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Invalid cast from IEnumerable to List

I'm working on a C# script within a Unity3D Project where I'm trying to take a list of strings and get a 2D list of the permutations. Using this answer's GetPermutations() in the following fashion:
List<string> ingredientList = new List<string>(new string[] { "ingredient1", "ingredient2", "ingredient3" });
List<List<string>> permutationLists = GetPermutations(ingredientList, ingredientList.Count);
But it throws an implicit conversion error:
IEnumerable<IEnumerable<string>> to List<List<string>> ... An explicit conversion exists (are you missing a cast)?
So I looked at a few places, such as here and came up with the following modification:
List<List<string>> permutationLists = GetPermutations(ingredientList, ingredientList.Count).Cast<List<string>>().ToList();
But it breaks at runtime, gets handled internally, and allows it to continue without indicating a failure – probably because it's running in Unity3D.
Here is what I see in Unity3D after I stop debugging the script:
InvalidCastException: Cannot cast from source type to destination type.
System.Linq.Enumerable+<CreateCastIterator>c__Iterator0`1[System.Collections.Generic.List`1[System.String]].MoveNext ()
System.Collections.Generic.List`1[System.Collections.Generic.List`1[System.String]].AddEnumerable (IEnumerable`1 enumerable) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:128)
System.Collections.Generic.List`1[System.Collections.Generic.List`1[System.String]]..ctor (IEnumerable`1 collection) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:65)
System.Linq.Enumerable.ToList[List`1] (IEnumerable`1 source)
Which I interpret as still casting incorrectly, so I also attempted the following approaches and more that I can't remember:
List<List<string>> permutationLists = GetPermutations(ingredientList, ingredientList.Count).Cast<List<List<string>>>();
List<List<string>> permutationLists = GetPermutations(ingredientList.AsEnumerable(), ingredientList.Count);
as well as explicitly casting with parenthesis before the method call like you would in C or Java, still to no avail.
So how should I be casting the results from the GetPermutations() function to get a List<List<string>>? Or alternatively, how could I modify the function to only return List<List<string>> since I don't need it to work for a generic type? I tried to modify the method myself to be the following:
List<List<string>> GetPermutations(List<string> items, int count)
{
int i = 0;
foreach(var item in items)
{
if(count == 1)
yield return new string[] { item };
else
{
foreach(var result in GetPermutations(items.Skip(i + 1), count - 1))
yield return new string[] { item }.Concat(result);
}
++i;
}
}
However, having removed the <T> from the function name it breaks stating that the body cannot be an iterator block. I have no prior experience with C# and I'm rusty with template functions in strongly typed languages, so any explanation/help is appreciated.
I wasn't sure how to look this issue up, so if this is a duplicate just post it here and I'll delete this post immediately.

So how should I be casting the results from the GetPermutations() function to get a List<List<string>>
Best solution: don't. Why do you need to turn the sequence into a list in the first place? Keep it as a sequence of sequences.
If you must though:
GetPermutations(...).Select(s => s.ToList()).ToList()
If you want to modify the original method, just do the same thing:
IEnumerable<List<string>> GetPermutations(List<string> items, int count)
{
int i = 0;
foreach(var item in items)
{
if(count == 1)
yield return new List<T>() { item };
else
{
foreach(var result in GetPermutations(items.Skip(i + 1), count - 1))
yield return (new string[] {item}.Concat(result)).ToList();
}
++i;
}
}
And then do GetPermutations(whatever).ToList() and you have a list of lists. But again, do not do this. Keep everything in sequences if you possibly can.
I want to turn the sequence into a list so that I can sort the elements alphabetically and re-join them as a sorted, single comma-delimited string.
OK, then do that. Let's rewrite your method as an extension method Permute(). And let's make some new one-liner methods:
static public string CommaSeparate(this IEnumerable<string> items) =>
string.Join(",", items);
static public string WithNewLines(this IEnumerable<string> items) =>
string.Join("\n", items);
static public IEnumerable<string> StringSort(this IEnumerable<string> items) =>
items.OrderBy(s => s);
Then we have the following -- I'll annotate the types as we go:
string result =
ingredients // List<string>
.Permute() // IEnumerable<IEnumerable<string>>
.Select(p => p.StringSort()) // IEnumerable<IEnumerable<string>>
.Select(p => p.CommaSeparate())// IEnumerable<string>
.WithNewLines(); // string
And we're done. Look at how clear and straightforward the code is when you make methods that do one thing and do it well. And look at how easy it is when you keep everything in sequences, as it should be.

