Attaching Behaviour to MetroWindow fails and results in wrong Style

Question

I have a simple test application that shows a simple Mahapps MetroWindow with an attached Behaviour. The problem is when attaching the Behaviour the outer border of the Mahapps MetroWindow is drawn.

<controls:MetroWindow x:Class="Desktop.Shell.Modern.ShellView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:controls="http://metro.mahapps.com/winfx/xaml/controls"
        xmlns:cal="http://www.caliburnproject.org"
        xmlns:modern="clr-namespace:Desktop.Shell.Modern"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        Height="350"
        Width="525"
        ResizeMode="CanResizeWithGrip"
        WindowTransitionsEnabled="True"
        ShowIconOnTitleBar="False"
        TitleCaps="False"
        GlowBrush="{DynamicResource WindowTitleColorBrush}" >
    <i:Interaction.Behaviors>
        <modern:SomeBehavior SomeKey="{Binding Key}" />
    </i:Interaction.Behaviors>
    <ContentControl cal:View.Model="{Binding ActiveItem}" />
</controls:MetroWindow>

When removing the Behaviour everything looks as expected:

... but the Behaviour itself did nothing (yet). Here is the code of the SomeBehaviour class:

public sealed class SomeBehavior : Behavior<Window>
{
    public static readonly DependencyProperty SomeKeyProperty =
      DependencyProperty.Register(
        "SomeKey",
        typeof(Key),
        typeof(SomeBehavior),
        new PropertyMetadata(default(Key)));

    public Key SomeKey
    {
        get
        {
            return (Key)this.GetValue(SomeKeyProperty);
        }

        set
        {
            this.SetValue(SomeKeyProperty, value);
        }
    }

    protected override void OnAttached()
    {
        base.OnAttached();

    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
    }
}

Am I doing something wrong? Should I attach the Behaviour in a different way than attaching them to "normal" Windows?

Answer 1

That's because Mahapps.Metro sets its required behaviours in the window style, see MetroWindow.xaml.

If you want to attach additional behaviors, you have to copy these behaviours to your window, e.g.

<controls:MetroWindow x:Class="Desktop.Shell.Modern.ShellView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="http://metro.mahapps.com/winfx/xaml/controls"
    xmlns:cal="http://www.caliburnproject.org"
    xmlns:modern="clr-namespace:Desktop.Shell.Modern"
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:Behaviours="http://metro.mahapps.com/winfx/xaml/shared"
    Height="350"
    Width="525"
    ResizeMode="CanResizeWithGrip"
    WindowTransitionsEnabled="True"
    ShowIconOnTitleBar="False"
    TitleCaps="False"
    GlowBrush="{DynamicResource WindowTitleColorBrush}" >
    <i:Interaction.Behaviors>
        <modern:SomeBehavior SomeKey="{Binding Key}" />
        <Behaviours:BorderlessWindowBehavior />
        <Behaviours:WindowsSettingBehaviour />
        <Behaviours:GlowWindowBehavior />
    </i:Interaction.Behaviors>
    <ContentControl cal:View.Model="{Binding ActiveItem}" />
</controls:MetroWindow>