Protobuf.net "The type cannot be changed once a serializer has been generated"

I have some fairly simple code using Protobuf.net, which is throwing a very odd exception. In MetaType.cs on line 167 it throws a InvalidOperationException "The type cannot be changed once a serializer has been generated". What does this mean and how do I fix it?

My code looks like this:

This method starts off all the serialising:

while (!Parallel.For(0, 100, (i) =>
{
    Widget w;
    lock (f) { w = f.CreateWidget(); }

    SerialiseWidget(w);

}).IsCompleted)
{
    Thread.Sleep(10);
}

Pretty simple, it just loops through everything in Parallel, and serialises 100 widgets.

The serialise method is also pretty s开发者_高级运维imple:

private byte[] SerialiseWidget(Widget w)
{
    using (MemoryStream m = new MemoryStream())
    {
        Serializer.Serialize<PacketChunk>(m, w);

        return m.ToArray();
    }
}

And finally, the widget class looks like this:

[ProtoContract]
private class Widget
{
    [ProtoMember(1)]
    public int a;

    [ProtoMember(2)]
    public byte[] b;

    [ProtoMember(3)]
    public Thing c; //Thing is itself a protocontract

    [ProtoMember(4)]
    public int d;

    [ProtoMember(5)]
    public int e;
}

Edit:: I suspect this may be to do with the fact I'm looping over in parallel. How threadsafe is Protobuf.net for that kind of thing?

How threadsafe is Protobuf.net for that kind of thing?

Well, it noticed something odd happening :)

Just add a call to Serializer.PrepareSerializer at some point before the theading (app startup being the obvious point) and it should forgive you...

Edit: thinking about it, there is a fairly simple change I can make to help that specific scenario; I'll tweak that when I get a chance. Also, this should only affect the unreleased "v2" code (from sorce code) - the pre-built dll is not affected by this AFAIK.