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C# "Generator" Method

I come from the world of Python and am trying to create a "generator" method in C#. I'm parsing a file in chunks of a specific buffer size, and only want to read and store the next chunk one at a time and yield it in a foreach loop. Here's what I have so far (simplified proof of concept):
class Page
{
public uint StartOffset { get; set; }
private uint currentOffset = 0;
public Page(MyClass c, uint pageNumber)
{
uint StartOffset = pageNumber * c.myPageSize;
if (StartOffset < c.myLength)
currentOffset = StartOffset;
else
throw new ArgumentOutOfRangeException("Page offset exceeds end of file");
while (currentOffset < c.myLength && currentOffset < (StartOffset + c.myPageSize))
// read data from page and populate members (not shown for MWE purposes)
. . .
}
}
class MyClass
{
public uint myLength { get; set; }
public uint myPageSize { get; set; }
public IEnumerator<Page> GetEnumerator()
{
for (uint i = 1; i < this.myLength; i++)
{
// start count at 1 to skip first page
Page p = new Page(this, i);
try
{
yield return p;
}
catch (ArgumentOutOfRangeException)
{
// end of available pages, how to signal calling foreach loop?
}
}
}
}
I know this is not perfect since it is a minimum working example (I don't allow many of these properties to be set publicly, but for keeping this simple I don't want to type private members and properties).
However, my main question is how do I let the caller looping over MyClass with a foreach statement know that there are no more items left to loop through? Is there an exception I throw to indicate there are no elements left?

As mentioned in the comments, you should use IEnumerable<T> instead of IEnumerator<T>. The enumerator is the technical object that is being used to enumerate over something. That something—in many cases–is an enumerable.
C# has special abilities to deal with enumerables. Most prominently, you can use a foreach loop with an enumerable (but not an enumerator; even though the loop actually uses the enumerator of the enumerable). Also, enumerables allow you to use LINQ which makes it even more easier to consume.
So you should change your class like this:
class MyClass
{
public uint myLength { get; set; }
public uint myPageSize { get; set; }
# note the modified signature
public IEnumerable<Page> GetPages()
{
for (uint i = 1; i < this.myLength; i++)
{
Page p;
try
{
p = new Page(this, i);
}
catch (ArgumentOutOfRangeException)
{
yield break;
}
yield return p;
}
}
}
In the end, this allows you to use it like this:
var obj = new MyClass();
foreach (var page in obj.GetPages())
{
// do whatever
}
// or even using LINQ
var pageOffsets = obj.GetPages().Select(p => p.currentOffset).ToList();
Of course, you should also change the name of the method to something meaningful. If you’re returning pages, GetPages is maybe a good first step in the right direction. The name GetEnumerator is kind of reserved for types implementing IEnumerable, where the GetEnumerator method is supposed to return an enumerator of the collection the object represents.

The two ways to do it is let the code execution reach the end of the GetEnumerator function or put in a yield break; in the code, this would behave the same as a return; in a function that returned void.
From the caller's perceptive the Enumerator returned from GetEnumerator() will start returning false for MoveNext(), that is how they tell that the enumerator is done.
To fix your "Can't yield a value inside the body of a try block with a catch clause" you put the try/catch around the wrong part of the code, the execption will be thrown on the new not the yield return. Your code should look like
public IEnumerator<Page> GetEnumerator()
{
for (uint i = 1; i < this.myLength; i++)
{
// start count at 1 to skip first page
Page p;
try
{
p = new Page(this, i);
}
catch (ArgumentOutOfRangeException)
{
yield break;
}
yield return p;
}
}

Use the yield break; statement to end the sequence that your iterator method is generating.

Need help understanding C# yield in IEnumerable

i am reading C# 2010 Accelerated. i dont get what is yield
When GetEnumerator is called, the code
in the method that contains the yield
statement is not actually executed at
that point in time. Instead, the
compiler generates an enumerator
class, and that class contains the
yield block code
public IEnumerator<T> GetEnumerator() {
foreach( T item in items ) {
yield return item;
}
}
i also read from Some help understanding “yield”
yield is a lazy producer of data, only
producing another item after the first
has been retrieved, whereas returning
a list will return everything in one
go.
does this mean that each call to GetEnumerator will get 1 item from the collection? so 1st call i get 1st item, 2nd, i get the 2nd and so on ... ?

