In my application there is a List<MyItem> with a getter only:
public List<MyItem> myList
{
get
{
MyHost.GetItemFromID(_i1); //this may be a long operation
MyHost.GetItemFromID(_i2);
MyHost.GetItemFromID(_i3);
MyHost.GetItemFromID(_i4);
MyHost.GetItemFromID(_i5);
}
}
This list needs sometimes to be retrieved as whole and other times only certain item has to be accessed: i.e. myList[3]. Is there a way of not building the entire list as I only need the fourth item?
You could wrap the list with a class, say 'myListContainer' and overload its '[]' operator so that you could do something like this:
myListContainer[3]
which will invoke the call
MyHost.GetItemFromID(_i3);
and return the desired list item.
I'll add a full example if needed
EDIT
public class myListContainer
{
public MyItem this[int i]
{
get
{
return MyHost.GetItemFromID(i);
}
}
}
and add method to get the entire list.
You could return IEnumerable<MyItem> and use yield return:
public IEnumerable<MyItem> MyItems
{
get
{
yield return MyHost.GetItemFromID(_i1); //this may be a long operation
yield return MyHost.GetItemFromID(_i2);
yield return MyHost.GetItemFromID(_i3);
yield return MyHost.GetItemFromID(_i4);
yield return MyHost.GetItemFromID(_i5);
}
}
Then it is using deferred execution and you can write:
var fourItems = MyItems.Take(4).ToList();
Note that it might be a good idea to change the order of execution if the order doesn't matter and only the first call of GetItemFromID takes more time than the others:
yield return MyHost.GetItemFromID(_i2);
yield return MyHost.GetItemFromID(_i3);
yield return MyHost.GetItemFromID(_i4);
yield return MyHost.GetItemFromID(_i5);
yield return MyHost.GetItemFromID(_i1); //this may be a long operation
I think i have misuderstood the requirement. I've read "sometimes I only need the fourth item" as "only need four items"
So you could use my aproach but with ElementAt:
MyItem fourthItem = MyItems.ElementAt(3);
If you don't know if there's a fourth use ElementAtOrdefault.
I don't know what type your keys for _i1 etc are, but let's assume they are ints. Then you could do this:
public IEnumerable<MyItem> GetItems(params int[] keys)
{
return keys.Select(key => MyHost.GetItemFromID(key));
}
Which you could call like this:
var myItems = GetItems(_i2, _i3, _i5).ToList();
or
var myItems = GetItems(_i3).ToArray();
and so on.
Note: If you only ever want a list returned, you can do the conversion inside GetItems() itself:
public List<MyItem> GetItems(params int[] keys)
{
return keys.Select(key => MyHost.GetItemFromID(key)).ToList();
}
This approach requires a method rather than a property, though.
Note:
This assumes by '4th item' you mean an ordinal number, not the actual id itself.
Build a map in advance and return only items you wish to.
List<int> map = new List<int>
{
_i1,
_i2,
_i3,
_i4,
_i5,
};
public IEnumerable<MyItem> myList(params int[] indices)
{
if (!indices.Any())
return map.Select(MyHost.GetItemFromID);
return indices.Select(i => MyHost.GetItemFromID(map[i]));
}
// so you call
myList(); // for all items; decide on this API, may be separate to two methods?
myList(0); // or
myList(4); // or
myList(1, 3); // all loaded only on demand
Use a dictionary if you want more control over indexing. Ideally, for some reason this looks like you should be passing all IDs to DB in one go and SQL directly handle it, if that is the case.
How much space is reserved to the underlying collection behind a method using yield return syntax WHEN I PERFORM a ToList() on it? There's a chance it will reallocate and thus decrease performance if compared to the standard approach where i create a list with predefined capacity?
The two scenarios:
public IEnumerable<T> GetList1()
{
foreach( var item in collection )
yield return item.Property;
}
public IEnumerable<T> GetList2()
{
List<T> outputList = new List<T>( collection.Count() );
foreach( var item in collection )
outputList.Add( item.Property );
return outputList;
}
yield return does not create an array that has to be resized, like what List does; instead, it creates an IEnumerable with a state machine.