Your question is related to several aspects of C# and .net types system. I will try to provide simple explanation and will provide links as more formal answers.
So, according to your description it looks like GetPermutations(ingredientList, ingredientList.Count); returns IEnumerable<IEnumerable<string>> but you are trying to assign this result to the variable of another type, in pseudo code:
List<List<string>> = IEnumerable<IEnumerable<string>>;
List<T> implements IEnumerable<T>, so in general it is possible to make this assignment:
IEnumerable<T> = List<T>;
but the problem is that in your case T on the left side differs from the T on the right side.
for IEnumerable<IEnumerable<string>> T is IEnumerable<string>.
for List<List<string>> T is List<string>
To fix your problem we should change the code to have the same T on the left and right sides i.e. convert T to either List<string> or IEnumerable<string>.
You can convert T to the List<string> this way:
IEnumerable<List<string> GetPermutationsList(List<string> items, int count)
{
return GetPermutations(items, count).Select(x=>x.ToList())
}
IEnumerable<List<string>> permutationLists = GetPermutations(ingredientList.AsEnumerable(), ingredientList.Count);
// or
List<List<string>> permutationLists = GetPermutations(ingredientList.AsEnumerable(), ingredientList.Count).ToList();
but in general it is not good idea to use List in all places. Use lists only where you really need it. Important points here:
IEnumerable<T> provides minimum functionality (enumeration only) that should be enougth for your goals.
IList <T> (List implements it ) provides maximum functionality (Add, Remove ,“random” access by index). Do you really need maximum functionality?
Also using ToList() can cause memory shortage problem for big data.
ToList() just forces immediate query evaluation and returns a List<T>
Some useful information: covariance-contr-variance, List, casting

List<T>.AddRange and the yield statement

I am aware that the yield keyword indicates that the method in which it appears is an iterator. I was just wondering how that works with something like List<T>.AddRange.
Let's use the below example:
static void Main()
{
foreach (int i in MyInts())
{
Console.Write(i);
}
}
public static IEnumerable<int> MyInts()
{
for (int i = 0; i < 255; i++)
{
yield return i;
}
}
So in the above example after each yield, a value is returned in the foreach loop in Main and is printed to the console.
If we change Main to this:
static void Main()
{
var myList = new List<int>();
myList.AddRange(MyInts());
}
how does that work? Does AddRange get called for each int returned by the yield statement or does it somehow wait for all 255 values before adding the entire range?

The implementation of AddRange will iterate over the IEnumerable input using the iterator's .MoveNext() method until all values have been produced by your yielding method. This can be seen here.
So myList.AddRange(MyInts()); is called once and its implementation forces MyInts to return all of it values before moving on.
AddRange exhausts all values of the iterator because of how is implemented, but the following hypothetic method would only evaluate the first value of the iterator:
public void AddFirst<T>(IEnumerable<T> collection)
{
Insert(collection.First());
}
An interesting experiment while you play around with this is to add a Console.WriteLine(i); line in your MyInts method to see when each number is generated.

Short answer: When you call AddRange, it will internally iterate every item in your IEnumerable and add to the list.
If you did something like this:
var myList = new List<int>();
myList.AddRange(MyInts());
foreach (int i in myList)
{
Console.Write(i);
}
Then your values would be iterated twice, from the start to the end:
Once when adding to your list
Then in your for loop
Playing a bit
Now, let's suppose you created your own extension method for AddRange like this:
public static IEnumerable<T> AddRangeLazily<T>(this ICollection<T> col, IEnumerable<T> values)
{
foreach (T i in values)
{
yield return i; // first we yield
col.Add(i); // then we add
}
}
Then you could use it like this:
foreach (int i in myList.AddRangeLazily(MyInts()))
{
Console.Write(i);
}
...and it would be iterated twice as well, without going from the start to the end both times. It would lazily add each value to the list/collection and at the same time allow you to do something else (like printing it to output) after every new item being added.
If you had some sort of logic to stop the adding to the list in the middle of the operation, this should be helpful somehow.
The downside if this AddRangeLazily is: values will only be added to the collection once you iterate over AddRangeLazily like my code sample. If you just do this:
var someList = new List<int>();
someList.AddRangeLazily(MyInts());
if (someList.Any())
// it wouldn't enter here...
...it won't add values at all. If you wanted that behaviour, you should use AddRange. Forcing the iterationg over AddRangeLazily method would work, though:
var someList = new List<int>();
someList.AddRangeLazily(MyInts());
if (someList.AddRangeLazily(MyInts()).Count())
// it would enter here...thus adding all values to the someList
...however, depending on how lazy is the method you calling, it wouldn't iterate everything. For example:
var someList = new List<int>();
someList.AddRangeLazily(MyInts());
if (someList.AddRangeLazily(MyInts()).Any())
// it would enter here, plus adding only the first value to someList
Since Any() is true as soon as any item exists, then Any() just needs one iterationg to return true, therefore it just needs the first item to be iterated over.
I actually don't remember having to do something like this, it was just to play around with yield.
Fiddle here!!!