Best way to think of it is when you first request an item from an IEnumerator (for example in a foreach), it starts running trough the method, and when it hits a yield return it pauses execution and returns that item for you to use in your foreach. Then you request the next item, it resumes the code where it left and repeats the cycle until it encounters either yield break or the end of the method.
public IEnumerator<string> enumerateSomeStrings()
{
yield return "one";
yield return "two";
var array = new[] { "three", "four" }
foreach (var item in array)
yield return item;
yield return "five";
}

Take a look at the IEnumerator<T> interface; that may well to clarify what's happening. The compiler takes your code and turns it into a class that implements both IEnumerable<T> and IEnumerator<T>. The call to GetEnumerator() simply returns the class itself.
The implementation is basically a state machine, which, for each call to MoveNext(), executes the code up until the next yield return and then sets Current to the return value. The foreach loop uses this enumerator to walk through the enumerated items, calling MoveNext() before each iteration of the loop. The compiler is really doing some very cool things here, making yield return one of the most powerful constructs in the language. From the programmer's perspective, it's just an easy way to lazily return items upon request.

Yes thats right, heres the example from MSDN that illustrates how to use it
public class List
{
//using System.Collections;
public static IEnumerable Power(int number, int exponent)
{
int counter = 0;
int result = 1;
while (counter++ < exponent)
{
result = result * number;
yield return result;
}
}
static void Main()
{
// Display powers of 2 up to the exponent 8:
foreach (int i in Power(2, 8))
{
Console.Write("{0} ", i);
}
}
}
/*
Output:
2 4 8 16 32 64 128 256
*/

If I understand your question correct then your understanding is incorrect I'm affraid. The yield statements (yield return and yield break) is a very clever compiler trick. The code in you method is actually compiled into a class that implements IEnumerable. An instance of this class is what the method will return. Let's Call the instance 'ins' when calling ins.GetEnumerator() you get an IEnumerator that for each Call to MoveNext() produced the next element in the collection (the yield return is responsible for this part) when the sequence has no more elements (e.g. a yield break is encountered) MoveNext() returns false and further calls results in an exception. So it is not the Call to GetEnumerator that produced the (next) element but the Call to MoveNext

It looks like you understand it.
yield is used in your class's GetEnumerator as you describe so that you can write code like this:
foreach (MyObject myObject in myObjectCollection)
{
// Do something with myObject
}
By returning the first item from the 1st call the second from the 2nd and so on you can loop over all elements in the collection.
yield is defined in MyObjectCollection.

The Simple way to understand yield keyword is we do not need extra class to hold the result of iteration when return using
yield return keyword. Generally when we iterate through the collection and want to return the result, we use collection object
to hold the result. Let's look at example.
public static List Multiplication(int number, int times)
{
List<int> resultList = new List<int>();
int result = number;
for(int i=1;i<=times;i++)
{
result=number*i;
resultList.Add(result);
}
return resultList;
}
static void Main(string[] args)
{
foreach(int i in Multiplication(2,10))
{
Console.WriteLine(i);
}
Console.ReadKey();
}
In the above example, I want to return the result of multiplication of 2 ten times. So I Create a method Multiplication
which returns me the multiplication of 2 ten times and i store the result in the list and when my main method calls the
multiplication method, the control iterates through the loop ten times and store result result in the list. This is without
using yield return. Suppose if i want to do this using yield return it looks like
public static IEnumerable Multiplication(int number, int times)
{
int result = number;
for(int i=1;i<=times;i++)
{
result=number*i;
yield return result;
}
}
static void Main(string[] args)
{
foreach(int i in Multiplication(2,10))
{
Console.WriteLine(i);
}
Console.ReadKey();
}
Now there is slight changes in Multiplication method, return type is IEnumerable and there is no other list to hold the
result because to work with Yield return type must be IEnumerable or IEnumerator and since Yield provides stateful iteration
we do not need extra class to hold the result. So in the above example, when Multiplication method is called from Main
method, it calculates the result in for 1st iteration and return the result to main method and come backs to the loop and
calculate the result for 2nd iteration and returns the result to main method.In this way Yield returns result to calling
method one by one in each iteration.There is other Keyword break used in combination with Yield that causes the iteration
to stop. For example in the above example if i want to calculate multiplication for only half number of times(10/2=5) then
the method looks like this:
public static IEnumerable Multiplication(int number, int times)
{
int result = number;
for(int i=1;i<=times;i++)
{
result=number*i;
yield return result;
if (i == times / 2)
yield break;
}
}
This method now will result multiplication of 2, 5 times.Hope this will help you understand the concept of Yield. For more
information please visit http://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx

