When using IEnumerable I'm trying to avoid multiple enumerations. I know I can just use LINQ's .ToList() and be done with it, but that can be a lot of unnecessary list creation. I'd like to:
check and see if the underlying type is a List, and if so return that instance, otherwise
.ToList() it and return the new List
My thought was to use something akin to:
public void Fee()
{
var list = new List<string>(); // I want to retrieve this instance in Foo
Foo(list);
}
public void Foo(IEnumerable<T> enumerable)
{
var list = enumerable as List<T> ?? enumerable.ToList();
// do stuff with original list
}
... but it appears from the documentation that the as operator just performs a cast, which would create a new List rather than returning the underlying one, would it not?
If so, how can I retrieve the underlying list instead of creating a new one?
The as operator does not create a new list. It only checks type and perform cast if type is compatible.
The code in the post is logically correct and matches how many LINQ methods are implemented (for example see source of Enumerable.Count which casts to ICollection to see if it can skip enumeration of items).
Note that it is important to cast to correct generic version of list or maybe one of its interfaces - IList would work if you must use non-generic version. Beware of the fact that List<T> is not co/contra-variant and type must match exactly unlike in case of covariant IEnumerable<out T> where you can cast parameter to IEnumerable<TBase> if IEnumerable<TDerived> passed.
Maybe you wanted to do this:
public void Fee()
{
var list = new List<string>(); // I want to retrieve this instance in Foo
Foo(list);
}
public void Foo<T>(IEnumerable<T> enumerable)
{
List<T> list = enumerable as List<T> ?? enumerable.ToList();
// do stuff with original list
}
I have a extension method for an enum
public static IEnumerable<T> GetFlags<T>(this T value) where T : struct
{
CheckIsEnum<T>(true);
foreach (T flag in Enum.GetValues(typeof(T)).Cast<T>())
{
if (value.IsFlagSet(flag))
yield return flag;
}
}
I try to get the result like this:
Zone_Status_ZoneConditionFlagEnum flags = (Zone_Status_ZoneConditionFlagEnum)flagsRaw;
List<Zone_Status_ZoneConditionFlagEnum> ZoneConditionFlags_List = (List<Zone_Status_ZoneConditionFlagEnum>)flags.GetFlags();
But I get
NX584(NX584Test)->Error parsing message: Cannot implicitly convert type [Digicom.NX584Engine.Messages.Zone_Status_ZoneConditionFlagEnum] to System.Collections.Generic.List`1[Digicom.NX584Engine.Messages.Zone_Status_ZoneConditionFlagEnum].
It's not clear to me why you're getting that error - but you can't cast the result of GetFlags<T> to a List<T>, because it doesn't return a List<T>. The simplest fix would be:
var ZoneConditionFlags_List = flags.GetFlags().ToList();
If that doesn't work, please give the new error message.
Alternatively, you could change GetFlags so it actually returned a List<T> rather than using an iterator block.
The first issue here is that a sequence is different to a list; if you need a list, either return a list, or add .ToList() after GetFlags(), i.e.
var ZoneConditionFlags_List = flags.GetFlags().ToList();
However, the bigger problem is that you can't use that IsFlagSet in that generic context; that method is not defined for an arbitrary T : struct.
Personally, I think you'd be better just to treat it as a [Flags] enum throughout; I assume you have existing code that wants a list rather than a single value?
GetFlags returns an IEnumerable<T>, not a List<T>, you cannot cast here.
You should, however, be able to construct a list from the results:
List<Zone_Status_ZoneConditionFlagEnum> ZoneConditionFlags_List = flags.GetFlags().ToList();
However, the error does not match the code here exactly, it should complain about an IEnumerable not be able to be cast, but instead it says the enum type. Are you sure this is the right code?
I have defined the following:
public ICollection<Item> Items { get; set; }
When I run this code:
Items = _item.Get("001");
I get the following message:
Error 3
Cannot implicitly convert type
'System.Collections.Generic.IEnumerable<Storage.Models.Item>' to
'System.Collections.Generic.ICollection<Storage.Models.Item>'.
