Client object model thread safety

Question

Is ClientContext thread-safe? I'm using Parallel framework to parallelize some work that creates new items on the list. Example:

using(var ctx = new ClientContext(siteUrl))
{
    var list = ctx.Web.GetList('/List/somelist')
    var subList = ctx.Web.GetList('/List/sublist')
    ctx.Load(list):
    ctx.Load(subList):
    ctx.ExecuteQuery();
    var myExternalItems = collection of items

    Parallel.ForEach(myExternalItems , new ParallelOptions() { MaxDegreeOfParallelism = 4 }, myitem =>
    {
        var item = list.AddItem(new ListItemCretionInformation());

        item["Title"] = myitem.Title; //just an example

        item.Update();
        ctx.ExecuteQuery();

        foreach(var sitem in myitem.SubItems)
        {
            var subitem = subList.AddItem(new ListItemCretionInformation());

            subitem["Title"] = sitem.Title;

            subitem.Update();
            ctx.ExecuteQuery();
        }
    });
}

It fails with:

An unhandled exception of type 'System.StackOverflowException' occurred in Microsoft.SharePoint.Client.Runtime.dll

And in Call Stack:

The maximum number of stack frames supported by Visual Studio has been exceeded.

It works if I replace Parallel.ForEach with a normal sequent foreach loop. It's not an error in field values or names, but something related to ClientContext's web request I guess. So is Client Object Model thread safe?

Answer 1

As the MSDN documentation states, instance members of ClientContext are not guaranteed to be thread-safe:

http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.client.clientcontext.aspx