Need help in terminating deferred execution

The following snippet prints 1 through 10 on the console, but does not terminate until variable 'i' reaches int.MaxValue. TIA for pointing out what I am missing.
class Program
{
public static IEnumerable<int> GetList()
{
int i = 0;
while (i < int.MaxValue)
{
i++;
yield return i;
}
}
static void Main(string[] args)
{
var q = from i in GetList() // keeps calling until i reaches int.MaxValue
where i <= 10
select i;
foreach (int i in q)
Console.WriteLine(i);
}
}

You could try:
var q = GetList ().TakeWhile ((i)=> i <=10);

The query that you defined in Main doesn't know anything about the ordering of your GetList method, and it must check every value of that list with the predicate i <= 10. If you want to stop processing sooner, you will you can use the Take extension method or use the TakeWhile extension method:
foreach (int i in GetList().Take(10))
Console.WriteLine(i);
foreach (int i in GetList().TakeWhile(x => x <= 10))
Console.WriteLine(i);

Your iterators limits are 0 through Int32.MaxValue, so it will process that whole range. Iterators are only smart enough to not pre-iterate the results of the range of data you design it to iterate. However they are not smart enough to know when the code that uses them no longer needs more unless you tell it so (i.e. you break out of a foreach loop.) The only way to allow the iterator to limit itself is to pass in the upper bound to the GetList function:
public static IEnumerable<int> GetList(int upperBound)
{
int i = 0;
while (i < upperBound)
{
i++;
yield return i;
}
}
You could also explicitly tell the iterator that you only wish to iterate the first 10 results:
var numbers = GetList().Take(10);

Consider using the LINQ extension method .Take() with your argument instead of having it in your where clause. More on Take.
var q = from i in GetList().Take(10)
select i;

Can someone demystify the yield keyword?

I have seen the yield keyword being used quite a lot on Stack Overflow and blogs. I don't use LINQ. Can someone explain the yield keyword?
I know that similar questions exist.
But none really explain what is its use in plain simple language.

By far the best explanation of this (that I've seen) is Jon Skeet's book - and that chapter is free! Chapter 6, C# in Depth. There is nothing I can add here that isn't covered.
Then buy the book; you will be a better C# programmer for it.
Q: Why didn't I write a longer answer here (paraphrased from comments); simple. As Eric Lippert observes (here), the yield construct (and the magic that goes behind it) is the single most complex bit of code in the C# compiler, and to try and describe it in a brief reply here is naïve at best. There are so many nuances to yield that IMO it is better to refer to a pre-existing (and fully qualified) resource.
Eric's blog now has 7 entries (and that is just the recent ones) discussing yield. I have a vast amount of respect for Eric, but his blog is probably more appropriate as a "further information" for people who are comfortable with the subject (yield in this case), as it typically describes a lot of the background design considerations. Best done in the context of a reasonable foundation.
(and yes, chapter 6 does download; I verified...)

The yield keyword is used with methods that return IEnumerable<T> or IEnumerator<T> and it makes the compiler generate a class that implements the necessary plumbing for using the iterator. E.g.
public IEnumerator<int> SequenceOfOneToThree() {
yield return 1;
yield return 2;
yield return 3;
}
Given the above the compiler will generate a class that implements IEnumerator<int>, IEnumerable<int> and IDisposable (actually it will also implement the non-generic versions of IEnumerable and IEnumerator).
This allows you to call the method SequenceOfOneToThree in a foreach loop like this
foreach(var number in SequenceOfOneToThree) {
Console.WriteLine(number);
}
An iterator is a state machine, so each time yield is called the position in the method is recorded. If the iterator is moved to the next element, the method resumes right after this position. So the first iteration returns 1 and marks that position. The next iterator resumes right after one and thus returns 2 and so forth.
Needless to say you can generate the sequence in any way you like, so you don't have to hard code the numbers like I did. Also, if you want to break the loop you can use yield break.

