CodeValue.CodeCommander.FilterManager.AddFilter C# (CSharp) Method

        public void AddFilter(IFilter filter)
        {
            lock (holdChanges)
            {
                _filtersChanged = true;
                Filters.Add(filter);
            }
        }

 public void AddFilter_AddingAFilterWhenFilterIsAlreadyAdded_FilterManagerHasOneFilter()
 {
     // Arrange
     FilterManager target = new FilterManager();
     var filter = Mock.Create<IFilter>();
     // Act
     target.AddFilter(filter);
     target.AddFilter(filter);
     // Assert
     Assert.IsTrue(target.CurrentFilters.Length == 1);
 }