Foreach (Obj is produced in the new list <Base> ())

Question

What does the following code do?

class Base { }
class Derived : Base { }
class Test
{
    void Foo(List<Base> list)
    {
        foreach (Derived obj in list)
        {
            // ...
        }
    }
}

I did not expect it to even compile, but it is.

Answer 1

The behavior you observe corresponds to section 8.8.4. Presentation of the foreach C # language specification. This section defines operator semantics foreachas follows:

[...] The above steps, if successful, uniquely create a collection Ctype, an enumerator type, Eand an element type T. A foreachform expression

foreach (V v in x) embedded-statement

then expand to:

{
    E e = ((C)(x)).GetEnumerator();
    try {
        V v;
        while (e.MoveNext()) {
            // here the current item will be casted                
            v = (V)(T)e.Current;
            embedded-statement
        }
    }
    finally {
        // Dispose e
    }
}

, , . # 1.0 , ,

ArrayList list = new ArrayList();
list.Add(1);
foreach (int i in list)
{
   ...
}

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Answer 2

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:

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• Derived, . - , .
• foreach, .

Edit
InvalidCastException, - , Foo, Derived.

Edit2
? foreach Object , foreach, Derived

Answer 3

foreach . , . Base Derived, , .

Answer 4

, , . , :

{
  List<Base> list = new List<Base>();  // creates new, empty list of Base
  foreach (Derived obj in list)
  {
    // ...
  }
}

, ?

EDIT .

InvalidCastException, , Derived. Derived ' "", .

Answer 5

:

Enumerator<Base> enumerator = new List<Base>().GetEnumerator();
while (enumerator.MoveNext())
{
    Derived obj = (Derived) enumerator.Current;
    // ...
}

, .

, InvalidCastException, - Derived.

Answer 6

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Derived , Derived, , Base. , Derived .

, , List<Derived>.

Answer 7

,

list.Cast<Derived>();

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