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ToList method in Linq

If I am not wrong, the ToList() method iterate on each element of provided collection and add them to new instance of List and return this instance.Suppose an example
//using linq
list = Students.Where(s => s.Name == "ABC").ToList();
//traditional way
foreach (var student in Students)
{
if (student.Name == "ABC")
list.Add(student);
}
I think the traditional way is faster, as it loops only once, where as of above of Linq iterates twice once for Where method and then for ToList() method.
The project I am working on now has extensive use of Lists all over and I see there is alot of such kind of use of ToList() and other Methods that can be made better like above if I take list variable as IEnumerable and remove .ToList() and use it further as IEnumerable.
Do these things make any impact on performance?

Do these things make any impact on performance?
That depends on your code. Most of the time, using LINQ does cause a small performance hit. In some cases, this hit can be significant for you, but you should avoid LINQ only when you know that it is too slow for you (i.e. if profiling your code showed that LINQ is reason why your code is slow).
But you're right that using ToList() too often can cause significant performance problems. You should call ToList() only when you have to. Be aware that there are also cases where adding ToList() can improve performance a lot (e.g. when the collection is loaded from database every time it's iterated).
Regarding the number of iterations: it depends on what exactly do you mean by “iterates twice”. If you count the number of times MoveNext() is called on some collection, then yes, using Where() this way leads to iterating twice. The sequence of operations goes like this (to simplify, I'm going to assume that all items match the condition):
Where() is called, no iteration for now, Where() returns a special enumerable.
ToList() is called, calling MoveNext() on the enumerable returned from Where().
Where() now calls MoveNext() on the original collection and gets the value.
Where() calls your predicate, which returns true.
MoveNext() called from ToList() returns, ToList() gets the value and adds it to the list.
…
What this means is that if all n items in the original collection match the condition, MoveNext() will be called 2n times, n times from Where() and n times from ToList().

var list = Students.Where(s=>s.Name == "ABC");
This will only create a query and not loop the elements until the query is used. By calling ToList() will first then execute the query and thus only loop your elements once.
List<Student> studentList = new List<Student>();
var list = Students.Where(s=>s.Name == "ABC");
foreach(Student s in list)
{
studentList.add(s);
}
this example will also only iterate once. Because its only used once. Keep in mind that list will iterate all students everytime its called.. Not only just those whose names are ABC. Since its a query.
And for the later discussion Ive made a testexample. Perhaps its not the very best implementation of IEnumable but it does what its supposed to do.
First we have our list
public class TestList<T> : IEnumerable<T>
{
private TestEnumerator<T> _Enumerator;
public TestList()
{
_Enumerator = new TestEnumerator<T>();
}
public IEnumerator<T> GetEnumerator()
{
return _Enumerator;
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
throw new NotImplementedException();
}
internal void Add(T p)
{
_Enumerator.Add(p);
}
}
And since we want to count how many times MoveNext is called we have to implement our custom enumerator aswel. Observe in MoveNext we have a counter that is static in our program.
public class TestEnumerator : IEnumerator
{
public Item FirstItem = null;
public Item CurrentItem = null;
public TestEnumerator()
{
}
public T Current
{
get { return CurrentItem.Value; }
}
public void Dispose()
{
}
object System.Collections.IEnumerator.Current
{
get { throw new NotImplementedException(); }
}
public bool MoveNext()
{
Program.Counter++;
if (CurrentItem == null)
{
CurrentItem = FirstItem;
return true;
}
if (CurrentItem != null && CurrentItem.NextItem != null)
{
CurrentItem = CurrentItem.NextItem;
return true;
}
return false;
}
public void Reset()
{
CurrentItem = null;
}
internal void Add(T p)
{
if (FirstItem == null)
{
FirstItem = new Item<T>(p);
return;
}
Item<T> lastItem = FirstItem;
while (lastItem.NextItem != null)
{
lastItem = lastItem.NextItem;
}
lastItem.NextItem = new Item<T>(p);
}
}
And then we have a custom item that just wraps our value
public class Item<T>
{
public Item(T item)
{
Value = item;
}
public T Value;
public Item<T> NextItem;
}
To use the actual code we create a "list" with 3 entries.
public static int Counter = 0;
static void Main(string[] args)
{
TestList<int> list = new TestList<int>();
list.Add(1);
list.Add(2);
list.Add(3);
var v = list.Where(c => c == 2).ToList(); //will use movenext 4 times
var v = list.Where(c => true).ToList(); //will also use movenext 4 times
List<int> tmpList = new List<int>(); //And the loop in OP question
foreach(var i in list)
{
tmpList.Add(i);
} //Also 4 times.
}
And conclusion? How does it hit performance?
The MoveNext is called n+1 times in this case. Regardless of how many items we have.
And also the WhereClause does not matter, he will still run MoveNext 4 times. Because we always run our query on our initial list.
The only performance hit we will take is the actual LINQ framework and its calls. The actual loops made will be the same.
And before anyone asks why its N+1 times and not N times. Its because he returns false the last time when he is out of elements. Making it the number of elements + end of list.

