Readonly local variable cannot be used as an assignment target

Question

If I have:

var myObjects = new ConcurrentBag<object>();

And try to remove objects via:

foreach (var myObject in myObjects.ToArray())
{
    myObjects.TryTake(out myObject);
}

The compiler complains with: "Readonly local variable cannot be used as an assignment target"

Yet, if I add a local reference within the foreach it compiles:

foreach (var myObject in myObjects.ToArray())
{
    var localReference = myObject;
    myObjects.TryTake(out localReference);
}

What exactly is going on here?

Answer 1

The iteration variable in a foreach (i.e. myObject) cannot be assigned a new value inside the foreach. It is not allowed.

In the first scenario, the out tries to do this. In the second scenario, you *never try to reassign to myObject, so it is fine.

To quote from the ECMA specification, 15.8.4, emphasis mine:

1. The type and identifier of a foreach statement declare the iteration variable of the statement.

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

3. During execution of a foreach statement, the iteration variable represents the collection element for which an iteration is currently being performed.

4. A compile-time error occurs if the embedded statement attempts to modify the iteration variable (via assignment or the ++ and --operators) or pass the iteration variable as a ref or out parameter.

Answer 2

If you want to assign the retrieved object to the array, use a for-statement instead, since you cannot assign a value to the loop variable

object[] arr = myObjects.ToArray();
for (int i = 0; i < arr.Length; i++) {
    myObjects.TryTake(out arr[i]); 
}

Now the array contains the retrieved objects or null where no object could be taken.

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UPDATE

After having read about ConcurrentBag<T>, I think I am beginning to understand what you have in mind. My first solution above runs into race conditions in multithreaded scenarios (the very and only reson for using a ConcurrentBag<T>). Try this instead

var removedObjects = new List<object>();
object obj;
while (myObjects.TryTake(out obj)) {
    removedObjects.Add(obj);
}

Note: Assigning the objects of the bag to an array and then looping through it is not a good idea, since other threads could have added or removed objects meanwhile. Therefore you must use this direct approach.

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If you only want to remove the objects without retrieving them at the same time:

object obj;
while (myObjects.TryTake(out obj)) {
}