Related
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 want to create a new instance of an object IEnumerable<object>
Can I do this?
IEnumerable<object> a = new IEnumerable<object>();
You can for example create an instance of List<object>, which implements IEnumerable<object>. Example:
List<object> list = new List<object>();
list.Add(1);
list.Add(4);
list.Add(5);
IEnumerable<object> en = list;
CallFunction(en);
Another solution would be to use Empty<T>.
msdn extract:
Returns an empty IEnumerable that has the specified type argument.
IEnumerable<object> a = Enumerable.Empty<object>();
There is a thread on SO about it: Is it better to use Enumerable.Empty() as opposed to new List to initialize an IEnumerable?
If you use an empty array or empty list, those are objects and they are stored in memory. The Garbage Collector has to take care of them. If you are dealing with a high throughput application, it could be a noticeable impact.
Enumerable.Empty does not create an object per call thus putting less load on the GC.
Since you now specified you want to add to it, what you want isn't a simple IEnumerable<T> but at least an ICollection<T>. I recommend simply using a List<T> like this:
List<object> myList=new List<object>();
myList.Add(1);
myList.Add(2);
myList.Add(3);
You can use myList everywhere an IEnumerable<object> is expected, since List<object> implements IEnumerable<object>.
(old answer before clarification)
You can't create an instance of IEnumerable<T> since it's a normal interface(It's sometimes possible to specify a default implementation, but that's usually used only with COM).
So what you really want is instantiate a class that implements the interface IEnumerable<T>. The behavior varies depending on which class you choose.
For an empty sequence use:
IEnumerable<object> e0=Enumerable.Empty<object>();
For an non empty enumerable you can use some collection that implements IEnumerable<T>. Common choices are the array T[], List<T> or if you want immutability ReadOnlyCollection<T>.
IEnumerable<object> e1=new object[]{1,2,3};
IEnumerable<object> e2=new List<object>(){1,2,3};
IEnumerable<object> e3=new ReadOnlyCollection(new object[]{1,2,3});
Another common way to implement IEnumerable<T> is the iterator feature introduced in C# 3:
IEnumerable<object> MyIterator()
{
yield return 1;
yield return 2;
yield return 3;
}
IEnumerable<object> e4=MyIterator();
No you can't since IEnumerable is an interface.
You should be able to create an empty instance of most non-interface types which implement IEnumerable, e.g.:-
IEnumerable<object> a = new object[] { };
or
IEnumerable<object> a = new List<object>();
No, You cannot do that. Use the following line of code instead:
IEnumerable<int> usersIds = new List<int>() {1, 2, 3}.AsEnumerable();
I hope it helps.
The main reason is we can't create object of an interface, and IEnumerable is an interface.
We need to create object of the class which implements the interface. This is the main reason we can't directly create object of IEnumerable.
You can do this:
IEnumerable<object> list = new List<object>(){1, 4, 5}.AsEnumerable();
CallFunction(list);
I wanted to create a new enumerable object or list and be able to add to it.
This comment changes everything. You can't add to a generic IEnumerable<T>. If you want to stay with the interfaces in System.Collections.Generic, you need to use a class that implements ICollection<T> like List<T>.
No IEnumerable is an interface, you can't create instance of interface
you can do something like this
IEnumerable<object> a = new object[0];
It's a pretty old question, but for the sake of newcomers, this is how we can protect an IEnumerable<T> from a null exception. Another word, to create an empty instance of a variable of type IEnumerable<T>
public IEnumerable<T> MyPropertyName { get; set; } = Enumerable.Empty<T>();
https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.empty?view=net-5.0
Cheers.
I have been Implementing IEnumerable of type IEnumerable<T> by this way
IEnumerable<T> MyEnum = new T[]{ /*object of type T*/ };
Example:
var Object = new Book{id = 1,name = "Hello World"};
IEnumerable<Book> MyEnum = new Book[]{ Object };
What's the standard way to get a typed, readonly empty list in C#, or is there one?
ETA: For those asking "why?": I have a virtual method that returns an IList (or rather, post-answers, an IEnumerable), and the default implementation is empty. Whatever the list returns should be readonly because writing to it would be a bug, and if somebody tries to, I want to halt and catch fire immediately, rather than wait for the bug to show up in some subtle way later.