For instance, let's take this method:
public static IEnumerable<int> Foo()
{
Console.WriteLine("Returning 1");
yield return 1;
Console.WriteLine("Returning 2");
yield return 2;
Console.WriteLine("Returning 3");
yield return 3;
}
Now let's call it and assign that enumerable to a variable:
var elems = Foo();
None of the code in Foo has executed yet. Nothing will be printed on the console. But if we iterate over it, like this:
foreach(var elem in elems)
{
Console.WriteLine( "Got " + elem );
}
On the first iteration of the foreach loop, the Foo method will be executed until the first yield return. Then, on the second iteration, the method will "resume" from where it left off (right after the yield return 1), and execute until the next yield return. Same for all subsequent elements.
At the end of the loop, the console will look like this:
Returning 1
Got 1
Returning 2
Got 2
Returning 3
Got 3
This means you can write methods like this:
public static IEnumerable<int> GetAnswers()
{
while( true )
{
yield return 42;
}
}
You can call the GetAnswers method, and every time you request an element, it'll give you 42; the sequence never ends. You couldn't do this with a List, because lists have to have a finite size.
How much space is reserved to the underlying collection behind a method using yield return syntax?
There's no underlying collection.
There's an object, but it isn't a collection. Just how much space it will take up depends on what it needs to keep track of.
There's a chance it will reallocate
No.
And thus decrease performance if compared to the standard approach where i create a list with predefined capacity?
It will almost certainly take up less memory than creating a list with a predefined capacity.
Let's try a manual example. Say we had the following code:
public static IEnumerable<int> CountToTen()
{
for(var i = 1; i != 11; ++i)
yield return i;
}
To foreach through this will iterate through the numbers 1 to 10 inclusive.
Now let's do this the way we would have to if yield did not exist. We'd do something like:
private class CountToTenEnumerator : IEnumerator<int>
{
private int _current;
public int Current
{
get
{
if(_current == 0)
throw new InvalidOperationException();
return _current;
}
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
if(_current == 10)
return false;
_current++;
return true;
}
public void Reset()
{
throw new NotSupportedException();
// We *could* just set _current back, but the object produced by
// yield won't do that, so we'll match that.
}
public void Dispose()
{
}
}
private class CountToTenEnumerable : IEnumerable<int>
{
public IEnumerator<int> GetEnumerator()
{
return new CountToTenEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
public static IEnumerable<int> CountToTen()
{
return new CountToTenEnumerable();
}
Now, for a variety of reasons this is quite different to the code you're likely to get from the version using yield, but the basic principle is the same. As you can see there are two allocations involved of objects (same number as if we had a collection and then did a foreach on that) and the storage of a single int. In practice we can expect yield to store a few more bytes than that, but not a lot.
Edit: yield actually does a trick where the first GetEnumerator() call on the same thread that obtained the object returns that same object, doing double service for both cases. Since this covers over 99% of use cases yield actually does one allocation rather than two.
Now let's look at:
public IEnumerable<T> GetList1()
{
foreach( var item in collection )
yield return item.Property;
}
While this would result in more memory used than just return collection, it won't result in a lot more; the only thing the enumerator produced really needs to keep track of is the enumerator produced by calling GetEnumerator() on collection and then wrapping that.
This is going to be massively less memory than that of the wasteful second approach you mention, and much faster to get going.
Edit:
You've changed your question to include "syntax WHEN I PERFORM a ToList() on it", which is worth considering.
Now, here we need to add a third possibility: Knowledge of the collection's size.
Here, there is the possibilty that using new List(capacity) will prevent allocations of the list being built. That can indeed be a considerable saving.
If the object that has ToList called on it implements ICollection<T> then ToList will end up first doing a single allocation of an internal array of T and then calling ICollection<T>.CopyTo().
This would mean that your GetList2 would result in a faster ToList() than your GetList1.
However, your GetList2 has already wasted time and memory doing what ToList() will do with the results of GetList1 anyway!
What it should have done here was just return new List<T>(collection); and be done with it.
If though we need to actually do something inside GetList1 or GetList2 (e.g. convert elements, filter elements, track averages, and so on) then GetList1 is going to be faster and lighter on memory. Much lighter if we never call ToList() on it, and slightly ligher if we do call ToList() because again, the faster and lighter ToList() is offset by GetList2 being slower and heavier in the first place by exactly the same amount.
