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

Question

In looking at this question which Jon did a fine job in answering... 'How to read a text file reversly with iterator'. And there was a similar question in which I answered using pointers hocus pocus..'.net is there a way to read a text file from bottom to top' before it got closed....

Now I did set out to try solve this using pointers, ok, it looks hackish and rough around the edges...

public class ReadChar : IEnumerable<char>
{
    private Stream _strm = null;
    private string _str = string.Empty;
    public ReadChar(string s)
    {
        this._str = s;
    }
    public ReadChar(Stream strm)
    {
        this._strm = strm;
    }
    public IEnumerator<char> GetEnumerator()
    {
        if (this._strm != null && this._strm.CanRead && this._strm.CanSeek)
        {
            return ReverseReadStream();
        }
        if (this._str.Length > 0)
        {
            return ReverseRead();
        }
        return null;
    }

    private IEnumerator<char> ReverseReadStream()
    {
        long lIndex = this._strm.Length;
        while (lIndex != 0 && this._strm.Position != 0)
        {
            this._strm.Seek(lIndex--, SeekOrigin.End);
            int nByte = this._strm.ReadByte();
            yield return (char)nByte; 
        }
    }

    private IEnumerator<char> ReverseRead()
    {
        unsafe
        {
            fixed (char* beg = this._str)
            {
                char* p = beg + this._str.Length;
                while (p-- != beg)
                {
                    yield return *p;
                }
            }
        }
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

but discovered that C# compiler cannot handle this using this implementation but was devastated when the C# compiler refused with an error CS1629 - 'Unsafe code may not appear in iterators'

Why is that so?

Answer 1

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

Answer 2

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?

Answer 3

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.