AttachedProperty cannot be assigned in a XAML file if it is declared in the same project.

• .NET Core Version: 5.0.203
• Windows version: 10.0.19043.985
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

Problem description:

Declare a class with an AttachedProperty:

public static class DependencyTest
{
    public static readonly System.Windows.DependencyProperty FooProperty = System.Windows.DependencyProperty.RegisterAttached(
        "Foo", typeof(bool), typeof(DependencyTest),
        new System.Windows.PropertyMetadata(default(bool)));

    public static bool GetFoo(System.Windows.DependencyObject element) => (bool)element.GetValue(FooProperty);

    public static void SetFoo(System.Windows.DependencyObject element, bool value) => element.SetValue(FooProperty, value);
}

Use the AttachedProperty in a XAML in the same project:

<Window x:Class="Walterlv.Demo.XamlProperties.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Walterlv.Demo.XamlProperties"
        xmlns:system="clr-namespace:System;assembly=System.Runtime">
    <local:DependencyTest.Foo>
        <system:Boolean>True</system:Boolean>
    </local:DependencyTest.Foo>
    <Grid>
    </Grid>
</Window>

Then the build will fail.

If the AttachedProperty is written in another project, the build succeeds.

Actual behavior:

The error message is:

error MC3089: The object 'Window' already has a child and cannot add ''. 'Window' can accept only one child.

Expected behavior:

There is no building error and it can be treated as an AttachedProperty.

Minimal repro:

See the repo: https://github.com/walterlv/Walterlv.Issues.XamlProperties

[Kuldeep-MS]

We can not find the Assembly in NamespaceMapEntry class when the AttachedProperty is written in the same project. And we can not find the know type in CompileBamlTag in XamlReaderHelper.cs

I have added a fix here. Can you please give it a try and let me know if it works.

[walterlv]

The AttachedProperty can be completely distinguished from nested classes because the nested class is not supported in a XAML.

See the official docs here:

• XAML and custom classes - WPF .NET Framework | Microsoft Docs

Your custom class must not be a nested class. Nested classes and the “dot” in their general CLR usage syntax interfere with other WPF and/or XAML features such as attached properties.