I would like to use an IEnumerable to generate a sequence of values -- specifically, a list of Excel-like column headers.
private IEnumerable<string> EnumerateSymbolNames()
{
foreach (var sym in _symbols)
{
yield return sym;
}
foreach (var sym1 in _symbols)
{
foreach (var sym2 in _symbols)
{
yield return sym1 + sym2;
}
}
yield break;
}
private readonly string[] _symbols = new string[] { "A", "B", "C", ...};
This works fine if I fetch the values from a foreach loop. But what I want is to use the iterator block as a state machine and fetch the next available column header in response to a user action. And this -- consuming the generated values -- is where I've run into trouble.
So far I've tried
return EnumerateSymbolNames().Take(1).FirstOrDefault();
return EnumerateSymbolNames().Take(1).SingleOrDefault();
return EnumerateSymbolNames().FirstOrDefault();
var enumerator = EnumerateSymbolNames().GetEnumerator();
enumerator.MoveNext();
return enumerator.Current;
... but none of these have worked. (All repeatedly return "A".)
Based on the responses to this question, I'm wondering what I want is even possible -- although several of the responses to that post suggest techniques similar to my last one.
And no, this is not a homework assignment :)
When you use GetEnumerator, you need to use the same enumerator for each iteration. If you call GetEnumerator a second time, it will start over at the beginning of the collection.
If you want to use Take, you must first Skip the number of records that have already been processed.
This code worked for me...
var states = EnumerateSymbolNames();
var stateMachine = states.GetEnumerator();
do
{
//something
} while (stateMachine.MoveNext());
When print the results within that loop, it successfully produced the following output:
A
B
C
...
AA
AB
AC
...
BA
BB
...
Which is what I think you intended...
As both #cadrell0's answer and #Mr Steak's comment point out, what I needed to do was retain a reference to the enumerator returned by EnumerateSymbolNames().GetEnumerator().
When you're in a foreach loop, this is done implicitly for you: the iterator variable wraps an enumerator, which (I'm assuming) is scoped locally to the loop. So -- and this is the key piece -- when the iterator block does (the equivalent of)
enumerator.MoveNext();
return enumerator.Current;
... you're always using the same enumerator. Whereas what I was doing was creating / obtaining a different (new) enumerator every time. And predictably, it always started out at the first position in the sequence. This was probably obvious to everyone but me; it seems obvious to me as well in hindsight. (I was thinking of the enumerator as sort of a singleton property of the sequence, assuming that I'd be getting the same enumerator back every time.)
The following does what I want:
public class SymbolGenerator
{
private readonly string[] _symbols = { "A", "B", "C", ... };
private readonly IEnumerator<string> _enumerator;
public SymbolGenerator()
{
_enumerator = EnumerateSymbols().GetEnumerator();
}
public string GetNextSymbol()
{
_enumerator.MoveNext();
return _enumerator.Current;
}
private IEnumerable<string> EnumerateSymbols()
{
// (unchanged)
}
}
What's the best practice to check if a collection has items?
Here's an example of what I have:
var terminalsToSync = TerminalAction.GetAllTerminals();
if(terminalsToSync.Any())
SyncTerminals(terminalsToSync);
else
GatewayLogAction.WriteLogInfo(Messages.NoTerminalsForSync);
The GetAllTerminals() method will execute a stored procedure and, if we return a result, (Any() is true), SyncTerminals() will loop through the elements; thus enumerating it again and executing the stored procedure for the second time.
What's the best way to avoid this?
I'd like a good solution that can be used in other cases too; possibly without converting it to List.
Thanks in advance.
I would probably use a ToArray call, and then check Length; you're going to enumerate all the results anyway so why not do it early? However, since you've said you want to avoid early realisation of the enumerable...
I'm guessing that SyncTerminals has a foreach, in which case you can write it something like this:
bool any = false;
foreach(var terminal in terminalsToSync)
{
if(!any)any = true;
//....
}
if(!any)
GatewayLogAction.WriteLogInfo(Messages.NoTerminalsForSync);
Okay, there's a redundant if after the first loop, but I'm guessing the cost of an extra few CPU cycles isn't going to matter much.
