WPFでWindowが閉じられたときにViewModelの後始末メソッドを呼ぶ

そういう動きをするビヘイビアを作ればOKです。こんな感じで。

public class ViewModelCleanupBehavior : Behavior<Window>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.Closed += this.WindowClosed;
    }

    private void WindowClosed(object sender, EventArgs e)
    {
        (this.AssociatedObject.DataContext as IDisposable)?.Dispose();
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        this.AssociatedObject.Closed -= this.WindowClosed;
    }
}

使い方はこんな感じ。

<Window x:Class="CloseAndDisposeSample.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:CloseAndDisposeSample"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        mc:Ignorable="d"
        Title="MainWindow"
        Height="350"
        Width="525">
    <Window.DataContext>
        <local:MainWindowViewModel />
    </Window.DataContext>
    <i:Interaction.Behaviors>
        <local:ViewModelCleanupBehavior />
    </i:Interaction.Behaviors>
    <Grid>
    </Grid>
</Window>