How do I use an array from code in the xaml file? C#

will be helpful for those in need This is because Visual Studio doesn't know they are associated. It's just a naming convention to have .xaml and .xaml.cs. IN the project file you need to enforce the relationship via:

<Compile Include="NodeControl.xaml.cs">
  <DependentUpon>NodeControl.xaml</DependentUpon>
</Compile>

this will help I want to merge one XAML file, that only contains styles into the Window.Resource of my application. I tried using MergedDictionaries as described in WPF Reference custom resource defined in another xaml file . , Here is a demo wpf project:

-WpfApplication
 |--App.xaml
 |--MyStyle[here is a floder]
   |---MyStyleDocument1.xaml
   |---MyStyleDocument2.xaml
   |---MyStyleDocument3.xaml

 <Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
              <!--add style file here like this-->
              <ResourceDictionary Source="/MyStyle/MyStyleDocument1.xaml" />
        </ResourceDictionary.MergedDictionaries>
    <ResourceDictionary>
 <Application.Resources>

I think the issue was by ths following , What are you trying to do here? The code you posted has got nothing to do with accessing "XAML objects in another file". The Application.Properties property is just a dictionary for sharing data, in a thread-safe fashion between different parts of your application. It is very rarely used in my experience - in fact I have never seen it used. You are getting a null exception because you probably haven't added the 'favoritesItem' to the dictionary first.
If you want to access a named XAML element from another class you will need to expose it via a public property. (Named XAML elements create private member fields in their defining class).

I wish this helpful for you A really simple solution would just be to house each of your rooms inside a user control and then place the control into your container grid when you need to change rooms. Here's the rough idea:

public partial class MainWindow : Window
{
    UserControl _currentRoom;  

    public MainWindow()
    {
        InitializeComponent();
    }

    protected override void OnPreviewMouseDown(MouseButtonEventArgs e)
    {
         this.areaContainer.Children.Clear();
         _currentRoom = null;

        if (e.LeftButton == MouseButtonState.Pressed)
            _currentRoom = new Room1(); 
        if(e.RightButton == MouseButtonState.Pressed) 
            _currentRoom = new Room2(); 

        this.areaContainer.Children.Add(_currentRoom); 
        base.OnPreviewMouseDown(e);
    }

}

<UserControl x:Class="Test.Room1"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
    <Button Height="100" Width="100">
        Hello world!
    </Button>   
</UserControl>

Hope this helps
You may perhaps bind the Content property of a ContentControl, with a Binding Converter that returns the result of XamlReader.Load.