I am still learning the basics of C# and found a task where you have to implement the method below, it is supposed to return the same sequence of strings but uppercase, and I assume by sequence it means an array.
But I am also somehow supposed to use IEnumerable for this.
I thought IEnumerable was an interface, how is it a type and a parameter in that method I am supposed to add logic into?
I searched and found that return type IEnumerable means it has to return something that can implement IEnumerable, but the parameters still confuse me, how do I use them to return uppercase? Do I use a foreach ?
using System;
using System.Collections.Generic;
namespace EnumerableTask
{
public class EnumerableManipulation
{
/// <summary> Transforms all strings to upper case.</summary>
/// <param name="data">Source string sequence.</param>
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
}
}
You can use Linq Select() which would return an IEnumerable. ie:
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
return data.Select(d => d.ToUpper());
}
EDIT: You can check Linq documentation or my blog starting from the oldest post (Feb 2009) at:
FoxSharp
You can also use foreach and yield:
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
foreach (var item in data)
yield return item?.ToUpper();
}
IEnumerable<T> is a generic interface. It represents a collection of objects of type T. In your case, data is a collection of string that can be enumerated.
Enumerate a IEnumerable
To enumerate through the 'data' and acccess the elements one after the others, there is several possibilities. Here are two examples.
GetEnumerator()
GetEnumerator() gives you a way to enumerate through the IEnumerable<T>. enumerator.Current allows you to access the current element and enumerator.MoveNext() gives you a way to move to the next element in the collection.
At first use call MoveNext() method to place the enumerator at the first element of the IEnumerable<T>. While there is still elements to enumerate, MoveNext() will return true. When the end is reached, MoveNext() returns false.
Here is an example of how it is typically used.
IEnumerator<string> enumerator = data.GetEnumerator();
while(enumerator.MoveNext())
{
string text = data.Current;
/* Do something */
}
enumerator.Dispose();
foreach
Using foreach takes care of all the complexity of the enumerator process. It is in fact recommended to use foreach instead of directly manipulating the enumerator (see Microsoft Docs, in Remarks).
foreach(string text in data)
{
/* Do something */
}
Return a IEnumerable
Because IEnumerable<T> is an interface, you cannot "construct" it as you would with a class or a struct. Hence to return a IEnumerable<T>, you have to instanciate an actual class or a struct that implements this interface. For example:
public IEnumerable<string> Foo()
{
List<string> list = new List<string>();
/* Do something on the list*/
return list;
}
or
public IEnumerable<string> Foo(int count)
{
string[] array = new string[count];
/* Do something on the array*/
return array;
}
You can do this because List<T> and Array implement the interface IEnumerable<string>.
Related
I was looking into the IEnumerable and trying the example given in this link. I understand that when we iterate using foreach the GetEnumerator() method gets call because my List class has implemented IEnumerable (or may be I am wrong).
public class List<T> : IEnumerable
{
private T[] _collection;
public List(T[] inputs)
{
_collection = new T[inputs.Length];
for (int i = 0; i < inputs.Length; i++)
{
_collection[i] = inputs[i];
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return (IEnumerator)GetEnumerator();
}
public CollectionEnum<T> GetEnumerator()
{
return new CollectionEnum<T>(_collection);
}
}
public class CollectionEnum<T> : IEnumerator
{
public T[] _collection ;
int position = -1;
public CollectionEnum(T[] list)
{
_collection = list;
}
public bool MoveNext()
{
position++;
return (position < _collection.Length);
}
//implementation on Current and Reset
}
Then, it is also mentioned that implementation of IEnumerable is not required to iterate using foreach. So, in the above code if I remove the implementation of IEnumerable the foreach must work. So my List<> class looks like
public class List<T>
{
private T[] _collection;
public List(T[] persons)
{
_collection = new T[persons.Length];
for (int i = 0; i < persons.Length; i++)
{
_collection[i] = persons[i];
}
}
public CollectionEnum<T> GetEnumerator()
{
return new CollectionEnum<T>(_collection);
}
}
And which does work. Now I did not understand how foreach knows my class has a method call GetEnumerator() which returns a IEnumerator type.
The article you linked to provided the explanation to you already in the remarks.
If your collection does not implement IEnumerable, you must still
follow the iterator pattern to support this syntax by providing a
GetEnumerator method that returns an interface, class or struct....that contains a Current property, and MoveNext and Reset methods as
described by IEnumerator, but the class does not have to implement
IEnumerator.
As Nkosi mentions, in interpreting foreach, the compiler looks for a pattern originally represented in IEnumerable and IEnumerator.