An explicit conversion exists (are you missing a cast?)
Can someone explain what I am doing wrong. I am very confused about the
difference between Enumerable, Collections and using the ToList()
Added information
Later in my code I have the following:
for (var index = 0; index < Items.Count(); index++)
Would I be okay to define Items as an IEnumerable?
ICollection<T> inherits from IEnumerable<T> so to assign the result of
IEnumerable<T> Get(string pk)
to an ICollection<T> there are two ways.
// 1. You know that the referenced object implements `ICollection<T>`,
// so you can use a cast
ICollection<T> c = (ICollection<T>)Get("pk");
// 2. The returned object can be any `IEnumerable<T>`, so you need to
// enumerate it and put it into something implementing `ICollection<T>`.
// The easiest is to use `ToList()`:
ICollection<T> c = Get("pk").ToList();
The second options is more flexible, but has a much larger performance impact. Another option is to store the result as an IEnumerable<T> unless you need the extra functionality added by the ICollection<T> interface.
Additional Performance Comment
The loop you have
for (var index = 0; index < Items.Count(); index++)
works on an IEnumerable<T> but it is inefficient; each call to Count() requires a complete enumeration of all elements. Either use a collection and the Count property (without the parenthesis) or convert it into a foreach loop:
foreach(var item in Items)
You cannot convert directly from IEnumerable<T> to ICollection<T>. You can use ToList method of IEnumerable<T> to convert it to ICollection<T>
someICollection = SomeIEnumerable.ToList();
Pending more information on the question:
please provide more information on the type of item and the signature of Get
Two things you can try are:
To cast the return value of _item.Get to (ICollection)
secondly to use _item.Get("001").ToArray() or _item.Get("001").ToList()
Please note the second will incur a performance hit for the array copy. If the signature (return type) of Get is not an ICollection then the first will not work, if it is not IEnumerable then the second will not work.
Following your clarification to question and in comments, I would personally declare the returning type of _item.Get("001") to ICollection. This means you won't have to do any casting or conversion (via ToList / ToArray) which would involve an unnecessary create/copy operation.
// Leave this the same
public ICollection<Item> Items { get; set; }
// Change function signature here:
// As you mention Item uses the same underlying type, just return an ICollection<T>
public ICollection<Item> Get(string value);
// Ideally here you want to call .Count on the collectoin, not .Count() on
// IEnumerable, as this will result in a new Enumerator being created
// per loop iteration
for (var index = 0; index < Items.Count(); index++)
Best regards,
As the easiest way to convert the IList<T1> to IList<BaseT1>?
IList<T1>.Count() is very large number!!!
class BaseT1 { };
class T1 : BaseT1
{
static public IList<BaseT1> convert(IList<T1> p)
{
IList<BaseT1> result = new List<BaseT1>();
foreach (BaseT1 baseT1 in p)
result.Add(baseT1);
return result;
}
}
You'll get much better performance in your implementation if you specify the size of the result list when it is initalized, and call the Add method on List<T> directly:
List<BaseT1> result = new List<BaseT1>(p.Count);
that way, it isn't resizing lots of arrays when new items get added. That should yield an order-of-magnitude speedup.
Alternatively, you could code a wrapper class that implements IList<BaseT1> and takes an IList<T1> in the constructor.
linq?
var baseList = derivedList.Cast<TBase>();
Edit:
Cast returns an IEnumerable, do you need it in a List? List can be an expensive class to deal with
IList<T1>.Count() is very large number!!!
Yes, which means that no matter what syntax sugar you use, the conversion is going to require O(n) time and O(n) storage. You cannot cast the list to avoid re-creating it. If that was possible, client code could add an element of BaseT1 to the list, violating the promise that list only contains objects that are compatible with T1.
