How can I add an item to a IEnumerable<T> collection?

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:

1. 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>;

2. Create a list from IEnumerable (e.g. via LINQ extension method toList():

var list = enumerableInstance.toList();

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.

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);
}