Personally, I think this is better than any of the other answers:
static readonly IList<T> EmptyList = new T[0];
Arrays implement IList<T>.
You cannot add to an array.
You cannot assign to an element in an empty array (because there is none).
This is, in my opinion, a lot simpler than new List<T>().AsReadOnly().
You still get to return an IList<T> (if you want).
Incidentally, this is what Enumerable.Empty<T>() actually uses under the hood, if I recall correctly. So theoretically you could even do (IList<T>)Enumerable.Empty<T>() (though I see no good reason to do that).
You can just create a list:
List<MyType> list = new List<MyType>();
If you want an empty IEnumerable<T>, use Enumerable.Empty<T>():
IEnumerable<MyType> collection = Enumerable.Empty<MyType>();
If you truly want a readonly list, you could do:
IList<MyType> readonlyList = (new List<MyType>()).AsReadOnly();
This returns a ReadOnlyCollection<T>, which implements IList<T>.
Starting with .net 4.6 you can also use:
IList<T> emptyList = Array.Empty<T>();
This does only create a new instance once for every different type you specify as T.
IList<T> list = new List<T>().AsReadOnly();
Or, if you want an IEnumerable<>:
IEnumerable<T> sequence = Enumerable.Empty<T>();
If you want a list whose contents can't be modified, you can do:
ReadOnlyCollection<Foo> foos = new List<Foo>().AsReadOnly();
Construct an instance of System.Collections.ObjectModel.ReadOnlyCollection from your list.
List<int> items = new List<int>();
ReadOnlyCollection<int> readOnlyItems = new ReadOnlyCollection<int>(items);
To expand on Dan Tao's answer, the following implementation can be used in the same way as Enumerable.Empty<T>(), by specifying List.Empty<T>() instead.
public static class List
{
public static IList<T> Empty<T>()
{
// Note that the static type is only instantiated when
// it is needed, and only then is the T[0] object created, once.
return EmptyArray<T>.Instance;
}
private sealed class EmptyArray<T>
{
public static readonly T[] Instance = new T[0];
}
}
Edit: I change the above code to reflect the outcome of a discussion with Dan Tao about lazy versus eager initialization of the Instance field.
What about:
readonly List<T> mylist = new List<T>();
Not sure why you want it readonly; that doesn't make much sense in most scenarios I can think of, though.
Let's say I have a class
public class MyObject
{
public int SimpleInt{get;set;}
}
And I have a List<MyObject>, and I ToList() it and then change one of the SimpleInt, will my change be propagated back to the original list. In other words, what would be the output of the following method?
public void RunChangeList()
{
var objs = new List<MyObject>(){new MyObject(){SimpleInt=0}};
var whatInt = ChangeToList(objs );
}
public int ChangeToList(List<MyObject> objects)
{
var objectList = objects.ToList();
objectList[0].SimpleInt=5;
return objects[0].SimpleInt;
}
Why?
P/S: I'm sorry if it seems obvious to find out. But I don't have compiler with me now...
Yes, ToList will create a new list, but because in this case MyObject is a reference type then the new list will contain references to the same objects as the original list.
Updating the SimpleInt property of an object referenced in the new list will also affect the equivalent object in the original list.
(If MyObject was declared as a struct rather than a class then the new list would contain copies of the elements in the original list, and updating a property of an element in the new list would not affect the equivalent element in the original list.)
From the Reflector'd source:
public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source)
{
if (source == null)
{
throw Error.ArgumentNull("source");
}
return new List<TSource>(source);
}
So yes, your original list won't be updated (i.e. additions or removals) however the referenced objects will.
ToList will always create a new list, which will not reflect any subsequent changes to the collection.
However, it will reflect changes to the objects themselves (Unless they're mutable structs).
In other words, if you replace an object in the original list with a different object, the ToList will still contain the first object.
However, if you modify one of the objects in the original list, the ToList will still contain the same (modified) object.
Yes, it creates a new list. This is by design.
The list will contain the same results as the original enumerable sequence, but materialized into a persistent (in-memory) collection. This allows you to consume the results multiple times without incurring the cost of recomputing the sequence.
