Moq: Veriy if SendAsync of a Targetblock object was called

I've a class that is initialised with a reference to a ITargetBlock<T> object. In my unit tests I wanted to check if the SendAsync<T> method of ITargetBlock<T> was called. So I used the following verification code:

   this.targetFake.Verify(mock => mock.SendAsync(It.IsAny<InternalMessage>()), Times.Never());

When calling this code I get the following exception:

System.NotSupportedException

HResult=0x80131515

Message=Invalid verify on a non-virtual (overridable in VB) member: mock => mock.SendAsync(It.IsAny())

Source=Moq

StackTrace:

at Moq.Mock.ThrowIfVerifyNonVirtual(Expression verify, MethodInfo method)

at Moq.Mock.Verify[T,TResult](Mock1 mock, Expression1 expression, Times times, String failMessage)

at Moq.Mock1.Verify[TResult](Expression1 expression, Times times)

After some research I found at that SendAsync is an extension method in ITargetBlock, which can't be mocked.

So I guess my unit test approach is not the correct one when working with TPL dataflow. Can anyone give me a hint how to test a class like:

 public class Detector
 {
     ...

     public Detector(ITargetBlock<FailureMessage> target, ITimer timer)
     {
         _target = target;
         _timer.Elapsed = TimerElapsed;
         ...
     }

     private async void TimerElapsed(object sender, ElapsedEventArgs e) 
     {
          bool errorCondition = false;

          // Perform checks that set errorCondition to true
          ...

          if (errorCondition)
          {
               var rv = await SendAsync(new FailureMessage());
               ...
          }

          ...
      }
 }

My idea was to inject an ITimer decorator. Using the ITimer interface allows me to simulate elapsed timers in unit test environments.

Thx