The only way to get ahead is to return an interface type that cannot change the list. Which would be IEnumerable<BaseT1> in this case. Allowing you to iterate the list, nothing else. That conversion is automatic in .NET 4.0 thanks to its support for covariance. You'll have to write a little glue code in earlier versions:
public static IEnumerable<BaseT1> enumerate(IList<T1> p) {
foreach (BaseT1 item in p) yield return item;
}
My question as title above. For example
IEnumerable<T> items = new T[]{new T("msg")};
items.ToList().Add(new T("msg2"));
but after all it only has 1 item inside. Can we have a method like items.Add(item) like the List<T>?
You cannot, because IEnumerable<T> does not necessarily represent a collection to which items can be added. In fact, it does not necessarily represent a collection at all! For example:
IEnumerable<string> ReadLines()
{
string s;
do
{
s = Console.ReadLine();
yield return s;
} while (!string.IsNullOrEmpty(s));
}
IEnumerable<string> lines = ReadLines();
lines.Add("foo") // so what is this supposed to do??
What you can do, however, is create a new IEnumerable object (of unspecified type), which, when enumerated, will provide all items of the old one, plus some of your own. You use Enumerable.Concat for that:
items = items.Concat(new[] { "foo" });
This will not change the array object (you cannot insert items into to arrays, anyway). But it will create a new object that will list all items in the array, and then "Foo". Furthermore, that new object will keep track of changes in the array (i.e. whenever you enumerate it, you'll see the current values of items).
The type IEnumerable<T> does not support such operations. The purpose of the IEnumerable<T> interface is to allow a consumer to view the contents of a collection. Not to modify the values.
When you do operations like .ToList().Add() you are creating a new List<T> and adding a value to that list. It has no connection to the original list.
What you can do is use the Add extension method to create a new IEnumerable<T> with the added value.
items = items.Add("msg2");
Even in this case it won't modify the original IEnumerable<T> object. This can be verified by holding a reference to it. For example
var items = new string[]{"foo"};
var temp = items;
items = items.Add("bar");
After this set of operations the variable temp will still only reference an enumerable with a single element "foo" in the set of values while items will reference a different enumerable with values "foo" and "bar".
EDIT
I contstantly forget that Add is not a typical extension method on IEnumerable<T> because it's one of the first ones that I end up defining. Here it is
public static IEnumerable<T> Add<T>(this IEnumerable<T> e, T value) {
foreach ( var cur in e) {
yield return cur;
}
yield return value;
}
Have you considered using ICollection<T> or IList<T> interfaces instead, they exist for the very reason that you want to have an Add method on an IEnumerable<T>.
IEnumerable<T> is used to 'mark' a type as being...well, enumerable or just a sequence of items without necessarily making any guarantees of whether the real underlying object supports adding/removing of items. Also remember that these interfaces implement IEnumerable<T> so you get all the extensions methods that you get with IEnumerable<T> as well.
In .net Core, there is a method Enumerable.Append that does exactly that.
The source code of the method is available on GitHub..... The implementation (more sophisticated than the suggestions in other answers) is worth a look :).
A couple short, sweet extension methods on IEnumerable and IEnumerable<T> do it for me:
public static IEnumerable Append(this IEnumerable first, params object[] second)
{
return first.OfType<object>().Concat(second);
}
public static IEnumerable<T> Append<T>(this IEnumerable<T> first, params T[] second)
{
return first.Concat(second);
}
public static IEnumerable Prepend(this IEnumerable first, params object[] second)
{
return second.Concat(first.OfType<object>());
}
public static IEnumerable<T> Prepend<T>(this IEnumerable<T> first, params T[] second)
{
return second.Concat(first);
}
Elegant (well, except for the non-generic versions). Too bad these methods are not in the BCL.
No, the IEnumerable doesn't support adding items to it. The alternative solution is
var myList = new List(items);
myList.Add(otherItem);
To add second message you need to -
IEnumerable<T> items = new T[]{new T("msg")};
items = items.Concat(new[] {new T("msg2")})
I just come here to say that, aside from Enumerable.Concat extension method, there seems to be another method named Enumerable.Append in .NET Core 1.1.1. The latter allows you to concatenate a single item to an existing sequence. So Aamol's answer can also be written as
IEnumerable<T> items = new T[]{new T("msg")};
items = items.Append(new T("msg2"));
Still, please note that this function will not change the input sequence, it just return a wrapper that put the given sequence and the appended item together.
