.RemoveAt(i); // Assuming it's a List<Gem>
OnGemCollected(gem.Value, Player);
}
}
If it's a dictionary<string, Gem> for example, you could iterate like this:
foreach(string s in gems.Keys.ToList())
{
if(gems[s].BoundingCircle.Intersects(Player.BoundingRectangle))
{
gems.Remove(s);
}
}
The easiest way is to do what #IV4 suggested:
foreach (var gem in gems.ToList())
The ToList() will convert the Dictionary to a list of KeyValuePair, so it will work fine.
The only time you wouldn't want to do it that way is if you have a big dictionary from which you are only removing relatively few items and you want to reduce memory use.
Only in that case would you want to use one of the following approaches:
Make a list of the keys as you find them, then have a separate loop to remove the items:
List<KeyType> keysToRemove = new List<KeyType>();
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
{
OnGemCollected(gem.Value, Player);
keysToRemove.Add(gem.Key);
}
}
foreach (var key in keysToRemove)
gems.Remove(key);
(Where KeyType is the type of key you're using. Substitute the correct type!)
Alternatively, if it is important that the gem is removed before calling OnGemCollected(), then (with key type TKey and value type TValue) do it like this:
var itemsToRemove = new List<KeyValuePair<TKey, TValue>>();
foreach (var gem in gems)
{
gem.Value.Update(gameTime);
if (gem.Value.BoundingCircle.Intersects(Player.BoundingRectangle))
itemsToRemove.Add(gem);
}
foreach (var item in itemsToRemove)
{
gems.Remove(item.Key);
OnGemCollected(item.Value, Player);
}
As the other answers say, a foreach is designed purely for iterating over a collection without modifying it as per the documenation:
The foreach statement is used to iterate through the collection to get
the desired information, but should not be used to change the contents
of the collection to avoid unpredictable side effects.
in order to do this you would need to use a for loop (storing the items of the collection you need to remove) and remove them from the collection afterwards.
However if you are using a List<T> you could do this:
lines.RemoveAll(line => line.FullfilsCertainConditions());
After going through all the answers, and being equally good. I faced a challenge where I had to modify a List and what I ended up doing worked quite well for me. So just in case anyone finds it useful. Can someone provide me feedback on how efficient it might be.
Action removeFromList;
foreach(var value in listOfValues){
if(whatever condition to remove is){
removeFromList+=()=>listOfValues.remove(value);
}
}
removeFromList?.Invoke();
removeFromList = null;
You should use the for loop instead of the foreach loop. Please refer here
Collections support foreach statement using Enumarator. Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException.
Use for loop for collection modifying.
My windows phone 7 silverlight app before placing pushpins on a map layer removes any that are previously there.
I was doing this in a foreach loop as follows:
//Clear previous pins
try
{
foreach (UIElement p in PushPinLayer.Children)
{
if(p.GetType() == typeof(Pushpin))
{
PushPinLayer.Children.Remove(p);
}
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
//TODO: For some reason the foreach loop above causes an invalid Operation exception.
//Cathing the error here until I can work out why it is happening.
}
This code removes any pushpins as required but after the last loop would throw an exception "Invalid Operation" I re-wrote it as a for loop:
for (int i = 0; i < PushPinLayer.Children.Count; i++)
{
if (PushPinLayer.Children[i].GetType() == typeof(Pushpin))
{
PushPinLayer.Children.RemoveAt(i);
}
}
Which works fine, however I can't see why the foreach is throwing an error.
This is very normal,
You cannot remove items from a list which you still use in the foreach list.
better then removing the item would be to create a new list, and every time it is not a pushpin type, add the object to the new list.
This way the original list is not altered and you won't get an exception.
I find it strange that the for loop works, but if it does, it would mean that the way they are itterated is different. The for loop will be copied to another memory location and used for the for loop so that the original one, in which you remove items, is not used anymore by the for loop. The foreach loop will get parameters from the list, the you remove items, so the list and the parameters become concurrent.
Your foreach loop uses and Enumerator to iterate the objects in your collection. When you delete an object from the collection the Enumerator is no longer valid as it references object that no longer exist. This causes a InvalidOperationException
The best way to remove them is using a for loop and even better doing it in reverse.
for (int i = PushPinLayer.Children.Count - 1; i >= 0 ; i--)
{
if (PushPinLayer.Children[i].GetType() == typeof(Pushpin))
{
PushPinLayer.Children.RemoveAt(i);
}
}
This will ensure that as the items are removed your Index i does not exceed the number of items in your collection.
Since others have already answered your question, I will only comment on your usage of xxx.GetType() == typeof(Pushpin). You could check whether something is a pushpin by simply using the C# reserved keyword is. e.g.:
if (p is Pushpin) {...}
When you change the content of the collection, the enumerator used in the foreach loop becomes invalid. You can't change a collection while you enumerate it.