Equally, you could do the iteration the old way and use a do...while loop and GetEnumerator; taking the first iteration out of the loop; that way there are literally no wasted operations:
var enumerator = terminalsToSync.GetEnumerator();
if(enumerator.MoveNext())
{
do
{
//sync enumerator.Current
} while(enumerator.MoveNext())
}
else
GatewayLogAction.WriteLogInfo(Messages.NoTerminalsForSync);
How about this, which still defers execution, but buffers it once executed:
var terminalsToSync = TerminalAction.GetAllTerminals().Lazily();
with:
public static class LazyEnumerable {
public static IEnumerable<T> Lazily<T>(this IEnumerable<T> source) {
if (source is LazyWrapper<T>) return source;
return new LazyWrapper<T>(source);
}
class LazyWrapper<T> : IEnumerable<T> {
private IEnumerable<T> source;
private bool executed;
public LazyWrapper(IEnumerable<T> source) {
if (source == null) throw new ArgumentNullException("source");
this.source = source;
}
IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
public IEnumerator<T> GetEnumerator() {
if (!executed) {
executed = true;
source = source.ToList();
}
return source.GetEnumerator();
}
}
}
Personally i wouldnt use an any here, foreach will simply not loop through any items if the collection is empty, so i would just do it like that. However i would recommend that you check for null.
If you do want to pre-enumerate the set use .ToArray() eg will only enumerate once:
var terminalsToSync = TerminalAction.GetAllTerminals().ToArray();
if(terminalsToSync.Any())
SyncTerminals(terminalsToSync);
var terminalsToSync = TerminalAction.GetAllTerminals().ToList();
if(terminalsToSync.Any())
SyncTerminals(terminalsToSync);
else
GatewayLogAction.WriteLogInfo(Messages.NoTerminalsForSync);
.Length or .Count is faster since it doesn't need to go through the GetEnumerator()/MoveNext()/Dispose() required by Any()
Here's another way of approaching this problem:
int count = SyncTerminals(terminalsToSync);
if(count == 0) GatewayLogAction.WriteLogInfo(Messages.NoTerminalsForSync);
where you change SyncTerminals to do:
int count = 0;
foreach(var obj in terminalsToSync) {
count++;
// some code
}
return count;
Nice and simple.
All the caching solutions here are caching all items when the first item is being retrieved. It it really lazy if you cache each single item while the items of the list are is iterated.
The difference can be seen in this example:
public class LazyListTest
{
private int _count = 0;
public void Test()
{
var numbers = Enumerable.Range(1, 40);
var numbersQuery = numbers.Select(GetElement).ToLazyList(); // Cache lazy
var total = numbersQuery.Take(3)
.Concat(numbersQuery.Take(10))
.Concat(numbersQuery.Take(3))
.Sum();
Console.WriteLine(_count);
}
private int GetElement(int value)
{
_count++;
// Some slow stuff here...
return value * 100;
}
}
If you run the Test() method, the _count is only 10. Without caching it would be 16 and with .ToList() it would be 40!
An example of the implementation of LazyList can be found here.
If you're seeing two procedure calls for the evaluation of whatever GetAllTerminals() returns, this means that the procedure's result isn't being cached. Without knowing what data-access strategy you're using, this is quite hard to fix in a general way.
The simplest solution, as you've alluded, is to copy the result of the call before you perform any other operations. If you wanted to, you could neatly wrap this behaviour up in an IEnumerable<T> which executes the inner enumerable call just once:
public class CachedEnumerable<T> : IEnumerable<T>
{
public CachedEnumerable<T>(IEnumerable<T> enumerable)
{
result = new Lazy<List<T>>(() => enumerable.ToList());
}
private Lazy<List<T>> result;
public IEnumerator<T> GetEnumerator()
{
return this.result.Value.GetEnumerator();
}
System.Collections.IEnumerable GetEnumerator()
{
return this.GetEnumerator();
}
}
Wrap the result in an instance of this type and it will not evaluate the inner enumerable multiple times.
In the How Can I Expose Only a Fragment of IList<> question one of the answers had the following code snippet:
IEnumerable<object> FilteredList()
{
foreach(object item in FullList)
{
if(IsItemInPartialList(item))
yield return item;
}
}
What does the yield keyword do there? I've seen it referenced in a couple places, and one other question, but I haven't quite figured out what it actually does. I'm used to thinking of yield in the sense of one thread yielding to another, but that doesn't seem relevant here.
