Registered CancellationToken callback invocation order

Question

If I register some callbacks to a CancellationToken before it is cancelled, it seems they will be invoked in reverse order when the token is cancelled. Is this invocation order guaranteed?

var cts = new CancellationTokenSource();
var token = cts.Token;
token.Register(() => Console.WriteLine("1"));
token.Register(() => Console.WriteLine("2"));
token.Register(() => Console.WriteLine("3"));
cts.Cancel();

This will output

3
2
1

Answer 1

Well, from source code of class CancellationToken it appears to be so. The method ExecuteCallbackHandlers(bool throwOnFirstException) is responsible for getting callbacks and executing them. It contains this fragment of code:

try
{
   for (int index = 0; index < callbackLists.Length; index++)
   {
      SparselyPopulatedArray<CancellationCallbackInfo> list = Volatile.Read<SparselyPopulatedArray<CancellationCallbackInfo>>(ref callbackLists[index]);
      if (list != null)
      {
         SparselyPopulatedArrayFragment<CancellationCallbackInfo> currArrayFragment = list.Tail;

         while (currArrayFragment != null)
         {
             for (int i = currArrayFragment.Length - 1; i >= 0; i--)
             {
                ... some other code
             }
        }
     }
   }
}

In the inner for loop it traverses the fragment of array of callbacks backwards from last to first element.

However as noted by @Nick this is not guaranteed in the documentation. A simple solution would be to add callbacks into one delegate - this way we have more control over the execution order:

token.Register(() => 
{ 
   Console.WriteLine("1"));
   Console.WriteLine("2"));
   Console.WriteLine("3"));
});