Here's a workaround:
List<UIElement> toRemove = new List<UIElement>();
foreach (UIElement p in PushPinLayer.Children)
{
if(p.GetType() == typeof(Pushpin))
{
toRemove.Add(p);
}
}
foreach(UIElement p in toRemove)
{
PushPinLayer.Children.Remove(p);
}
Alternatively, you could use the RemoveAll method, which takes a predicate as a parameter:
PushPinLayer.Children.RemoveAll(p => p is Pushpin);
Others have already given you the reason for the problem so I just thought I'd post a LINQ version of a work around
var toRemove = PushPinLayer.Children.OfType<Pushpin>().ToList();
// since toRemove is a separate collection, it's safe to do this now:
foreach (var child in toRemove)
PushPinLayer.Children.Remove(child)
You must not change the collection while foreaching through it.
I have a loop that iterates through elements in a list. I am required to remove elements from this list within the loop based on certain conditions. When I try to do this in C#, I get an exception. apparently, it is not allowed to remove elements from the list which is being iterated through. The problem was observed with a foreach loop. Is there any standard way to get around this problem?
Note : One solution I could think of is to create a copy of the list solely for iteration purpose and to remove elements from the original list within the loop. I am looking for a better way of dealing with this.
When using List<T> the ToArray() method helps in this scenario vastly:
List<MyClass> items = new List<MyClass>();
foreach (MyClass item in items.ToArray())
{
if (/* condition */) items.Remove(item);
}
The alternative is to use a for loop instead of a foreach, but then you have to decrement the index variable whenever you remove an element i.e.
List<MyClass> items = new List<MyClass>();
for (int i = 0; i < items.Count; i++)
{
if (/* condition */)
{
items.RemoveAt(i);
i--;
}
}
If your list is an actual List<T> then you can use the built-in RemoveAll method to delete items based on a predicate:
int numberOfItemsRemoved = yourList.RemoveAll(x => ShouldThisItemBeDeleted(x));
You could use LINQ to replace the initial list by a new list by filtering out items:
IEnumerable<Foo> initialList = FetchList();
initialList = initialList.Where(x => SomeFilteringConditionOnElement(x));
// Now initialList will be filtered according to the condition
// The filtered elements will be subject to garbage collection
This way you don't have to worry about loops.
You can use integer indexing to remove items:
List<int> xs = new List<int> { 1, 2, 3, 4 };
for (int i = 0; i < xs.Count; ++i)
{
// Remove even numbers.
if (xs[i] % 2 == 0)
{
xs.RemoveAt(i);
--i;
}
}
This can be weird to read and tough to maintain, though, especially if the logic in the loop gets any more complex.
Another trick is to loop through the list backwards.. removing an item won't affect any of the items you are going to encounter in the rest of the loop.
I'm not recommending this or anything else though. Everything you need this for can probably be done using LINQ statements to filter the list on your requirements.
You can iterate with foreach this way:
List<Customer> custList = Customer.Populate();
foreach (var cust in custList.ToList())
{
custList.Remove(cust);
}
Note: ToList on the list of variables, this iterates through the list created by the ToList but removes the items from the original list.
Hope this helps.
The recommended solution is to put all your elements you want to remove in a separate list and after the first loop, put a second loop where you iterate over the remove-list and remove those elements form the first list.
The reason you get an error is because you're using a foreach loop. If you think about how a foreach loop works this makes sense. The foreach loop calls the GetEnumerator method on the List. If you where to change the number of elements in the List, the Enumerator the foreach loop holds wouldn't have the correct number of elements. If you removed an element a null exception error would be thrown, and if you added an element the loop would miss an item.
If you like Linq and Lamda expressions I would recommend Darin Dimitrov solution, otherwise I would use the solution provided by Chris Schmich.
I have a problem with a foreach statement in my project. So far I have the code:
foreach(object i in listboxFiles.Items)
{
if (i == ".ftpquota")
{
listboxFiles.Items.Remove(i);
}
if (i == ".")
{
listboxFiles.Items.Remove(i);
}
if (i == "..")
{
listboxFiles.Items.Remove(i);
}
}
I have this in a 1 second timer. It gets the item name all right, but when it gets to the if statements it says that they do not match, but they do?
First thing, you are changing a collection while iterating over it. This cannot work, so your code is fundamentally broken.
There are several ways to fix this; the simplest in your case would be to copy the items collection, iterating over the copy and changing (= removing from) the original:
var items = new System.Collections.ArrayList(listboxFiles.Items);
foreach (var item in items) {
if (item.Equals("."))
listboxFiles.Items.remove(item);
…
}
Secondly, you are comparing an object to a string, hence the == operator does reference equality checking rather than testing for string equality. Either use Equals or do an appropriate cast.
The equality check is not working because you should cast to string first and do an appropriate string comparison.
e.g.
if (string.Equals((string)i, ".ftpquota", StringComparison.Ordinal))
If you remove items from a collection of items whilst iterating through the collection, you may well run into trouble. One way to get around this problem is to start with the last item and count backwards, thus any removals you do will not affect the remaining items of the collection, e.g.
for(var i = listboxFiles.Items.Count - 1; i >= 0; --i)
{
var item = listboxFiles[i];
if (...)
{
listboxFiles.Items.RemoveAt(i);
}
}