Earlier versions of the C# compiler required the implementation of these interfaces. However, with the advent of LINQ, the language and compiler have been adjusted to do more of recognizing patterns.
For instance, you can create a LINQ query around an object that has a Where method but doesn't implement IEnumerable.
Similarly, you can await any object that follows the Awaitable Pattern not just Task or Task.
Once you understand the particular methods the compiler is looking for to satisfy a pattern it becomes easier to wrap your head around why IEnumerable is not required for foreach. Remember also that foreach is really just syntactic sugar for traversing a collection of objects.
I would like to implement a generic IEnumerator<T> extension method that returns the next element in the IEnumerable as followed:
var list = new List<MyClass>();
var enumerator = list.GetEnumerator();
var nextItem1 = enumerator.GetNext();
var nextItem2 = enumerator.GetNext();
// Instead of calling enumerator.MoveNext() and then reading the current item.
I tried to do this but it won't compile:
public static T GetNext(this IEnumerator<T> list)
{
list.MoveNext();
return list.Current;
}
Where did I go wrong? Thanks.
You appear to be missing the generic type definition on the method signature:
public static T GetNext<T>(this IEnumerator<T> list)
{
// ---- this bit---^^^
}
It needs to be there assuming there is no class level generic type with the same name. Otherwise it looks fine.
However...
I personally wouldn't do it this way as you are not listening to the result of MoveNext. Unless you can make assumptions about the expected values, there is no way for you to tell if the iterator reached the end of the set. For most common uses, this won't be a viable approach.
I actually went the other route and implemented an AsEnumerable extension method that simply iterates manually, but from the outside allows you to do stuff like foreach (var item in iterator.AsEnumerable()) and doesn't ignore the MoveNext result:
public static IEnumerable<TSource> AsEnumerable<TSource>(this IEnumerator<TSource> iterator)
{
while (iterator.MoveNext())
{
yield return iterator.Current;
}
}
When given an d you could be dealing with a fixed sequence like a list or array, an AST that will enumerate some external datasource, or even an AST on some existing collection. Is there a way to safely "materialize" the enumerable so that enumeration operations like foreach, count, etc. don't execute the AST each time?
I've often used .ToArray() to create this represenation but if the underlying storage is already a list or other fixed sequence, that seems like wasted copying. It would be nice if i could do
var enumerable = someEnumerable.Materialize();
if(enumberable.Any() {
foreach(var item in enumerable) {
...
}
} else {
...
}
Without having to worry that .Any() and foreach try to enumerate the sequence twice and without it unccessarily copying the enumerable.
Easy enough:
public static IList<TSource> Materialize<TSource>(this IEnumerable<TSource> source)
{
if (source is IList<TSource>)
{
// Already a list, use it as is
return (IList<TSource>)source;
}
else
{
// Not a list, materialize it to a list
return source.ToList();
}
}
Original answer:
Same as Thomas's answer, just a bit better according to me:
public static ICollection<T> Materialize<T>(this IEnumerable<T> source)
{
// Null check...
return source as ICollection<T> ?? source.ToList();
}
Please note that this tend to return the existing collection itself if its a valid collection type, or produces a new collection otherwise. While the two are subtly different, I don't think it could be an issue.
Edit:
Today this is a better solution:
public static IReadOnlyCollection<T> Materialize<T>(this IEnumerable<T> source)
{
// Null check...
switch (source)
{
case ICollection<T> collection:
return new ReadOnlyCollectionAdapter<T>(collection);
case IReadOnlyCollection<T> readOnlyCollection:
return readOnlyCollection;
default:
return source.ToList();
}
}
public class ReadOnlyCollectionAdapter<T> : IReadOnlyCollection<T>
{
readonly ICollection<T> m_source;
public ReadOnlyCollectionAdapter(ICollection<T> source) => m_source = source;
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
public int Count => m_source.Count;
public IEnumerator<T> GetEnumerator() => m_source.GetEnumerator();
}
Check out this blog post I wrote a couple of years ago: http://www.fallingcanbedeadly.com/posts/crazy-extention-methods-tolazylist
In it, I define a method called ToLazyList that effectively does what you're looking for.
As written, it will eventually make a full copy of the input sequence, although you could tweak it so that instances of IList don't get wrapped in a LazyList, which would prevent this from happening (this action, however, would carry with it the assumption that any IList you get is already effectively memoized).