Interesting question.
The behavior is different if the enumerable is for a class that implements ICollection, such as another list or an array, but let's say it doesn't since your example doesn't. AddRange() simply uses the enumerator to insert items into the list one at a time.
using(IEnumerator<T> en = collection.GetEnumerator()) {
while(en.MoveNext()) {
Insert(index++, en.Current);
If the type of the enumerator is ICollection then AddRange first expands the list and then does a block copy.
If you want to see the code yourself:
https://referencesource.microsoft.com/#mscorlib/system/collections/generic/list.cs,51decd510e5bfe6e

Foreach and enumerable

Why we can iterate item ex
mList.ForEach((item)
{
item.xyz ....
}
and for a simple array we need to force foreach loop?
foreach(int i in arr)
i.xyz
or use delegate type ?
Action<int> action = new Action<int>(myfunc);
Array.ForEach(intArray, action);
What is the differemce?

The first syntax is not correct. It should be like this:
mList.ForEach(item =>
{
// item.xyz
});
The ForEach is a method of List<T> that enables you for each item in a list to call an Action<T>.
On the other hand the foreach
statement repeats a group of embedded statements for each element in
an array or an object collection that implements the
System.Collections.IEnumerable or
System.Collections.Generic.IEnumerable interface.
That being said, ForEach can be called only on lists and foreach can be called on any object that implements either IEnumerable or IEnumerable. That's the big difference here.
Regarding the delegate type, there isn't any difference. Actually, lambda expressions item=>{ item.xyz = ...} are a shorthand for delegates.

The language defines foreach as an operation of IEnumerable. Therefore, everything which implements IEnumerable is iteratable. However, not all IEnumerables 'make sense' when using a ForEach block.
Take this for example:
public static IEnumerable<MyObject> GetObjects()
{
var i = 0;
while(i < 30)
yield return new MyObject { Name = "Object " + i++ };
}
And then you do something like this:
var objects = GetObjects();
objects.ForEach(o => o.Name = "Rob");
foreach (var obj in objects)
Console.WriteLine(obj.Name);
IF that compiled, it would print out Object 0 to Object 29 - NOT Rob 30 times.
The reason for this is that the iterator is reset each time you iterate the enumerable. It makes sense for ForEach on a list, as the enumerable has been materialized, and objects are not re-created every time you iterate it.
In order to make ForEach work on an enumerable, you'd need to materialize the collection as well (such as putting it into a list), but even that is not always possible, as you can have an enumerable with no defined end:
public static IEnumerable<MyObject> GetObjects()
{
while(true)
yield return new MyObject { Name = "Object " };
}
It also makes sense to have ForEach on Array - but for reasons I'm unaware of, it was defined as Array.ForEach(arr) rather than arr.ForEach()
Moral of the story is, if you think you need a ForEach block, you probably want to materialize the enumerable first, usually to a List<T> or an array (T[]).

What's the Best Way to Add One Item to an IEnumerable<T>?

Here's how I would add one item to an IEnumerable object:
//Some IEnumerable<T> object
IEnumerable<string> arr = new string[] { "ABC", "DEF", "GHI" };
//Add one item
arr = arr.Concat(new string[] { "JKL" });
This is awkward. I don't see a method called something like ConcatSingle() however.
Is there a cleaner way to add a single item to an IEnumerable object?

Nope, that's about as concise as you'll get using built-in language/framework features.
You could always create an extension method if you prefer:
arr = arr.Append("JKL");
// or
arr = arr.Append("123", "456");
// or
arr = arr.Append("MNO", "PQR", "STU", "VWY", "etc", "...");
// ...
public static class EnumerableExtensions
{
public static IEnumerable<T> Append<T>(
this IEnumerable<T> source, params T[] tail)
{
return source.Concat(tail);
}
}

IEnumerable is immutable collection, it means you cannot add, or remove item. Instead, you have to create a new collection for this, simply to convert to list to add:
var newCollection = arr.ToList();
newCollection.Add("JKL"); //is your new collection with the item added

Write an extension method ConcatSingle :)
public static IEnumerable<T> ConcatSingle<T>(this IEnumerable<T> source, T item)
{
return source.Concat(new [] { item } );
}
But you need to be more careful with your terminology.
You can't add an item to an IEnumerable<T>. Concat creates a new instance.
Example:
var items = Enumerable.Range<int>(1, 10)
Console.WriteLine(items.Count()); // 10
var original= items;
items = items.ConcatSingle(11);
Console.WriteLine(original.Count()); // 10
Console.WriteLine(items.Count()); // 11
As you can see, the original enumeration - which we saved in original didn't change.

