AddRange in GeneratedBuilder iterates twice. about protobuf-csharp-port HOT 4 CLOSED

I think it's reasonable to iterate over a List<T> twice, so we could run over 
the sequence to check for nulls in that case, just before calling list.AddRange.

Alternatively, we could build a List<T> from the source to start with if it 
wasn't already a list, check everything and then call AddRange in all cases. 
The advantage of this would be that if we were going to throw an exception, 
we'd do so *before* adding anything to the target. It does mean creating an 
extra list, admittedly...

The interesting thing here is that we always wind up in the else clause...

            else
            {
                foreach (T element in source)
                {
                    destination.Add(element);
                }
            }

... since the target list is never a List<T>, but our own PopsicleList<T>.  If 
we are going to foreach through the array we can test elements for null at that 
time.

I would recommend we move the null assertion to inside the PopsicleList.  
PopsicleList's current Add() and this[] {set;} methods do not assert that the 
value is non-null.  So I can still break it with the following:

    MyMessage.Builder b;
    b.RepeatedValueList.Add(null);


I'll put together a fix branch for this, I think this is critical enough given 
today's world of Linq and yield statements.

Fixed on branch 'issue-26', ready for review.  The change involved a few 
things....

1. Removed protected member "AddRange<T>" of GeneratedBuilder
2. Generator now calls PopsicleList<T>.Add(IEnumerable<T>)
3. PopsicleList now checks for null on all add/set/insert methods
4. The double-enumeration now only happens if the provided IEnumerable<T> also 
implements ICollection<T>

            bool CheckForNull = default(T) == null;
            ...
            if (!CheckForNull || collection is PopsicleList<T>)
            {
                items.AddRange(collection);
            }
            else
            {
                // Assumption, it's ok to enumerate collections more than once.
                if (collection is ICollection<T>)
                {
                    ThrowHelper.ThrowIfAnyNull(collection);
                    items.AddRange(collection);
                }
                else
                {
                    foreach (T item in collection)
                    {
                        ThrowHelper.ThrowIfNull(item);
                        items.Add(item);
                    }
                }
            }