The yield contextual keyword actually does quite a lot here.
The function returns an object that implements the IEnumerable<object> interface. If a calling function starts foreaching over this object, the function is called again until it "yields". This is syntactic sugar introduced in C# 2.0. In earlier versions you had to create your own IEnumerable and IEnumerator objects to do stuff like this.
The easiest way understand code like this is to type-in an example, set some breakpoints and see what happens. Try stepping through this example:
public void Consumer()
{
foreach(int i in Integers())
{
Console.WriteLine(i.ToString());
}
}
public IEnumerable<int> Integers()
{
yield return 1;
yield return 2;
yield return 4;
yield return 8;
yield return 16;
yield return 16777216;
}
When you step through the example, you'll find the first call to Integers() returns 1. The second call returns 2 and the line yield return 1 is not executed again.
Here is a real-life example:
public IEnumerable<T> Read<T>(string sql, Func<IDataReader, T> make, params object[] parms)
{
using (var connection = CreateConnection())
{
using (var command = CreateCommand(CommandType.Text, sql, connection, parms))
{
command.CommandTimeout = dataBaseSettings.ReadCommandTimeout;
using (var reader = command.ExecuteReader())
{
while (reader.Read())
{
yield return make(reader);
}
}
}
}
}
Iteration. It creates a state machine "under the covers" that remembers where you were on each additional cycle of the function and picks up from there.
Yield has two great uses,
It helps to provide custom iteration without creating temp collections.
It helps to do stateful iteration.
In order to explain above two points more demonstratively, I have created a simple video you can watch it here
Recently Raymond Chen also ran an interesting series of articles on the yield keyword.
The implementation of iterators in C# and its consequences (part 1)
The implementation of iterators in C# and its consequences (part 2)
The implementation of iterators in C# and its consequences (part 3)
The implementation of iterators in C# and its consequences (part 4)
While it's nominally used for easily implementing an iterator pattern, but can be generalized into a state machine. No point in quoting Raymond, the last part also links to other uses (but the example in Entin's blog is esp good, showing how to write async safe code).
At first sight, yield return is a .NET sugar to return an IEnumerable.
Without yield, all the items of the collection are created at once:
class SomeData
{
public SomeData() { }
static public IEnumerable<SomeData> CreateSomeDatas()
{
return new List<SomeData> {
new SomeData(),
new SomeData(),
new SomeData()
};
}
}
Same code using yield, it returns item by item:
class SomeData
{
public SomeData() { }
static public IEnumerable<SomeData> CreateSomeDatas()
{
yield return new SomeData();
yield return new SomeData();
yield return new SomeData();
}
}
The advantage of using yield is that if the function consuming your data simply needs the first item of the collection, the rest of the items won't be created.
The yield operator allows the creation of items as it is demanded. That's a good reason to use it.
A list or array implementation loads all of the items immediately whereas the yield implementation provides a deferred execution solution.
In practice, it is often desirable to perform the minimum amount of work as needed in order to reduce the resource consumption of an application.
For example, we may have an application that process millions of records from a database. The following benefits can be achieved when we use IEnumerable in a deferred execution pull-based model:
Scalability, reliability and predictability are likely to improve since the number of records does not significantly affect the application’s resource requirements.
Performance and responsiveness are likely to improve since processing can start immediately instead of waiting for the entire collection to be loaded first.
Recoverability and utilisation are likely to improve since the application can be stopped, started, interrupted or fail. Only the items in progress will be lost compared to pre-fetching all of the data where only using a portion of the results was actually used.
Continuous processing is possible in environments where constant workload streams are added.
Here is a comparison between build a collection first such as a list compared to using yield.