To answer this completely, it depends on the implementation. If you are talking about LINQ to SQL/EF, there will be only one iteration in this case when .ToList is called, which internally calls .GetEnumerator. The query expression is then parsed into TSQL and passed to the database. The resulting rows are then iterated over (once) and added to the list.
In the case of LINQ to Objects, there is only one pass through the data as well. The use of yield return in the where clause sets up a state machine internally which keeps track of where the process is in the iteration. Where does NOT do a full iteration creating a temporary list and then passing those results to the rest of the query. It just determines if an item meets a criteria and only passes on those that match.

First of all, Why are you even asking me? Measure for yourself and see.
That said, Where, Select, OrderBy and the other LINQ IEnumerable extension methods, in general, are implemented as lazy as possible (the yield keyword is used often). That means that they do not work on the data unless they have to. From your example:
var list = Students.Where(s => s.Name == "ABC");
won't execute anything. This will return momentarily even if Students is a list of 10 million objects. The predicate won't be called at all until the result is actually requested somewhere, and that is practically what ToList() does: It says "Yes, the results - all of them - are required immediately".
There is however, some initial overhead in calling of the LINQ methods, so the traditional way will, in general, be faster, but composability and the ease-of-use of the LINQ methods, IMHO, more than compensate for that.
If you like to take a look at how these methods are implemented, they are available for reference from Microsoft Reference Sources.

Manually increment an enumerator inside foreach loop

I have a nested while loop inside a foreach loop where I would like to advance the enumerator indefinitately while a certain condition is met. To do this I try casting the enumerator to IEnumerator< T > (which it must be if it is in a foreach loop) then calling MoveNext() on the casted object but it gives me an error saying I cannot convert it.
Cannot convert type 'System.DateTime' to System.Collections.Generic.IEnumerator via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion.
foreach (DateTime time in times)
{
while (condition)
{
// perform action
// move to next item
(time as IEnumerator<DateTime>).MoveNext(); // will not let me do this
}
// code to execute after while condition is met
}
What is the best way to manually increment the IEnumerator inside of the foreach loop?
EDIT:
Edited to show there is code after the while loop that I would like executed once the condition is met which is why I wanted to manually increment inside the while then break out of it as opposed to continue which would put me back at the top. If this isn't possible I believe the best thing is to redesign how I am doing it.

Many of the other answers recommend using continue, which may very well help you do what you need to do. However, in the interests of showing manually moving the enumerator, first you must have the enumerator, and that means writing your loop as a while.
using (var enumerator = times.GetEnumerator())
{
DateTime time;
while (enumerator.MoveNext())
{
time = enumerator.Current;
// pre-condition code
while (condition)
{
if (enumerator.MoveNext())
{
time = enumerator.Current;
// condition code
}
else
{
condition = false;
}
}
// post-condition code
}
}
From your comments:
How can the foreach loop advance it if it doesn't implement the IEnumerator interface?
In your loop, time is a DateTime. It is not the object that needs to implement an interface or pattern to work in the loop. times is a sequence of DateTime values, it is the one that must implement the enumerable pattern. This is generally fulfilled by implementing the IEnumerable<T> and IEnumerable interfaces, which simply require T GetEnumerator() and object GetEnumerator() methods. The methods return an object implementing IEnumerator<T> and IEnumerator, which define a bool MoveNext() method and a T or object Current property. But time cannot be cast to IEnumerator, because it is no such thing, and neither is the times sequence.

You cannot modify the enumerator from inside the for loop. The language does not permit this. You need to use the continue statement in order to advance to the next iteration of a loop.
However, I'm not convinced that your loop even needs a continue. Read on.
In the context of your code you would need to convert the while to an if in order to make the continue refer to the foreach block.
foreach (DateTime time in times)
{
if (condition)
{
// perform action
continue;
}
// code to execute if condition is not met
}
But written like this it is clear that the following equivalent variant is simpler still
foreach (DateTime time in times)
{
if (condition)
{
// perform action
}
else
{
// code to execute if condition is not met
}
}
This is equivalent to your pseudo-code because the part marked code to execute after while condition is met is executed for each item for which condition is false.
My assumption in all of this is that condition is evaluated for each item in the list.

Perhaps you can use continue?

You would use the continue statement:
continue;

This is just a guess, but it sounds like what you're trying to do is take a list of datetimes and move past all of them which meet a certain criteria, then perform an action on the rest of the list. If that's what you're trying to do, you probably want something like SkipWhile() from System.Linq. For example, the following code takes a series of datetimes and skips past all of them which are before the cutoff date; then it prints out the remaining datetimes:
var times = new List<DateTime>()
{
DateTime.Now.AddDays(1), DateTime.Now.AddDays(2), DateTime.Now.AddDays(3), DateTime.Now.AddDays(4)
};
var cutoff = DateTime.Now.AddDays(2);
var timesAfterCutoff = times.SkipWhile(datetime => datetime.CompareTo(cutoff) < 1)
.Select(datetime => datetime);
foreach (var dateTime in timesAfterCutoff)
{
Console.WriteLine(dateTime);
}
Console.ReadLine();
Is that the sort of thing you're trying to do?