Not only can you not add items like you state, but if you add an item to a List<T> (or pretty much any other non-read only collection) that you have an existing enumerator for, the enumerator is invalidated (throws InvalidOperationException from then on).
If you are aggregating results from some type of data query, you can use the Concat extension method:
Edit: I originally used the Union extension in the example, which is not really correct. My application uses it extensively to make sure overlapping queries don't duplicate results.
IEnumerable<T> itemsA = ...;
IEnumerable<T> itemsB = ...;
IEnumerable<T> itemsC = ...;
return itemsA.Concat(itemsB).Concat(itemsC);
Others have already given great explanations regarding why you can not (and should not!) be able to add items to an IEnumerable. I will only add that if you are looking to continue coding to an interface that represents a collection and want an add method, you should code to ICollection or IList. As an added bonanza, these interfaces implement IEnumerable.
you can do this.
//Create IEnumerable
IEnumerable<T> items = new T[]{new T("msg")};
//Convert to list.
List<T> list = items.ToList();
//Add new item to list.
list.add(new T("msg2"));
//Cast list to IEnumerable
items = (IEnumerable<T>)items;
Easyest way to do that is simply
IEnumerable<T> items = new T[]{new T("msg")};
List<string> itemsList = new List<string>();
itemsList.AddRange(items.Select(y => y.ToString()));
itemsList.Add("msg2");
Then you can return list as IEnumerable also because it implements IEnumerable interface
Instances implementing IEnumerable and IEnumerator (returned from IEnumerable) don't have any APIs that allow altering collection, the interface give read-only APIs.
The 2 ways to actually alter the collection:
If the instance happens to be some collection with write API (e.g. List) you can try casting to this type:
IList<string> list = enumerableInstance as IList<string>;
Create a list from IEnumerable (e.g. via LINQ extension method toList():
var list = enumerableInstance.toList();
IEnumerable items = Enumerable.Empty(T);
List somevalues = new List();
items.ToList().Add(someValues);
items.ToList().AddRange(someValues);
Sorry for reviving really old question but as it is listed among first google search results I assume that some people keep landing here.
Among a lot of answers, some of them really valuable and well explained, I would like to add a different point of vue as, to me, the problem has not be well identified.
You are declaring a variable which stores data, you need it to be able to change by adding items to it ? So you shouldn't use declare it as IEnumerable.
As proposed by #NightOwl888
For this example, just declare IList instead of IEnumerable: IList items = new T[]{new T("msg")}; items.Add(new T("msg2"));
Trying to bypass the declared interface limitations only shows that you made the wrong choice.
Beyond this, all methods that are proposed to implement things that already exists in other implementations should be deconsidered.
Classes and interfaces that let you add items already exists. Why always recreate things that are already done elsewhere ?
This kind of consideration is a goal of abstracting variables capabilities within interfaces.
TL;DR : IMO these are cleanest ways to do what you need :
// 1st choice : Changing declaration
IList<T> variable = new T[] { };
variable.Add(new T());
// 2nd choice : Changing instantiation, letting the framework taking care of declaration
var variable = new List<T> { };
variable.Add(new T());
When you'll need to use variable as an IEnumerable, you'll be able to. When you'll need to use it as an array, you'll be able to call 'ToArray()', it really always should be that simple. No extension method needed, casts only when really needed, ability to use LinQ on your variable, etc ...
Stop doing weird and/or complex things because you only made a mistake when declaring/instantiating.
Maybe I'm too late but I hope it helps anyone in the future.
You can use the insert function to add an item at a specific index.
list.insert(0, item);
Sure, you can (I am leaving your T-business aside):
public IEnumerable<string> tryAdd(IEnumerable<string> items)
{
List<string> list = items.ToList();
string obj = "";
list.Add(obj);
return list.Select(i => i);
}