Why you cannot use an unsafe keyword in an iterator context?

Answer 1

It's just part of the C# spec:

26.1 Iterator blocks ... It is a compile-time error for an iterator block to contain an unsafe context (§27.1). An iterator block always defines a safe context, even when its declaration is nested in an unsafe context.

Presumably, the code that gets generated by the compiler needs to be verifiable in order to not have to be labelled 'unsafe'. If you want to use pointers, you'll have to implement IEnumerator yourself.

Answer 2

Eric Lippert has an excellent blog post on this topic here: Iterator Blocks, Part Six: Why no unsafe code?

Answer 3

What I want to know is why you would use pointers for this at all. Why not simply say:

private IEnumerator<char> ReverseRead()
{
    int len = _str.Length;
    for(int i = 0; i < len; ++i)
        yield return _str[len - i - 1];
}

What's the compelling benefit of messing around with pointers?