How to add support Adding/Removing Item/Items in Xaml in Runtime?

Question

How to add support Adding/Removing Item/Items in Xaml in Runtime?

Mahdi Hosseini 310

we can easily add/remove items in combobox in runtime like:

<ComboBox>
    <ComboBoxItem/>
    <ComboBoxItem/>
    <ComboBoxItem/>
</ComboBox>

now i want to support current approach in my control.

i started with a single item but i get error in xaml in runtime

User's image

this is my Renderer property:

public RendererBase Renderer
{
    get { return (RendererBase)GetValue(RendererProperty); }
    set { SetValue(RendererProperty, value); }
}

public static readonly DependencyProperty RendererProperty =
    DependencyProperty.Register(nameof(Renderer), typeof(RendererBase), typeof(ShaderBackground), new PropertyMetadata(null, OnRendererChanged));

private static void OnRendererChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var ctl = (ShaderBackground)d;
    ctl.rendererBase = (RendererBase)e.NewValue;
}

and this is my RendererBase:

public class RendererBase : DependencyObject, IRenderer

and IRenderer:

internal interface IRenderer : IDisposable

Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-04T10:54:46.1033333+00:00

Hi @Mahdi Hosseini ,

I will look into this and I will get back to you as soon as possible.

2 answers

Your answer

Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-04T10:54:46.1033333+00:00

Hi @Mahdi Hosseini ,

I will look into this and I will get back to you as soon as possible.

Answer 1

RendererBase and its concrete implementations must be creatable by the XAML loader. The error “Type X cannot be created in XAML” means the type does not satisfy the requirements for XAML construction.

To support usage like:

<local:ShaderBackground x:Name="ShaderSample">
    <local:ShaderBackground.Renderer>
        <local:SnowRenderer />
    </local:ShaderBackground.Renderer>
</local:ShaderBackground>

ensure all of the following:

RendererBase must be public

public class RendererBase : DependencyObject, IRenderer
{
}

IRenderer must be public (or remove it from the public API surface of the property)

public interface IRenderer : IDisposable
{
}

The concrete renderer (SnowRenderer) must be public and have a public parameterless constructor

public sealed class SnowRenderer : RendererBase
{
    public SnowRenderer() { }
}

The dependency property type must be the base type that XAML can instantiate

public static readonly DependencyProperty RendererProperty =
    DependencyProperty.Register(
        nameof(Renderer),
        typeof(RendererBase),
        typeof(ShaderBackground),
        new PropertyMetadata(null, OnRendererChanged));

public RendererBase Renderer
{
    get => (RendererBase)GetValue(RendererProperty);
    set => SetValue(RendererProperty, value);
}

The namespace where SnowRenderer is declared must be mapped in XAML, for example:

xmlns:local="using:App1"

After these changes and a full rebuild, the XAML loader can create SnowRenderer and assign it to the Renderer property at runtime, similar to how ComboBoxItem elements are created and added to a ComboBox.

For adding/removing multiple items at runtime (like ComboBox items), expose a collection property (for example, an ObservableCollection<RendererBase>) and manipulate it in code, or bind it to a collection in a view model. The XAML loader will populate the collection from nested elements in XAML, and at runtime items can be added/removed from that collection.

References:

Answer 2

Nancy Vo (WICLOUD CORPORATION) 8,155 Microsoft External Staff Moderator

Hello @Mahdi Hosseini ,

Thanks for your question.

Your control only has a single Renderer property, so XAML doesn't know where to put child items. To support adding/removing multiple renderers like ComboBox does, I recommend switching to a collection. You can refer to the following steps and code examples:

Add [ContentProperty] and replace Renderer with a collection.

