ActionMock.cs

public class ActionMock<T>
{
    private int TimesExecuted { get; set; }
    public Action<T> Action { get; private set; }

    public ActionMock()
    {
        this.Action = _ => this.TimesExecuted++;
    }

    public void AssertWasCalledExactly(int expectedTimes)
    {
        Assert.Equal(expectedTimes, this.TimesExecuted);
    }
}