Does foreach() iterate by reference?

Consider this:

List<MyClass> obj_list = get_the_list();
foreach( MyClass obj in obj_list )
{
    obj.property = 42;
}

Is obj a reference to the corresponding object within the list so that when I change the property the change will persist in the object instance once constructed somewhere?

Solution 1:

Yes, obj is a reference to the current object in the collection (assuming MyClass is in fact a class). If you change any properties via the reference, you're changing the object, just like you would expect.

Be aware however, that you cannot change the variable obj itself as it is the iteration variable. You'll get a compile error if you try. That means that you can't null it and if you're iterating value types, you can't modify any members as that would be changing the value.

The C# language specification states (8.8.4)

"The iteration variable corresponds to a read-only local variable with a scope that extends over the embedded statement."

Solution 2:

Yes, until you change the generic type from List to IEnumerable..

Solution 3:

You've asked 2 different questions here, lets take them in order.

Does a foreach loop iterate by reference?

If you mean in the same sense as a C++ for loop by reference, then no. C# does not have local variable references in the same sense as C++ and hence doesn't support this type of iteration.

Will the change be persisted

Assuming that MyClass is a reference type, the answer is yes. A class is a reference type in .Net and hence the iteration variable is a reference to the one variable, not a copy. This would not be true for a value type.

Solution 4:

Well, it happened to me that my changes were not updated in a foreach loop when I iterated through var collection:

var players = this.GetAllPlayers();
foreach (Player player in players)
{
    player.Position = 1;
}

When I changed var to List it started working.

Solution 5:

You can in this instance (using a List<T>) but if you were to be iterating over the generic IEnumerable<T> then it becomes dependant on its implementation.

If it was still a List<T> or T[] for instance, all would work as expected. The big gotcha comes when you are working with an IEnumerable<T> that was constructed using yield. In this case, you can no longer modify properties of T within an iteration and expect them to be present if you iterate the same IEnumerable<T> again.