[ACCEPTED]-Why is foreach loop Read-Only in C#-loops

Accepted answer

Score: 11

That is because foreach is meant to iterate 20 over a container, making sure each item 19 is visited exactly once, without changing 18 the container, to avoid nasty side effects.

See: foreach in MSDN

If 17 you meant why would changes to an element 16 like an integer not affect a container of 15 integers, well this is because the variable 14 of iteration in this case would be a value 13 type and is copied, e.g.:

// Warning: Does not compile
foreach (int i in ints)
{
  ++i; // Would not change the int in ints
}

Even if the variable 12 of iteration was a reference type, whose 11 operations returned a new object, you wouldn't 10 be changing the original collection, you 9 would just be reassigning to this variable 8 most of the time:

// Warning: Does not compile
foreach (MyClass ob in objs)
{
  ob=ob+ob; // Reassigning to local ob, not changing the one from the original 
            // collection of objs
}

The following example has 7 the potential to actually modify the object 6 in the original collection by calling a 5 mutating method:

// Warning: Does not compile
foreach (MyClass ob in objs)
{
  ob.ChangeMe(); // This could modify the object in the original collection
}

To avoid confusion with regard to 4 value vs reference types and the scenarios 3 mentioned above (along with some reasons 2 related to optimization), MS chose to make 1 the variable of iteration readonly.

Score: 9

Because the current element is returned 10 by value(i.e. copied). And modifying the 9 copy is useless. If it is a reference type 8 you can modify the content of that object, but 7 can't replace the reference.

Perhaps you 6 should read the documentation of IEnumerable<T> and IEnumerator<T>. That 5 should make it clearer. The most important 4 bit is that IEnumerable<T> has a property Current of type T. And 3 this property has only a getter, but no 2 setter.

But what would happen if it had a 1 setter?

It would work well with arrays and Lists
It wouldn't work well with complex containers like hashtables, ordered list because the change causes larger changes in the container(for example a reordering), and thus invalidates the iterator. (Most collections invalidate the iterators if they get modified to avoid inconsistent state in the iterators.)
In LINQ it does make no sense at all. For example with select(x=>f(x)) the values are results of a function and have no permanent storage associated.
With iterators written with the yield return syntax it doesn't make sense either

Score: 0

foreach is designed to visit each item in a collection 7 exactly once, and does not use an explicit 6 "loop index"; if you want more control over 5 the loop and have a loop index, use for.

EDIT: You 4 can change the items in the collection being 3 iterated on inside a foreach loop. For example:

foreach(Chair ch in mychairs)
{
    ch.PaintColour = Colour.Green; //this alters the chair object *in* the collection.
}

You 2 cannot, however, add or remove items to/from 1 the collection.

Source: stackoverflow.com

[ACCEPTED]-Why is foreach loop Read-Only in C#-loops

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