[ContentProperty(nameof(Renderers))]
public class ShaderBackground : Control
{
    public ObservableCollection<RendererBase> Renderers { get; }
        = new ObservableCollection<RendererBase>();

    public ShaderBackground()
    {
        Renderers.CollectionChanged += OnRenderersChanged;
    }

    private void OnRenderersChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.NewItems != null)
            foreach (RendererBase r in e.NewItems)
            { /* attach/start renderer */ }

        if (e.OldItems != null)
            foreach (RendererBase r in e.OldItems)
                r.Dispose();
    }
}

Use it in XAML like ComboBox

<local:ShaderBackground>
    <local:GlowRenderer />
    <local:BlurRenderer />
    <local:NoiseRenderer />
</local:ShaderBackground>

Add or remove at runtime

myShaderBackground.Renderers.Add(new GlowRenderer());

myShaderBackground.Renderers.Remove(glowRenderer);

I hope this addresses your question. If this response was helpful, please consider following the guidance to provide feedback.

Mahdi Hosseini 310 Reputation points

2026-06-05T07:41:44.17+00:00

Thank you, but did you test it your self?

because i see same error again:
Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-05T08:25:39.65+00:00
Hi @Mahdi Hosseini ,

I tested this on my end. Please press Ctrl + Shift + B to build first, then use TestRender in XAML. Also make sure TestRender has a public parameterless constructor, because XAML requires it to instantiate any class in markup:

public class TestRender : RendererBase { public TestRender() { } }

Please try this and let me know the results. I’d be happy to investigate further if needed.
Mahdi Hosseini 310 Reputation points

2026-06-05T09:19:37.9266667+00:00

Unfortunately, you did not pay attention to my question. Your approach only works if the app is not compiled or run. However, when you run the app in debug mode and try to add the renderer at runtime, XAML throws an error. Please test this. my all classes and renderer are public and ctor.

Nancy Vo (WICLOUD CORPORATION) 8,155 Microsoft External Staff Moderator

Hi @Mahdi Hosseini ,

Sorry for the late response. I tested again and changed RendererBase base class from DependencyObject → Freezable, collection from ObservableCollection<T> → FreezableCollection<T>. You can refer to my code example and let me know results.

RenderBase.cs:

public abstract class RendererBase : Freezable, IRenderer
{
    protected override Freezable CreateInstanceCore()
        => throw new NotImplementedException();

    public abstract void Dispose();
}

ShaderBackground.cs:

[ContentProperty(nameof(Renderers))]
public class ShaderBackground : Control
{
    public FreezableCollection<RendererBase> Renderers { get; }
        = new FreezableCollection<RendererBase>();

    public ShaderBackground()
    {
        ((INotifyCollectionChanged)Renderers).CollectionChanged += OnRenderersChanged;
    }

    private void OnRenderersChanged(object? sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.NewItems != null)
            foreach (RendererBase r in e.NewItems)
            { /* attach renderer */ }

        if (e.OldItems != null)
            foreach (RendererBase r in e.OldItems)
                r.Dispose();
    }
}

Each renderer class:

public class GlowRenderer : RendererBase
{
    public GlowRenderer() { }

    protected override Freezable CreateInstanceCore()
        => new GlowRenderer();

    public override void Dispose() { }
}

Mahdi Hosseini 310 Reputation points

2026-06-09T12:27:34.3266667+00:00

there is no Freezable in WinUI!

are you using AI to answer my questions?
Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-10T02:35:43.0833333+00:00
I sincerely apologize for the confusion. I completely assumed your project was WPF without asking first, which led me to provide a wrong solution. I should have asked about your framework from the very beginning before giving any advice. I created a project for testing purposes, but I did not verify the framework thoroughly. I am truly sorry for this mistake.

Since you are using WinUI 3, I recommend the following:

Base class: keep DependencyObject

Collection: use ObservableCollection<T>

ContentProperty syntax: use [ContentProperty(Name = nameof(...))]
Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-12T07:55:05.66+00:00

Hi @Mahdi Hosseini ,

If you're still interested or have any questions, feel free to reach out whenever it works best for you.
Nancy Vo (WICLOUD CORPORATION) 8,155 Reputation points Microsoft External Staff Moderator

2026-06-16T03:30:23.2166667+00:00

Hi @Mahdi Hosseini ,

If you still need assistance, please don’t hesitate to reach out at any time. I’ll be happy to help.