WPF&MVVM: Library System.Windows.Interactivity in not available anymore?

I need to add `System.Windows.Interactivity.dll` library via `Reference Manager`. In `Visual Studio 2017`, I did not find it. All search results that begins from `System.Windows` showed on the screenshot below: I thought that this library becomes build-in in WPF project of current Visual Studio version, so when I added the bellow `C#` code, no warnings from IDE has been displayed: ``` private RelayCommand doubleCommand; public RelayCommand DoubleCommand { get { return doubleCommand ?? (doubleCommand = new RelayCommand(obj => { Phone phone = obj as Phone; if (phone != null) { Phone phoneCopy = new Phone { Company = phone.Company, Price = phone.Price, Title = phone.Title }; Phones.Insert(0, phoneCopy); } })); } } ``` However, when I added the following `XAML` markup, it says that `Interaction`, `EventTrigger` and `InvokeCommandAction` has not been found in `clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity` namespace: ``` <Window x:Class="MVVM_Tutorial.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:local="clr-namespace:MVVM_Tutorial" xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity" mc:Ignorable="d" Title="MainWindow" Height="350" Width="525"> <!-- ... --> <Button Content="2x"> <i:Interaction.Triggers> <i:EventTrigger EventName="MouseDoubleClick"> <i:InvokeCommandAction Command="{Binding DoubleCommand}" CommandParameter="{Binding SelectedPhone}" /> </i:EventTrigger> </i:Interaction.Triggers> </Button> <!-- ... --> </Window> ``` Has it relationship with `System.Windows.Interactivity` library or no, what have I do to solve this problem?