Currently for each UI's code behind I have following code:

#if UNITY_5_3_OR_NEWER
#define NOESIS
using Noesis;
#else
using System.Windows.Controls;
#endif

    public partial class Testing : UserControl
    {
        public Testing()
        {
            InitializeComponent();
        }

#if NOESIS
        private void InitializeComponent()
        {
            Noesis.GUI.LoadComponent(this, "Assets/Root/UI/Testing.xaml");
        }
#endif
    }

I wanted to avoid that, and for that I've created my custom UserControl class:

#if UNITY_5_3_OR_NEWER
#define NOESIS
#endif

namespace Alchemy.UI.Misc
{
    public class UserControl : 
#if NOESIS
        Noesis.UserControl
#else
        System.Windows.Controls.UserControl
#endif
    {
#if NOESIS
        protected void InitializeComponent()
        {
            var filepath = $"Assets/Root/UI/{GetType().Name}.xaml";
            Noesis.GUI.LoadComponent(this, filepath);
        }
#endif
    }
}

And this way I don't have to use #if directives in my UIs. Xaml in that case looks following:

<misc:UserControl x:Class="Alchemy.UI.Testing"
             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"
             xmlns:local="clr-namespace:Alchemy.UI"
             xmlns:misc="clr-namespace:Alchemy.UI.Misc"
             mc:Ignorable="d"
             d:DesignHeight="1080"
             d:DesignWidth="1920"
             d:DataContext="{d:DesignInstance {x:Type local:TestingViewModel}}">
    <Grid>
    </Grid>
</misc:UserControl>

And that works perfectly in WPF (in blend solution). But in Unity (in Noesis) I'm getting following error:

[noesis] Assets/Root/UI/Testing.xaml(1): Unknown type 'Alchemy.UI.Misc.UserControl'.
UnityEngine.Debug:LogError(Object)
NoesisUnity:UnityLog(Int32, String) (at Assets/NoesisGUI/Plugins/NoesisUnity.cs:141)
Noesis.GUI:Noesis_LoadXaml(String)
Noesis.GUI:LoadXaml(String) (at Assets/NoesisGUI/Plugins/API/Core/NoesisGUI.cs:244)
NoesisXaml:Load() (at Assets/NoesisGUI/Plugins/NoesisXaml.cs:30)
NoesisPostprocessor:ImportXaml(String, Boolean, Boolean, Boolean&) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:372)
NoesisPostprocessor:ImportXaml(String, Boolean, Boolean) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:299)
NoesisPostprocessor:ImportAssets(String[], Boolean, UpdateProgress) (at Assets/NoesisGUI/Plugins/Editor/NoesisPostprocessor.cs:78)