Prevent duplicating code in subclass using Timer Elapsed

I have a base class which performs a Timer Elapsed call. Subclasses of this class also perform a Timer Elapsed call after the base class has finished. The base class sets a protected boolean which the subclass uses to determine if it needs to continue. See code below.

    protected virtual async void OnTimerElapsed(object sender, ElapsedEventArgs e)
    {
      if (someCondition) {
          isValid = false;
      }
      else {
          isValid = true;
      }

      if (!isValid)
      {
         return;
      }
    }

    protected override async void OnTimerElapsed(object sender, ElapsedEventArgs e)
    {
      base.OnTimerElapsed(sender, e);

      if (!isValid)
      {
        return;
      }
    }

I want to somehow eliminate the need for the if statement in the override function in the subclass, but I suspect there is no clean way to do this.

Does this appear to be the best way of doing this?

I've tried thinking about different ways of writing this, but because the Timer.Elapsed function is natively void, I couldn't think of other ways...

You can make OnTimerElapsed private and make the subsequent processing virtual.
Example code (I ignore async for simplicity):

public class BaseClass
{
    private bool _isValid;

    private bool Validate() { ... }

    private void OnTimerElapsed(object sender, ElapsedEventArgs e)
    {
        _isValid = Validate();
        if (!_isValid)
        {
            return;
        }

        DoProcessingWhenValid();
    }

    protected virtual void DoProcessingWhenValid() { ... }
}

public class DerivedClass : BaseClass
{
    protected override void DoProcessingWhenValid();
    {
        base.DoProcessingWhenValid();
        // More work here.
    }
}

Note that technically you don't even need _isValid in this simple scenario, but it might make sense to still have the field, and potentially for it to remain protected (or even protected { get; private set; } if you want to get fancy).

The net result of this is that you only check _isValid once, in the base class. All derived classes can assume that DoProcessingWhenValid is only called when _isValid is true. If desired, you can pass the sender and ElapsedEventArgs into DoProcessingWhenValid, if the processing needs some of the information.