.NET MAUI False XC0103 Warning for CommunityToolkit.Maui Converters

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I am encountering false warnings about markup extension annotations in my .NET MAUI project, even though the annotations are present in the CommunityToolkit.Maui converters.

For example, using the CommunityToolkit.Maui.Converters.IsNullConverter produces the warning:

XC0103 Consider attributing the markup extension "CommunityToolkit.Maui.Converters.InvertedBoolConverter" with [RequireService] or [AcceptEmptyServiceProvider] if it doesn't require any.

The community toolkit source confirms these annotations are applied, as seen in the source code.

The warning appears in the XAML where the converters are added as resources:

XAML
<ContentPage
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
    xmlns:vm="clr-namespace:MenuTest"
    x:DataType="vm:MainViewModel"
    x:Class="Test.MainPage">

    <ContentPage.Resources>
        <toolkit:IsNullConverter x:Key="IsNullConverter"/>
        <!-- WARNING HERE -->
        <toolkit:IsNotNullConverter x:Key="IsNotNullConverter"/>  <!-- WARNING HERE -->
    </ContentPage.Resources>

    <ScrollView>
...
    </ScrollView>

</ContentPage>

I am using:

Visual Studio Version 17.12.4
CommunityToolkit.Maui 9.1.1

Steps to reproduce:

Download the Example
Rebuild the project
Observe the warnings in the Error List window

For reference, see this MAUI issue.

Since the classes are already annotated, I do not expect these warnings to appear. Any insight or workaround would be appreciated.

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is an explanation of what’s happening and some approaches you can take.

What’s Happening

Even though the Community Toolkit converters already include the appropriate markup extension attributes (such as [AcceptEmptyServiceProvider] or [RequireService]), the XAML analyzer in Visual Studio 17.12.4 is incorrectly flagging them as missing. The analyzer inspects the type’s metadata at build time, and in this case it appears that for some reason it isn’t picking up the annotations from the shared source files. This is a known issue documented in the MAUI issue you referenced as well as various discussions in the community.

Why It Appears

Shared Code Setup: The Community Toolkit converters are typically implemented as partial classes (with a “.shared.cs” file) so that platform-specific implementations can be provided. The attribute may live in the shared part, but the analyzer sometimes fails to resolve it correctly.
Tooling Bug: The warning (XC0103) is a false positive triggered by the current XAML analyzer in Visual Studio 17.12.4. This is not an issue with your usage or the source code in CommunityToolkit.Maui.

What You Can Do

Update Visual Studio:
Check if an update (for example, 17.12.6 or later) is available as the issue may have been fixed in a more recent version of Visual Studio.
Suppress the Warning:
Since it’s a false positive, you might choose to suppress the analyzer warning. There are a few ways to do this:
- Global Suppressions:
  You can add a GlobalSuppressions.cs file in your project with content similar to:
```
CSHARP
[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Xamarin", "XC0103", Justification = "False positive due to VS analyzer bug")]
```
- Project File Suppression:
  Alternatively, add a property to your project file to disable that specific analyzer check for markup extensions if available.
Monitor the Issue:
Keep an eye on the official MAUI documentation or issue tracker. The workaround may be lifted or fixed once the Visual Studio tooling is updated to correctly recognize the attributes on markup extensions from community toolkit assemblies.

Summary

Even though the converters are properly annotated in the CommunityToolkit.Maui source, a tooling bug in VS 17.12.4 leads to false warnings. Updating Visual Studio or suppressing these specific warnings in your project are the best ways to work around the issue.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by StellarPioneer428 • 1 month ago

I cloned your project and reproduced the problem. I have checked the dependnecies of the nuget package and found that the CommunityToolkit.Maui 9.1.1 didn't have the dpendency for .net 9.0.

And the CommunityToolkit.Maui 10.0.0 and CommunityToolkit.Maui 11.0.0 both have dependency for .net 9.0.

When I used the two versions of the CommunityToolkit.Maui nuget package and rebuild the project, the warning in your example disappeared.

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Discussion

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.NET MAUI False XC0103 Warning for CommunityToolkit.Maui Converters

2 Answers

What’s Happening

Why It Appears

What You Can Do

Summary

Discussion

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