I definitely do not condone what I am about to suggest, but you can create a wrapper around the original IEnumerable to transform it into something that returns items which can be used to navigate the underlying the enumerator. The end result might look like the following.
public static void Main(string[] args)
{
IEnumerable<DateTime> times = GetTimes();
foreach (var step in times.StepWise())
{
while (condition)
{
step.MoveNext();
}
Console.WriteLine(step.Current);
}
}
Then we need to create our StepWise extension method.
public static class EnumerableExtension
{
public static IEnumerable<Step<T>> StepWise<T>(this IEnumerable<T> instance)
{
using (IEnumerator<T> enumerator = instance.GetEnumerator())
{
while (enumerator.MoveNext())
{
yield return new Step<T>(enumerator);
}
}
}
public struct Step<T>
{
private IEnumerator<T> enumerator;
public Step(IEnumerator<T> enumerator)
{
this.enumerator = enumerator;
}
public bool MoveNext()
{
return enumerator.MoveNext();
}
public T Current
{
get { return enumerator.Current; }
}
}
}

You could use a func as your iterator and keep the state that you are changing in that delegate to be evaluated each iteration.
public static IEnumerable<T> FunkyIEnumerable<T>(this Func<Tuple<bool, T>> nextOrNot)
{
while(true)
{
var result = nextOrNot();
if(result.Item1)
yield return result.Item2;
else
break;
}
yield break;
}
Func<Tuple<bool, int>> nextNumber = () =>
Tuple.Create(SomeRemoteService.CanIContinueToSendNumbers(), 1);
foreach(var justGonnaBeOne in nextNumber.FunkyIEnumerable())
Console.Writeline(justGonnaBeOne.ToString());

One alternative not yet mentioned is to have an enumerator return a wrapper object which allows access to itself in addition to the data element being enumerated. For sample:
struct ControllableEnumeratorItem<T>
{
private ControllableEnumerator parent;
public T Value {get {return parent.Value;}}
public bool MoveNext() {return parent.MoveNext();}
public ControllableEnumeratorItem(ControllableEnumerator newParent)
{parent = newParent;}
}
This approach could also be used by data structures that want to allow collections to be modified in controlled fashion during enumeration (e.g. by including "DeleteCurrentItem", "AddBeforeCurrentItem", and "AddAfterCurrentItem" methods).

How to properly check IEnumerable for existing results

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.

When to use Yield?

When should I use return yield and when should I use return only?

Use yield when you are returning an enumerable, and you don't have all the results at that point.
Practically, I've used yield when I want to iterate through a large block of information (database, flat file, etc.), and I don't want to load everything in memory first. Yield is a nice way to iterate through the block without loading everything at once.

The yield keyword is incredibly powerful. It basically allows you to quickly return IEnumerable and IEnumerator objects without explicitly coding them.
Consider a scenario where you want to return the intersection of two IEnumerable objects. Here is how you would do it using the yield keyword.
public static class Program
{
public static void Main()
{
IEnumerable<object> lhs = new List<int> { 1, 2, 3, 4, 5 };
IEnumerable<object> rhs = new List<int> { 3, 4, 5, 6, 7 };
foreach (object item in IntersectExample.Intersect(lhs, rhs))
{
Console.WriteLine(item);
break;
}
}
}
public static class IntersectExample
{
public static IEnumerable<object> Intersect(IEnumerable<object> lhs, IEnumerable<object> rhs)
{
var hashset = new HashSet<object>();
foreach (object item in lhs)
{
if (!hashset.Contains(item))
{
hashset.Add(item);
}
}
foreach (object item in rhs)
{
if (hashset.Contains(item))
{
yield return item;
}
}
}
}
It is hard to appreciate this until you fully realize what is going on. Normally when you intersect two sets you complete the entire operation before returning the result to the caller. The means the runtime complexity of the operation is O(m + n), where m and n are the sizes of the collections being intersected, regardless of what you do with the result afterwards. But, in my example I just wanted to pick off the first item from the result. Using an IEnumerable that was created by the yield keyword makes it super easy to delay part of the processing until it is actually required. My example runs in O(m). The alternative is to code the IEnumerable and maintain the state in it manually. The power of the yield keyword is that it creates that state machine for you.

Yield is for iterators.
It lets you process a list in small swallows, which is nice for big lists.
The magical thing about Yield is that it remembers where you're up to between invocations.
If you're not iterating you don't need Yield.

The yield construct is used to create an iterator that can produce multiple values in succession:
IEnumerable<int> three_numbers() {
yield return 1;
yield return 2;
yield return 3;
}
...
foreach (var i in three_numbers()) {
// i becomes 1, 2 and 3, in turn.
}

Yield Return will continue the method from that point. For example, you want to loop over an array or list and return each element at the time for the caller to process. So you will use yield return. If you want to return everything and then done, you don't need to do that

It is explained here:
C# Language Reference
yield (C# Reference)
The method called will return every single value so that they can be enumerated by the caller.
This means that you will need to use yield when you want every possible result returned by an iteration.

What is the yield keyword used for in C#?

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];
}