.Net Maui Save error: 'x' is an undeclared prefix if using MauiAllowImplicitXmlnsDeclaration

[alistair-ocad]

Describe the bug
When saving a .Net Maui xaml file the error " 'x' is an undeclared prefix " appears. This is when using implicit namespaces.

To Reproduce

1. Create a .Net Maui project

2. Add this to the project file:
   <PropertyGroup> <!-- First, enable preview feature flag --> <EnablePreviewFeatures>true</EnablePreviewFeatures> <!-- Then, opt-in for the implicit namespace feature --> <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants> </PropertyGroup>

3. Create a client page/view.

4. Remove:
   xmlns="http://schemas.microsoft.com/dotnet/2021/maui" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"

5. According to the documentation these should not be necessary.
   See: https://devblogs.microsoft.com/dotnet/simpler-xaml-in-dotnet-maui-10/

6. Save, and the error message appears.

7. What's more, the code compiles and runs perfectly without these two lines.

Expected behavior
The file will save, accepting the missing "x:..." declaration, preferably understanding that "MauiAllowImplicitXmlnsDeclaration" is being used. Or have an option in the XamlStyler to ignore these.

Screenshots

Version Info (please complete the following information):

• Windows 11
• Visual Studio: 2026
• XAML Styler: version 3.2501.8
• .Net 10

[Mikabishop64]

Hello ! Up ? Is the project still alive ?

[alistair-ocad]

Hello ! Up ? Is the project still alive ?

Who knows.... my xaml is all formatted manually again :-(

[j-sich-fiducial]

I can confirm this issue on a .NET MAUI 10 project.

The project builds and runs correctly with implicit XAML namespaces, but XAML Styler no longer formats the XAML files unless xmlns:x is explicitly declared.

Could you please consider fixing this issue as soon as possible? This feature is very useful for MAUI projects.