I have a class which has two HashSet<String> collections as private members. Other classes in my code would like to be able to iterate over those HashSets and read their contents. I don't want to write a standard getter because another class could still do something like myClass.getHashSet().Clear(); Is there any other way to expose the elements of my HashSets to iteration without exposing the reference to the HashSet itself? I'd love to be able to do this in a way that is compatible with for-each loops.
Assuming you're using .NET 3.5, one alternative to writing the yield code yourself is to call a LINQ method. For example:
public IEnumerable<string> HashSet
{
get { return privateMember.Select(x => x); }
}
or
public IEnumerable<string> HashSet
{
get { return privateMember.Skip(0); }
}
There are various LINQ operators which could be used like this - using Skip(0) is probably the most efficient, as after the initial "skip 0 values" loop, it's probably just the foreach/yield return loop shown in the other answers. The Select version will call the no-op projection delegate for each item yielded. The chances of this difference being significant are astronomically small, however - I suggest you go with whatever makes the code clearest to you.
Expose a IEnumerable<T> property:
public IEnumerable<whatevertype> MyHashSet {
get {
return this.myHashSet;
}
}
Of course, the user of this code can cast that IEnumerable<T> to a HashSet<T> and edit elements, so to be on the safe side (while hurting performance), you can do:
public IEnumerable<whatevertype> MyHashSet {
get {
return this.myHashSet.ToArray();
}
}
or:
public IEnumerable<whatevertype> MyHashSet {
get {
foreach(var item in this.myHashSet) {
yield return item;
}
}
}
A more performant method of protection, but less convenient to the caller, is to return an IEnumerator<T>:
public IEnumerator<whatevertype> GetMyHashSetEnumerator() {
return this.myHashSet.GetEnumerator();
}
Add a method/property like this to avoid exposing the actual container:
public IEnumerable EnumerateFirst()
{
foreach( var item in hashSet )
yield return item;
}
You can also use the Select method to create a wrapper than can't be cast back to HashSet<T>:
public IEnumerable<int> Values
{
get { return _values.Select(value => value);
}
This avoids iterating over _values twice, as you would with .ToArray(), but keeps the implementation to a single clean line.
You may also provide a sequence like this:
public IEnumerable<string> GetHashSetOneValues()
{
foreach (string value in hashSetOne)
yield return value;
}
This method may then be called within a foreach loop:
foreach (string value in myObject.GetHashSetOneValues())
DoSomething(value);
This might be quite a bit too late to the party but the easiest way today would be to use Linq. Instead of writing
public IEnumerable<string> GetValues()
{
foreach(var elem in list)
yield return elem;
}
you can write
public IEnumerable<string> GetValues() => list;
Make your getter expose the HashSet as IEnumerable.
private HashSet<string> _mine;
public IEnumerable<string> Yours
{
get { return _mine; }
}
If the generic type is mutable, then that can still be modified, but no items can be added or removed from your HashSet.
I have a class property exposing an internal IList<> through
System.Collections.ObjectModel.ReadOnlyCollection<>
How can I pass a part of this ReadOnlyCollection<> without copying elements into a new array (I need a live view, and the target device is short on memory)? I'm targetting Compact Framework 2.0.
Try a method that returns an enumeration using yield:
IEnumerable<T> FilterCollection<T>( ReadOnlyCollection<T> input ) {
foreach ( T item in input )
if ( /* criterion is met */ )
yield return item;
}
These foreach samples are fine, though you can make them much more terse if you're using .NET 3.5 and LINQ:
return FullList.Where(i => IsItemInPartialList(i)).ToList();
You can always write a class that implements IList and forwards all calls to the original list (so it doesn't have it's own copy of the data) after translating the indexes.
You could use yield return to create a filtered list
IEnumerable<object> FilteredList()
{
foreach( object item in FullList )
{
if( IsItemInPartialList( item )
yield return item;
}
}
Depending on how you need to filter the collection, you may want to create a class that implements IList (or IEnumerable, if that works for you) but that mucks about with the indexing and access to only return the values you want. For example
class EvenList: IList
{
private IList innerList;
public EvenList(IList innerList)
{
this.innerList = innerList;
}
public object this[int index]
{
get { return innerList[2*i]; }
set { innerList[2*i] = value; }
}
// and similarly for the other IList methods
}
How do the filtered elements need to be accessed? If it's through an Iterator then maybe you could write a custom iterator that skips the elements you don't want publicly visible?
If you need to provide a Collection then you might need to write your own Collection class, which just proxies to the underlying Collection, but prevents access to the elements you don't want publicly visible.
(Disclaimer: I'm not very familiar with C#, so these are general answers. There may be more specific answers to C# that work better)