List Example
public class ContactListStore : IStore<ContactModel>
{
public IEnumerable<ContactModel> GetEnumerator()
{
var contacts = new List<ContactModel>();
Console.WriteLine("ContactListStore: Creating contact 1");
contacts.Add(new ContactModel() { FirstName = "Bob", LastName = "Blue" });
Console.WriteLine("ContactListStore: Creating contact 2");
contacts.Add(new ContactModel() { FirstName = "Jim", LastName = "Green" });
Console.WriteLine("ContactListStore: Creating contact 3");
contacts.Add(new ContactModel() { FirstName = "Susan", LastName = "Orange" });
return contacts;
}
}
static void Main(string[] args)
{
var store = new ContactListStore();
var contacts = store.GetEnumerator();
Console.WriteLine("Ready to iterate through the collection.");
Console.ReadLine();
}
Console Output
ContactListStore: Creating contact 1
ContactListStore: Creating contact 2
ContactListStore: Creating contact 3
Ready to iterate through the collection.
Note: The entire collection was loaded into memory without even asking for a single item in the list
Yield Example
public class ContactYieldStore : IStore<ContactModel>
{
public IEnumerable<ContactModel> GetEnumerator()
{
Console.WriteLine("ContactYieldStore: Creating contact 1");
yield return new ContactModel() { FirstName = "Bob", LastName = "Blue" };
Console.WriteLine("ContactYieldStore: Creating contact 2");
yield return new ContactModel() { FirstName = "Jim", LastName = "Green" };
Console.WriteLine("ContactYieldStore: Creating contact 3");
yield return new ContactModel() { FirstName = "Susan", LastName = "Orange" };
}
}
static void Main(string[] args)
{
var store = new ContactYieldStore();
var contacts = store.GetEnumerator();
Console.WriteLine("Ready to iterate through the collection.");
Console.ReadLine();
}
Console Output
Ready to iterate through the collection.
Note: The collection wasn't executed at all. This is due to the "deferred execution" nature of IEnumerable. Constructing an item will only occur when it is really required.
Let's call the collection again and obverse the behaviour when we fetch the first contact in the collection.
static void Main(string[] args)
{
var store = new ContactYieldStore();
var contacts = store.GetEnumerator();
Console.WriteLine("Ready to iterate through the collection");
Console.WriteLine("Hello {0}", contacts.First().FirstName);
Console.ReadLine();
}
Console Output
Ready to iterate through the collection
ContactYieldStore: Creating contact 1
Hello Bob
Nice! Only the first contact was constructed when the client "pulled" the item out of the collection.
yield return is used with enumerators. On each call of yield statement, control is returned to the caller but it ensures that the callee's state is maintained. Due to this, when the caller enumerates the next element, it continues execution in the callee method from statement immediately after the yield statement.
Let us try to understand this with an example. In this example, corresponding to each line I have mentioned the order in which execution flows.
static void Main(string[] args)
{
foreach (int fib in Fibs(6))//1, 5
{
Console.WriteLine(fib + " ");//4, 10
}
}
static IEnumerable<int> Fibs(int fibCount)
{
for (int i = 0, prevFib = 0, currFib = 1; i < fibCount; i++)//2
{
yield return prevFib;//3, 9
int newFib = prevFib + currFib;//6
prevFib = currFib;//7
currFib = newFib;//8
}
}
Also, the state is maintained for each enumeration. Suppose, I have another call to Fibs() method then the state will be reset for it.
Intuitively, the keyword returns a value from the function without leaving it, i.e. in your code example it returns the current item value and then resumes the loop. More formally, it is used by the compiler to generate code for an iterator. Iterators are functions that return IEnumerable objects. The MSDN has several articles about them.
If I understand this correctly, here's how I would phrase this from the perspective of the function implementing IEnumerable with yield.
Here's one.
Call again if you need another.
I'll remember what I already gave you.
I'll only know if I can give you another when you call again.
Here is a simple way to understand the concept:
The basic idea is, if you want a collection that you can use "foreach" on, but gathering the items into the collection is expensive for some reason (like querying them out of a database), AND you will often not need the entire collection, then you create a function that builds the collection one item at a time and yields it back to the consumer (who can then terminate the collection effort early).
Think of it this way: You go to the meat counter and want to buy a pound of sliced ham. The butcher takes a 10-pound ham to the back, puts it on the slicer machine, slices the whole thing, then brings the pile of slices back to you and measures out a pound of it. (OLD way).
With yield, the butcher brings the slicer machine to the counter, and starts slicing and "yielding" each slice onto the scale until it measures 1-pound, then wraps it for you and you're done. The Old Way may be better for the butcher (lets him organize his machinery the way he likes), but the New Way is clearly more efficient in most cases for the consumer.
