I have data (a 'DogWalk') that I'm accessing with EF. At runtime, I'm determining the distance of the DogWalk relative to an inputted postcode. This works fine, however, I'm having trouble returning an IEnumerable sorted collection to the ListView.
The code is part of the method public IEnumerable<DogWalk> ListView1_GetData(). I have a list which contains all the walks
List<DogWalk> grabAllWalks = (from w in db.DogWalks.Include("Pictures")
select w).ToList();
I loop through the walks and add them to a struct, which stores {double distance, DogWalk theWalk}. I also create a List to store the anonymous struct objects:
var inRangeWalks = new List<InRangeWalks>();
foreach (var walk in grabAllWalks)
{
double dis = ....;
if (dis <= radius) //if within range, add to list
{
inRangeWalks.Add(new InRangeWalks(dis, walk));
}
}
I now sort the List based on postcodes distances:
inRangeWalks.Sort((x, y) => x.DistanceFromPostcode.CompareTo(y.DistanceFromPostcode));
The problem is that I need to return an IEnumerable collection. I could do something like:
IEnumerable<DogWalk> sortedWalks = inRangeWalks.Select(x => x.Walk);
But with IEnumerable not guaranteeing insertion order, this could be problematic.
What are my options?
Returning the result of your sort as an Enumerable like this:
var result = inRangeWalks.Sort((x, y) =>
x.DistanceFromPostcode.CompareTo(y.DistanceFromPostcode)).AsEnumerable();
should preserve the sort order.
However, List<T> implements IEnumerable so you should actually be able to use the list directly as the data source for your ListView.
We begin with a List<X>. Every object of X has an attribute x of type Y. Could you propose an elegant way to construct a List which is composed of the Z.x for every element Z of some List<X>?
I'm sure "manual" iteration over List<X> isn't necessary. Thanks for any advice.
If this is List<T>, then:
var newList = oldList.ConvertAll(item => item.x);
or with LINQ:
var newList = oldList.Select(item => item.x).ToList();
Note that in C# 2.0 the first version might need the generic type mentioned explicitly:
List<Y> newList = oldList.ConvertAll<Y>(delegate (X item) { return item.x; });
(but that is actually 100% identical to the first line)
There is also a static Array.ConvertAll which behaves similarly, but for arrays.
This is a long shot, I know...
Let's say I have a collection
List<MyClass> objects;
and I want to run the same method on every object in the collection, with or without a return value. Before Linq I would have said:
List<ReturnType> results = new List<ReturnType>();
List<int> FormulaResults = new List<int>();
foreach (MyClass obj in objects) {
results.Add(obj.MyMethod());
FormulaResults.Add(ApplyFormula(obj));
}
I would love to be able to do something like this:
List<ReturnType> results = new List<ReturnType>();
results.AddRange(objects.Execute(obj => obj.MyMethod()));
// obviously .Execute() above is fictitious
List<int> FormulaResults = new List<int>();
FormulaResults.AddRange(objects.Execute(obj => ApplyFormula(obj)));
I haven't found anything that will do this. Is there such a thing?
If there's nothing generic like I've posited above, at least maybe there's a way of doing it for the purposes I'm working on now: I have a collection of one object that has a wrapper class:
class WrapperClass {
private WrappedClass wrapped;
public WrapperClass(WrappedClass wc) {
this.wrapped = wc;
}
}
My code has a collection List<WrappedClass> objects and I want to convert that to a List<WrapperClass>. Is there some clever Linq way of doing this, without doing the tedious
List<WrapperClass> result = new List<WrapperClass>();
foreach (WrappedClass obj in objects)
results.Add(new WrapperClass(obj));
Thanks...
Would:
results.AddRange(objects.Select(obj => ApplyFormula(obj)));
do?
or (simpler)
var results = objects.Select(obj => ApplyFormula(obj)).ToList();
I think that the Select() extension method can do what you're looking for:
objects.Select( obj => obj.MyMethod() ).ToList(); // produces List<Result>
objects.Select( obj => ApplyFormula(obj) ).ToList(); // produces List<int>
Same thing for the last case:
objects.Select( obj => new WrapperClass( obj ) ).ToList();
If you have a void method which you want to call, here's a trick you can use with IEnumerable, which doesn't have a ForEach() extension, to create a similar behavior without a lot of effort.
objects.Select( obj => { obj.SomeVoidMethod(); false; } ).Count();
The Select() will produce a sequence of [false] values after invoking SomeVoidMethod() on each [obj] in the objects sequence. Since Select() uses deferred execution, we call the Count() extension to force each element in the sequence to be evaluated. It works quite well when you want something like a ForEach() behavior.
If the method MyMethod that you want to apply returns an object of type T then you can obtain an IEnumerable<T> of the result of the method via:
var results = objects.Select(o => o.MyMethod());
If the method MyMethod that you want to apply has return type void then you can apply the method via:
objects.ForEach(o => o.MyMethod());
This assumes that objects is of generic type List<>. If all you have is an IEnumerable<> then you can roll your own ForEach extension method or apply objects.ToList() first and use the above syntax .
The C# compiler maps a LINQ select onto the .Select extension method, defined over IEnumerable (or IQueryable which we'll ignore here). Actually, that .Select method is exactly the kind of projection function that you're after.
LBushkin is correct, but you can actually use LINQ syntax as well...
var query = from o in objects
select o.MyMethod();
You can also run a custom method using the marvelous Jon Skeet's morelinq library
For example if you had a text property on your MyClass that you needed to change in runtime using a method on the same class:
objects = objects.Pipe<MyClass>(class => class.Text = class.UpdateText()).ToList();
This method will now be implemented on every object in your list. I love morelinq!
http://www.hookedonlinq.com/UpdateOperator.ashx has an extended Update method you can use. Or you can use a select statement as posted by others.
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);
}
What is the best way to convert a List to SortedList? Any good way to do it without cycling through it? Any clever way to do it with an OrderBy()?
WRAP UP
Please read all answers and comments.
Do you mean:
you have a List<T> and wish it to be sorted in place?
you have a List<T> and wish to create another 'list' which is itself sorted
you have a List<T> and wish to make a SortedList<T,T> where the key is the same as the value
Assuming input:
var x = new List<int>() { 3, 2, 1 };
1 is trivial
x.Sort();
2 is trivial
// sx is an IOrderedEnumerable<T>, you can call ToList() on it if you want
var sx = x.OrderBy(i => i);
3 is trivial with a copy
var s = new SortedList<int,int>(t.ToDictionary(i => i));
and more efficiently:
var s = new SortedList<int,int>();
foreach (var i in x) { s[i] = [i]; }
I can't see why you would want to do 3 but there you go.
var list = new List<string>();
var sortedList = new SortedList<string, string>(list.ToDictionary(s => s));
Now I have no clue how efficient this is, but it's one line of code :) Also, in this example I just used the string itself as the selector. In a real scenario, you should know ahead of time what you'd like to use as a selector.
Understand that a List<T> is a smart array, and a SortedList<T, U> is a key/value binary tree. Since there's no relationship between their structures, there can't possibly be a more effective way to do it rather than simply taking each element from the list and putting it into the tree.
If you mean "sorted list" instead of "SortedList," then it's trivial to sort your list via either List.Sort() or an appropriate OrderBy().
List unsortedPersons = new List();
// ... Populate unsortedPersons ...
var sorted = from person in unsortedPersons
orderby person.Name
select person;
The LINQ gives you an ISortedEnumerable i believe, which may be good enough for your purposes.