Since IEnumerable is read-only, you need to convert to list.
var new_one = arr.ToList().Add("JKL");
Or you can get a extension method like;
public static IEnumerable<T> Append<T>(this IEnumerable<T> source, params T[] item)
{
return source.Concat(item);
}

Append() - is exactly what you need, it has been added to the .NET Standard (in 2017), so you no longer need to write your own extension methods. You can simply do this:
arr = arr.Append("JKL");
Since .NET is open source, here you can look on the implementation (it is more sophisticated than custom methods suggested above):
https://github.com/dotnet/runtime/blob/master/src/libraries/System.Linq/src/System/Linq/AppendPrepend.cs

You're assigning an array to an IEnumerable. Why don't you use the Array type instead of IEnumerable?
Otherwise you can use IList (or List) if you want to change the collection.
I use IEnumerable only for methods params when I need to read and IList (or List) when I need to change items in it.

Intelligent way of removing items from a List<T> while enumerating in C#

I have the classic case of trying to remove an item from a collection while enumerating it in a loop:
List<int> myIntCollection = new List<int>();
myIntCollection.Add(42);
myIntCollection.Add(12);
myIntCollection.Add(96);
myIntCollection.Add(25);
foreach (int i in myIntCollection)
{
if (i == 42)
myIntCollection.Remove(96); // The error is here.
if (i == 25)
myIntCollection.Remove(42); // The error is here.
}
At the beginning of the iteration after a change takes place, an InvalidOperationException is thrown, because enumerators don’t like when the underlying collection changes.
I need to make changes to the collection while iterating. There are many patterns that can be used to avoid this, but none of them seems to have a good solution:
Do not delete inside this loop, instead keep a separate “Delete List”, that you process after the main loop.
This is normally a good solution, but in my case, I need the item to be gone instantly as “waiting” till after
the main loop to really delete the item changes the logic flow of my code.
Instead of deleting the item, simply set a flag on the item and mark it as inactive. Then add the functionality of pattern 1 to clean up the list.
This would work for all of my needs, but it means that a lot of code will have to change in order to check the inactive flag every time an item is accessed. This is far too much administration for my liking.
Somehow incorporate the ideas of pattern 2 in a class that derives from List<T>. This Superlist will handle the inactive flag, the deletion of objects after the fact and also will not expose items marked as inactive to enumeration consumers. Basically, it just encapsulates all the ideas of pattern 2 (and subsequently pattern 1).
Does a class like this exist? Does anyone have code for this? Or is there a better way?
I’ve been told that accessing myIntCollection.ToArray() instead of myIntCollection will solve the problem and allow me to delete inside the loop.
This seems like a bad design pattern to me, or maybe it’s fine?
Details:
The list will contain many items and I will be removing only some of them.
Inside the loop, I will be doing all sorts of processes, adding, removing etc., so the solution needs to be fairly generic.
The item that I need to delete may not be the current item in the loop. For example, I may be on item 10 of a 30 item loop and need to remove item 6 or item 26. Walking backwards through the array will no longer work because of this. ;o(

The best solution is usually to use the RemoveAll() method:
myList.RemoveAll(x => x.SomeProp == "SomeValue");
Or, if you need certain elements removed:
MyListType[] elems = new[] { elem1, elem2 };
myList.RemoveAll(x => elems.Contains(x));
This assume that your loop is solely intended for removal purposes, of course. If you do need to additional processing, then the best method is usually to use a for or while loop, since then you're not using an enumerator:
for (int i = myList.Count - 1; i >= 0; i--)
{
// Do processing here, then...
if (shouldRemoveCondition)
{
myList.RemoveAt(i);
}
}
Going backwards ensures that you don't skip any elements.
Response to Edit:
If you're going to have seemingly arbitrary elements removed, the easiest method might be to just keep track of the elements you want to remove, and then remove them all at once after. Something like this:
List<int> toRemove = new List<int>();
foreach (var elem in myList)
{
// Do some stuff
// Check for removal
if (needToRemoveAnElement)
{
toRemove.Add(elem);
}
}
// Remove everything here
myList.RemoveAll(x => toRemove.Contains(x));

If you must both enumerate a List<T> and remove from it then I suggest simply using a while loop instead of a foreach
var index = 0;
while (index < myList.Count) {
if (someCondition(myList[index])) {
myList.RemoveAt(index);
} else {
index++;
}
}