The yield keyword allows you to create an IEnumerable<T> in the form on an iterator block. This iterator block supports deferred executing and if you are not familiar with the concept it may appear almost magical. However, at the end of the day it is just code that executes without any weird tricks.
An iterator block can be described as syntactic sugar where the compiler generates a state machine that keeps track of how far the enumeration of the enumerable has progressed. To enumerate an enumerable, you often use a foreach loop. However, a foreach loop is also syntactic sugar. So you are two abstractions removed from the real code which is why it initially might be hard to understand how it all works together.
Assume that you have a very simple iterator block:
IEnumerable<int> IteratorBlock()
{
Console.WriteLine("Begin");
yield return 1;
Console.WriteLine("After 1");
yield return 2;
Console.WriteLine("After 2");
yield return 42;
Console.WriteLine("End");
}
Real iterator blocks often have conditions and loops but when you check the conditions and unroll the loops they still end up as yield statements interleaved with other code.
To enumerate the iterator block a foreach loop is used:
foreach (var i in IteratorBlock())
Console.WriteLine(i);
Here is the output (no surprises here):
Begin
1
After 1
2
After 2
42
End
As stated above foreach is syntactic sugar:
IEnumerator<int> enumerator = null;
try
{
enumerator = IteratorBlock().GetEnumerator();
while (enumerator.MoveNext())
{
var i = enumerator.Current;
Console.WriteLine(i);
}
}
finally
{
enumerator?.Dispose();
}
In an attempt to untangle this I have crated a sequence diagram with the abstractions removed:
The state machine generated by the compiler also implements the enumerator but to make the diagram more clear I have shown them as separate instances. (When the state machine is enumerated from another thread you do actually get separate instances but that detail is not important here.)
Every time you call your iterator block a new instance of the state machine is created. However, none of your code in the iterator block is executed until enumerator.MoveNext() executes for the first time. This is how deferred executing works. Here is a (rather silly) example:
var evenNumbers = IteratorBlock().Where(i => i%2 == 0);
At this point the iterator has not executed. The Where clause creates a new IEnumerable<T> that wraps the IEnumerable<T> returned by IteratorBlock but this enumerable has yet to be enumerated. This happens when you execute a foreach loop:
foreach (var evenNumber in evenNumbers)
Console.WriteLine(eventNumber);
If you enumerate the enumerable twice then a new instance of the state machine is created each time and your iterator block will execute the same code twice.
Notice that LINQ methods like ToList(), ToArray(), First(), Count() etc. will use a foreach loop to enumerate the enumerable. For instance ToList() will enumerate all elements of the enumerable and store them in a list. You can now access the list to get all elements of the enumerable without the iterator block executing again. There is a trade-off between using CPU to produce the elements of the enumerable multiple times and memory to store the elements of the enumeration to access them multiple times when using methods like ToList().
One major point about Yield keyword is Lazy Execution. Now what I mean by Lazy Execution is to execute when needed. A better way to put it is by giving an example
Example: Not using Yield i.e. No Lazy Execution.
public static IEnumerable<int> CreateCollectionWithList()
{
var list = new List<int>();
list.Add(10);
list.Add(0);
list.Add(1);
list.Add(2);
list.Add(20);
return list;
}
Example: using Yield i.e. Lazy Execution.
public static IEnumerable<int> CreateCollectionWithYield()
{
yield return 10;
for (int i = 0; i < 3; i++)
{
yield return i;
}
yield return 20;
}
Now when I call both methods.
var listItems = CreateCollectionWithList();
var yieldedItems = CreateCollectionWithYield();
you will notice listItems will have a 5 items inside it (hover your mouse on listItems while debugging).
Whereas yieldItems will just have a reference to the method and not the items.
That means it has not executed the process of getting items inside the method. A very efficient way of getting data only when needed.
Actual implementation of yield can be seen in ORM like Entity Framework and NHibernate etc.
The C# yield keyword, to put it simply, allows many calls to a body of code, referred to as an iterator, that knows how to return before it's done and, when called again, continues where it left off - i.e. it helps an iterator become transparently stateful per each item in a sequence that the iterator returns in successive calls.
