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.