c# - 只读局部变量不能用作赋值目标

Question

如果我有:

var myObjects = new ConcurrentBag<object>();

并尝试通过以下方式删除对象:

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

编译器提示:“Readonly local variable cannot be used as an assignment target”

但是，如果我在 foreach 中添加本地引用，它会编译:

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

这里到底发生了什么？

Answer 1

foreach 中的迭代变量(即 myObject)不能在 foreach 中分配新值.这是不允许的。

在第一种情况下，out 尝试这样做。在第二种情况下，您*永远不会尝试重新分配给 myObject，所以没问题。

引用 ECMA 规范 15.8.4，强调我的:

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.