In an effort to demystify I'll avoid talking about iterators, since they could be part of the mystery themselves.
the yield return and yield break statements are most often used to provide "deferred evaluation" of the collection.
What this means is that when you get the value of a method that uses yield return, the collection of things you are trying to get don't exist together yet (it's essentially empty). As you loop through them (using foreach) it will execute the method at that time and get the next element in the enumeration.
Certain properties and methods will cause the entire enumeration to be evaluated at once (such as "Count").
Here's a quick example of the difference between returning a collection and returning yield:
string[] names = { "Joe", "Jim", "Sam", "Ed", "Sally" };
public IEnumerable<string> GetYieldEnumerable()
{
foreach (var name in names)
yield return name;
}
public IEnumerable<string> GetList()
{
var list = new List<string>();
foreach (var name in names)
list.Add(name);
return list;
}
// we're going to execute the GetYieldEnumerable() method
// but the foreach statement inside it isn't going to execute
var yieldNames = GetNamesEnumerable();
// now we're going to execute the GetList() method and
// the foreach method will execute
var listNames = GetList();
// now we want to look for a specific name in yieldNames.
// only the first two iterations of the foreach loop in the
// GetYieldEnumeration() method will need to be called to find it.
if (yieldNames.Contains("Jim")
Console.WriteLine("Found Jim and only had to loop twice!");
// now we'll look for a specific name in listNames.
// the entire names collection was already iterated over
// so we've already paid the initial cost of looping through that collection.
// now we're going to have to add two more loops to find it in the listNames
// collection.
if (listNames.Contains("Jim"))
Console.WriteLine("Found Jim and had to loop 7 times! (5 for names and 2 for listNames)");
This can also be used if you need to get a reference to the Enumeration before the source data has values. For example if the names collection wasn't complete to start with:
string[] names = { "Joe", "Jim", "Sam", "Ed", "Sally" };
public IEnumerable<string> GetYieldEnumerable()
{
foreach (var name in names)
yield return name;
}
public IEnumerable<string> GetList()
{
var list = new List<string>();
foreach (var name in names)
list.Add(name);
return list;
}
var yieldNames = GetNamesEnumerable();
var listNames = GetList();
// now we'll change the source data by renaming "Jim" to "Jimbo"
names[1] = "Jimbo";
if (yieldNames.Contains("Jimbo")
Console.WriteLine("Found Jimbo!");
// Because this enumeration was evaluated completely before we changed "Jim"
// to "Jimbo" it isn't going to be found
if (listNames.Contains("Jimbo"))
// this can't be true
else
Console.WriteLine("Couldn't find Jimbo, because he wasn't there when I was evaluated.");

The yield keyword is a convenient way to write an IEnumerator. For example:
public static IEnumerator<int> Range(int from, int to)
{
for (int i = from; i < to; i++)
{
yield return i;
}
}
is transformed by the C# compiler to something similiar to:
public static IEnumerator<int> Range(int from, int to)
{
return new RangeEnumerator(from, to);
}
class RangeEnumerator : IEnumerator<int>
{
private int from, to, current;
public RangeEnumerator(int from, int to)
{
this.from = from;
this.to = to;
this.current = from;
}
public bool MoveNext()
{
this.current++;
return this.current < this.to;
}
public int Current
{
get
{
return this.current;
}
}
}

Take a look at the MSDN documentation and the example. It is essentially an easy way to create an iterator in C#.
public class List
{
//using System.Collections;
public static IEnumerable Power(int number, int exponent)
{
int counter = 0;
int result = 1;
while (counter++ < exponent)
{
result = result * number;
yield return result;
}
}
static void Main()
{
// Display powers of 2 up to the exponent 8:
foreach (int i in Power(2, 8))
{
Console.Write("{0} ", i);
}
}
}

Eric White's series on functional programming it well worth the read in it's entirety, but the entry on Yield is as clear an explanation as I've seen.

yield is not directly related to LINQ, but rather to iterator blocks. The linked MSDN article gives great detail on this language feature. See especially the Using Iterators section. For deep details of iterator blocks, see Eric Lippert's recent blog posts on the feature. For the general concept, see the Wikipedia article on iterators.

I came up with this to overcome a .NET shortcoming having to manually deep copy List.
I use this:
static public IEnumerable<SpotPlacement> CloneList(List<SpotPlacement> spotPlacements)
{
foreach (SpotPlacement sp in spotPlacements)
{
yield return (SpotPlacement)sp.Clone();
}
}
And at another place:
public object Clone()
{
OrderItem newOrderItem = new OrderItem();
...
newOrderItem._exactPlacements.AddRange(SpotPlacement.CloneList(_exactPlacements));
...
return newOrderItem;
}
I tried to come up with oneliner that does this, but it's not possible, due to yield not working inside anonymous method blocks.
EDIT:
Better still, use a generic List cloner:
class Utility<T> where T : ICloneable
{
static public IEnumerable<T> CloneList(List<T> tl)
{
foreach (T t in tl)
{
yield return (T)t.Clone();
}
}
}