The beauty of LINQ sequences is that they are composable. Often, the IEnumerable<T> you get is the result of combining multiple filtering, ordering, and/or projection operations. Extension methods like ToList() and ToArray() allow you to convert the computed sequence into a standard collection.
The accepted answer correctly addresses the OP's question based on his example. However, it only applies when ToList is applied to a concrete collection; it does not hold when the elements of the source sequence have yet to be instantiated (due to deferred execution). In case of the latter, you might get a new set of items each time you call ToList (or enumerate the sequence).
Here is an adaptation of the OP's code to demonstrate this behaviour:
public static void RunChangeList()
{
var objs = Enumerable.Range(0, 10).Select(_ => new MyObject() { SimpleInt = 0 });
var whatInt = ChangeToList(objs); // whatInt gets 0
}
public static int ChangeToList(IEnumerable<MyObject> objects)
{
var objectList = objects.ToList();
objectList.First().SimpleInt = 5;
return objects.First().SimpleInt;
}
Whilst the above code may appear contrived, this behaviour can appear as a subtle bug in other scenarios. See my other example for a situation where it causes tasks to get spawned repeatedly.
A new list is created but the items in it are references to the orginal items (just like in the original list). Changes to the list itself are independent, but to the items will find the change in both lists.
Just stumble upon this old post and thought of adding my two cents. Generally, if I am in doubt, I quickly use the GetHashCode() method on any object to check the identities. So for above -
public class MyObject
{
public int SimpleInt { get; set; }
}
class Program
{
public static void RunChangeList()
{
var objs = new List<MyObject>() { new MyObject() { SimpleInt = 0 } };
Console.WriteLine("objs: {0}", objs.GetHashCode());
Console.WriteLine("objs[0]: {0}", objs[0].GetHashCode());
var whatInt = ChangeToList(objs);
Console.WriteLine("whatInt: {0}", whatInt.GetHashCode());
}
public static int ChangeToList(List<MyObject> objects)
{
Console.WriteLine("objects: {0}", objects.GetHashCode());
Console.WriteLine("objects[0]: {0}", objects[0].GetHashCode());
var objectList = objects.ToList();
Console.WriteLine("objectList: {0}", objectList.GetHashCode());
Console.WriteLine("objectList[0]: {0}", objectList[0].GetHashCode());
objectList[0].SimpleInt = 5;
return objects[0].SimpleInt;
}
private static void Main(string[] args)
{
RunChangeList();
Console.ReadLine();
}
And answer on my machine -
objs: 45653674
objs[0]: 41149443
objects: 45653674
objects[0]: 41149443
objectList: 39785641
objectList[0]: 41149443
whatInt: 5
So essentially the object that list carries remain the same in above code. Hope the approach helps.
I think that this is equivalent to asking if ToList does a deep or shallow copy. As ToList has no way to clone MyObject, it must do a shallow copy, so the created list contains the same references as the original one, so the code returns 5.
ToList will create a brand new list.
If the items in the list are value types, they will be directly updated, if they are reference types, any changes will be reflected back in the referenced objects.
In the case where the source object is a true IEnumerable (i.e. not just a collection packaged an as enumerable), ToList() may NOT return the same object references as in the original IEnumerable. It will return a new List of objects, but those objects may not be the same or even Equal to the objects yielded by the IEnumerable when it is enumerated again
var objectList = objects.ToList();
objectList[0].SimpleInt=5;
This will update the original object as well. The new list will contain references to the objects contained within it, just like the original list. You can change the elements either and the update will be reflected in the other.
Now if you update a list (adding or deleting an item) that will not be reflected in the other list.
I don't see anywhere in the documentation that ToList() is always guaranteed to return a new list. If an IEnumerable is a List, it may be more efficient to check for this and simply return the same List.
The worry is that sometimes you may want to be absolutely sure that the returned List is != to the original List. Because Microsoft doesn't document that ToList will return a new List, we can't be sure (unless someone found that documentation). It could also change in the future, even if it works now.
new List(IEnumerable enumerablestuff) is guaranteed to return a new List. I would use this instead.
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);
}