I know this post is old, but I thought I'd share what worked for me.
Create a copy of the list for enumerating, and then in the for each loop, you can process on the copied values, and remove/add/whatever with the source list.
private void ProcessAndRemove(IList<Item> list)
{
foreach (var item in list.ToList())
{
if (item.DeterminingFactor > 10)
{
list.Remove(item);
}
}
}

When you need to iterate through a list and might modify it during the loop then you are better off using a for loop:
for (int i = 0; i < myIntCollection.Count; i++)
{
if (myIntCollection[i] == 42)
{
myIntCollection.Remove(i);
i--;
}
}
Of course you must be careful, for example I decrement i whenever an item is removed as otherwise we will skip entries (an alternative is to go backwards though the list).
If you have Linq then you should just use RemoveAll as dlev has suggested.

As you enumerate the list, add the one you want to KEEP to a new list. Afterward, assign the new list to the myIntCollection
List<int> myIntCollection=new List<int>();
myIntCollection.Add(42);
List<int> newCollection=new List<int>(myIntCollection.Count);
foreach(int i in myIntCollection)
{
if (i want to delete this)
///
else
newCollection.Add(i);
}
myIntCollection = newCollection;

Let's add you code:
List<int> myIntCollection=new List<int>();
myIntCollection.Add(42);
myIntCollection.Add(12);
myIntCollection.Add(96);
myIntCollection.Add(25);
If you want to change the list while you're in a foreach, you must type .ToList()
foreach(int i in myIntCollection.ToList())
{
if (i == 42)
myIntCollection.Remove(96);
if (i == 25)
myIntCollection.Remove(42);
}

For those it may help, I wrote this Extension method to remove items matching the predicate and return the list of removed items.
public static IList<T> RemoveAllKeepRemoved<T>(this IList<T> source, Predicate<T> predicate)
{
IList<T> removed = new List<T>();
for (int i = source.Count - 1; i >= 0; i--)
{
T item = source[i];
if (predicate(item))
{
removed.Add(item);
source.RemoveAt(i);
}
}
return removed;
}

How about
int[] tmp = new int[myIntCollection.Count ()];
myIntCollection.CopyTo(tmp);
foreach(int i in tmp)
{
myIntCollection.Remove(42); //The error is no longer here.
}

If you're interested in high performance, you can use two lists. The following minimises garbage collection, maximises memory locality and never actually removes an item from a list, which is very inefficient if it's not the last item.
private void RemoveItems()
{
_newList.Clear();
foreach (var item in _list)
{
item.Process();
if (!item.NeedsRemoving())
_newList.Add(item);
}
var swap = _list;
_list = _newList;
_newList = swap;
}

Just figured I'll share my solution to a similar problem where i needed to remove items from a list while processing them.
So basically "foreach" that will remove the item from the list after it has been iterated.
My test:
var list = new List<TempLoopDto>();
list.Add(new TempLoopDto("Test1"));
list.Add(new TempLoopDto("Test2"));
list.Add(new TempLoopDto("Test3"));
list.Add(new TempLoopDto("Test4"));
list.PopForEach((item) =>
{
Console.WriteLine($"Process {item.Name}");
});
Assert.That(list.Count, Is.EqualTo(0));
I solved this with a extension method "PopForEach" that will perform a action and then remove the item from the list.
public static class ListExtensions
{
public static void PopForEach<T>(this List<T> list, Action<T> action)
{
var index = 0;
while (index < list.Count) {
action(list[index]);
list.RemoveAt(index);
}
}
}
Hope this can be helpful to any one.

Currently you are using a list. If you could use a dictionary instead, it would be much easier. I'm making some assumptions that you are really using a class instead of just a list of ints. This would work if you had some form of unique key. In the dictionary, object can be any class you have and int would be any unique key.
Dictionary<int, object> myIntCollection = new Dictionary<int, object>();
myIntCollection.Add(42, "");
myIntCollection.Add(12, "");
myIntCollection.Add(96, "");
myIntCollection.Add(25, "");
foreach (int i in myIntCollection.Keys)
{
//Check to make sure the key wasn't already removed
if (myIntCollection.ContainsKey(i))
{
if (i == 42) //You can test against the key
myIntCollection.Remove(96);
if (myIntCollection[i] == 25) //or you can test against the value
myIntCollection.Remove(42);
}
}
Or you could use
Dictionary<myUniqueClass, bool> myCollection; //Bool is just an empty place holder
The nice thing is you can do anything you want to the underlying dictionary and the key enumerator doesn't care, but it also doesn't update with added or removed entries.