Let me add to all of this. Yield is not a keyword.
It will only work if you use "yield return" other than that it will work like a normal variable.
It's uses to return iterator from a function. You can search further on that.
I recommend searching for "Returning Array vs Iterator"

Some help understanding "yield"

In my everlasting quest to suck less I'm trying to understand the "yield" statement, but I keep encountering the same error.
The body of [someMethod] cannot be an iterator block because
'System.Collections.Generic.List< AClass>' is not an iterator interface type.
This is the code where I got stuck:
foreach (XElement header in headersXml.Root.Elements()){
yield return (ParseHeader(header));
}
What am I doing wrong? Can't I use yield in an iterator? Then what's the point?
In this example it said that List<ProductMixHeader> is not an iterator interface type.
ProductMixHeader is a custom class, but I imagine List is an iterator interface type, no?
--Edit--
Thanks for all the quick answers.
I know this question isn't all that new and the same resources keep popping up.
It turned out I was thinking I could return List<AClass> as a return type, but since List<T> isn't lazy, it cannot. Changing my return type to IEnumerable<T> solved the problem :D
A somewhat related question (not worth opening a new thread): is it worth giving IEnumerable<T> as a return type if I'm sure that 99% of the cases I'm going to go .ToList() anyway? What will the performance implications be?

A method using yield return must be declared as returning one of the following two interfaces:
IEnumerable<SomethingAppropriate>
IEnumerator<SomethingApropriate>
(thanks Jon and Marc for pointing out IEnumerator)
Example:
public IEnumerable<AClass> YourMethod()
{
foreach (XElement header in headersXml.Root.Elements())
{
yield return (ParseHeader(header));
}
}
yield is a lazy producer of data, only producing another item after the first has been retrieved, whereas returning a list will return everything in one go.
So there is a difference, and you need to declare the method correctly.
For more information, read Jon's answer here, which contains some very useful links.

It's a tricky topic. In a nutshell, it's an easy way of implementing IEnumerable and its friends. The compiler builds you a state machine, transforming parameters and local variables into instance variables in a new class. Complicated stuff.
I have a few resources on this:
Chapter 6 of C# in Depth (free download from that page)
Iterators, iterator blocks and data pipelines (article)
Iterator block implementation details (article)

"yield" creates an iterator block - a compiler generated class that can implement either IEnumerable[<T>] or IEnumerator[<T>]. Jon Skeet has a very good (and free) discussion of this in chapter 6 of C# in Depth.
But basically - to use "yield" your method must return an IEnumerable[<T>] or IEnumerator[<T>]. In this case:
public IEnumerable<AClass> SomeMethod() {
// ...
foreach (XElement header in headersXml.Root.Elements()){
yield return (ParseHeader(header));
}
}

List implements Ienumerable.
Here's an example that might shed some light on what you are trying to learn. I wrote this about 6 months
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace YieldReturnTest
{
public class PrimeFinder
{
private Boolean isPrime(int integer)
{
if (0 == integer)
return false;
if (3 > integer)
return true;
for (int i = 2; i < integer; i++)
{
if (0 == integer % i)
return false;
}
return true;
}
public IEnumerable<int> FindPrimes()
{
int i;
for (i = 1; i < 2147483647; i++)
{
if (isPrime(i))
{
yield return i;
}
}
}
}
class Program
{
static void Main(string[] args)
{
PrimeFinder primes = new PrimeFinder();
foreach (int i in primes.FindPrimes())
{
Console.WriteLine(i);
Console.ReadLine();
}
Console.ReadLine();
Console.ReadLine();
}
}
}

I highly recommend using Reflector to have a look at what yield actually does for you. You'll be able to see the full code of the class that the compiler generates for you when using yield, and I've found that people understand the concept much more quickly when they can see the low-level result (well, mid-level I guess).