In JavaScript, the same concept is called Generators.
It is a very simple and easy way to create an enumerable for your object. The compiler creates a class that wraps your method and that implements, in this case, IEnumerable<object>. Without the yield keyword, you'd have to create an object that implements IEnumerable<object>.
It's producing enumerable sequence. What it does is actually creating local IEnumerable sequence and returning it as a method result
This link has a simple example
Even simpler examples are here
public static IEnumerable<int> testYieldb()
{
for(int i=0;i<3;i++) yield return 4;
}
Notice that yield return won't return from the method. You can even put a WriteLine after the yield return
The above produces an IEnumerable of 4 ints 4,4,4,4
Here with a WriteLine. Will add 4 to the list, print abc, then add 4 to the list, then complete the method and so really return from the method(once the method has completed, as would happen with a procedure without a return). But this would have a value, an IEnumerable list of ints, that it returns on completion.
public static IEnumerable<int> testYieldb()
{
yield return 4;
console.WriteLine("abc");
yield return 4;
}
Notice also that when you use yield, what you are returning is not of the same type as the function. It's of the type of an element within the IEnumerable list.
You use yield with the method's return type as IEnumerable. If the method's return type is int or List<int> and you use yield, then it won't compile. You can use IEnumerable method return type without yield but it seems maybe you can't use yield without IEnumerable method return type.
And to get it to execute you have to call it in a special way.
static void Main(string[] args)
{
testA();
Console.Write("try again. the above won't execute any of the function!\n");
foreach (var x in testA()) { }
Console.ReadLine();
}
// static List<int> testA()
static IEnumerable<int> testA()
{
Console.WriteLine("asdfa");
yield return 1;
Console.WriteLine("asdf");
}
Nowadays you can use the yield keyword for async streams.
C# 8.0 introduces async streams, which model a streaming source of data. Data streams often retrieve or generate elements asynchronously. Async streams rely on new interfaces introduced in .NET Standard 2.1. These interfaces are supported in .NET Core 3.0 and later. They provide a natural programming model for asynchronous streaming data sources.
Source: Microsoft docs
Example below
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
public class Program
{
public static async Task Main()
{
List<int> numbers = new List<int>() { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
await foreach(int number in YieldReturnNumbers(numbers))
{
Console.WriteLine(number);
}
}
public static async IAsyncEnumerable<int> YieldReturnNumbers(List<int> numbers)
{
foreach (int number in numbers)
{
await Task.Delay(1000);
yield return number;
}
}
}
Simple demo to understand yield
using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApp_demo_yield {
class Program
{
static void Main(string[] args)
{
var letters = new List<string>() { "a1", "b1", "c2", "d2" };
// Not yield
var test1 = GetNotYield(letters);
foreach (var t in test1)
{
Console.WriteLine(t);
}
// yield
var test2 = GetWithYield(letters).ToList();
foreach (var t in test2)
{
Console.WriteLine(t);
}
Console.ReadKey();
}
private static IList<string> GetNotYield(IList<string> list)
{
var temp = new List<string>();
foreach(var x in list)
{
if (x.Contains("2")) {
temp.Add(x);
}
}
return temp;
}
private static IEnumerable<string> GetWithYield(IList<string> list)
{
foreach (var x in list)
{
if (x.Contains("2"))
{
yield return x;
}
}
}
}
}
It's trying to bring in some Ruby Goodness :)
Concept: This is some sample Ruby Code that prints out each element of the array
rubyArray = [1,2,3,4,5,6,7,8,9,10]
rubyArray.each{|x|
puts x # do whatever with x
}
The Array's each method implementation yields control over to the caller (the 'puts x') with each element of the array neatly presented as x. The caller can then do whatever it needs to do with x.
However .Net doesn't go all the way here.. C# seems to have coupled yield with IEnumerable, in a way forcing you to write a foreach loop in the caller as seen in Mendelt's response. Little less elegant.
//calling code
foreach(int i in obCustomClass.Each())
{
Console.WriteLine(i.ToString());
}
// CustomClass implementation
private int[] data = {1,2,3,4,5,6,7,8,9,10};
public IEnumerable<int> Each()
{
for(int iLooper=0; iLooper<data.Length; ++iLooper)
yield return data[iLooper];
}