this.WhenAny not working for ObservableCollection

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MVVM

this.WhenyAny is not working for me when I try to check the count of ObservableCollection.

SampleCommand = new ReactiveCommand(this.WhenAny(c => c.TaskList, ((task) => task.Value != null && task.Value.Count > 0)));
 ......

 private ObservableCollection<Task> _taskList;

 public ObservableCollection<Task> TaskList
    {
        get { return _taskList; }
        set { this.RaiseAndSetIfChanged(ref _taskList, value); }
    }

I believe the condition is never satisfied as the functionality inside the command is never executed.

My original requirement is whenever the properties of this collection changes, I want to assign it to ObservablePropertyHelper type- isTaskUpdationInitiated. I have implemented the same as below

this.WhenAny(x => x.TaskList,  (z) => z.Value.Count > 0 && ( z.Value.Any(y =>
            ( y.Id  == TaskId &&  y.Prop1 != MyProp1Value &&
             y.Prop2 != MyProp2Value && y.Prop3 != MyProp3Value)
             ))).ToProperty( this, x=>x.IsTaskUpdationInitiated , out isTaskUpdationInitiated );

So whenever these conditions are satisfied, isTaskUpdationInitiated should be set to true. As this was not working, i tried a sample code that I have given intially. Please help!