To understand yield, you need to understand when to use IEnumerator and IEnumerable (because you have to use either of them). The following examples help you to understand the difference.
First, take a look at the following class, it implements two methods - one returning IEnumerator<int>, one returning IEnumerable<int>. I'll show you that there is a big difference in usage, although the code of the 2 methods is looking similar:
// 2 iterators, one as IEnumerator, one as IEnumerable
public class Iterator
{
public static IEnumerator<int> IterateOne(Func<int, bool> condition)
{
for(var i=1; condition(i); i++) { yield return i; }
}
public static IEnumerable<int> IterateAll(Func<int, bool> condition)
{
for(var i=1; condition(i); i++) { yield return i; }
}
}
Now, if you're using IterateOne you can do the following:
// 1. Using IEnumerator allows to get item by item
var i=Iterator.IterateOne(x => true); // iterate endless
// 1.a) get item by item
i.MoveNext(); Console.WriteLine(i.Current);
i.MoveNext(); Console.WriteLine(i.Current);
// 1.b) loop until 100
int j; while (i.MoveNext() && (j=i.Current)<=100) { Console.WriteLine(j); }
1.a) prints:
1
2
1.b) prints:
3
4
...
100
because it continues counting right after the 1.a) statements have been executed.
You can see that you can advance item by item using MoveNext().
In contrast, IterateAll allows you to use foreach and also LINQ statements for bigger comfort:
// 2. Using IEnumerable makes looping and LINQ easier
var k=Iterator.IterateAll(x => x<100); // limit iterator to 100
// 2.a) Use a foreach loop
foreach(var x in k){ Console.WriteLine(x); } // loop
// 2.b) LINQ: take 101..200 of endless iteration
var lst=Iterator.IterateAll(x=>true).Skip(100).Take(100).ToList(); // LINQ: take items
foreach(var x in lst){ Console.WriteLine(x); } // output list
2.a) prints:
1
2
...
99
2.b) prints:
101
102
...
200
Note: Since IEnumerator<T> and IEnumerable<T> are Generics, they can be used with any type. However, for simplicity I have used int in my examples for type T.
This means, you can use one of the return types IEnumerator<ProductMixHeader> or IEnumerable<ProductMixHeader> (the custom class you have mentioned in your question).
The type List<ProductMixHeader> does not implement any of these interfaces, which is the reason why you can't use it that way. But Example 2.b) is showing how you can create a list from it.
If you're creating a list by appending .ToList() then the implication is, that it will create a list of all elements in memory, while an IEnumerable allows lazy creation of its elements - in terms of performance, it means that elements are enumerated just in time - as late as possible, but as soon as you're using .ToList(), then all elements are created in memory. LINQ tries to optimize performance this way behind the scenes.
DotNetFiddle of all examples

#Ian P´s answer helped me a lot to understand yield and why it is used. One (major) use case for yield is in "foreach" loops after the "in" keyword not to return a fully completed list. Instead of returning a complete list at once, in each "foreach" loop only one item (the next item) is returned. So you will gain performance with yield in such cases.
I have rewritten #Ian P´s code for my better understanding to the following:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace YieldReturnTest
{
public class PrimeFinder
{
private Boolean isPrime(int integer)
{
if (0 == integer)
return false;
if (3 > integer)
return true;
for (int i = 2; i < integer; i++)
{
if (0 == integer % i)
return false;
}
return true;
}
public IEnumerable<int> FindPrimesWithYield()
{
int i;
for (i = 1; i < 2147483647; i++)
{
if (isPrime(i))
{
yield return i;
}
}
}
public IEnumerable<int> FindPrimesWithoutYield()
{
var primes = new List<int>();
int i;
for (i = 1; i < 2147483647; i++)
{
if (isPrime(i))
{
primes.Add(i);
}
}
return primes;
}
}
class Program
{
static void Main(string[] args)
{
PrimeFinder primes = new PrimeFinder();
Console.WriteLine("Finding primes until 7 with yield...very fast...");
foreach (int i in primes.FindPrimesWithYield()) // FindPrimesWithYield DOES NOT iterate over all integers at once, it returns item by item
{
if (i > 7)
{
break;
}
Console.WriteLine(i);
//Console.ReadLine();
}
Console.WriteLine("Finding primes until 7 without yield...be patient it will take lonkg time...");
foreach (int i in primes.FindPrimesWithoutYield()) // FindPrimesWithoutYield DOES iterate over all integers at once, it returns the complete list of primes at once
{
if (i > 7)
{
break;
}
Console.WriteLine(i);
//Console.ReadLine();
}
Console.ReadLine();
Console.ReadLine();
}
}
}

What does the method you're using this in look like? I don't think this can be used in just a loop by itself.
For example...
public IEnumerable<string> GetValues() {
foreach(string value in someArray) {
if (value.StartsWith("A")) { yield return value; }
}
}