[Bug]: Unable to use the [Reactive] attribute with error that attribute is inaccessible due to its protection level

[matthewacme]

Describe the bug 🐞

Trying to convert my system over to use the new SourceGenerator nuget package instead of the ReactiveUI Fody nuget package.

Removed Fody assembly from references
Added the ReactiveUI.SourceGenerators nuget to the project

Tried to build and got hundreds of these errors

Step to reproduce

1. Open a project that was created using ReactiveUI.Fody and that is currently working correctly
2. Confirm the project is set to use <TargetFramework>net8.0-windows10.0.26100.0</TargetFramework>
3. Confirm the project is set to use <LangVersion>preview</LangVersion>
4. Remove Fody nuget reference
5. Add ReactiveUI.SourceGenerators nuget package reference
6. Open class that inherits from ReactiveObject

• [Mod to steps]
• I forgot to add the step,
  - Set the class and the properties that have the Reactive attribute to be partial

7. Clean Solution
8. close Visual Studio and clean up all the build artifacts (bin / obj / debug, etc artifacts)
9. Reopen Visual Studio and the solution
10. Try to build
11. You get errors everywhere the [Reactive] attribute is declared with an error or 'ReactiveAttribute' is inaccessible due to its protection level
12. Right mouse click on an instance where the error is shown for [Reactive] attribute and select "Go to definition"
13. The ReactiveUI.SourceGenerator is generating the ReactiveAttribute as an internal sealed class.

Reproduction repository

https://github.com/reactiveui/ReactiveUI

Expected behavior

I should complete steps 1 thru 8 successfully and then when open and built, the errors should show up only for those properties that don't match the new format system for the Reactive attribute usage

Screenshots 🖼️

No response

IDE

Visual Studio 2022

Operating system

Windows11 - 10.0.26200.7171

Version

No response

Device

Microsoft Surface Book

ReactiveUI Version

22.3.1 SourceGenerators - 2.5.1

Additional information ℹ️

No response

[glennawatson]

Are you adding references to the source generator NuGet for each project that uses it? Eg any project you use the reactive attribute will require a reference.

[matthewacme]

Short answer... Yes I actually have a solution that only has three projects in it. And only one of the projects has a reference to the ReactiveUI new stuff. [cid:d979255e-8259-4529-b2b5-7ad6cb129bf5] Matthew C. Hintzen [he-him-his] | Windows Washer • w – ***@***.******@***.***> p – 321.234.8839 •

…

________________________________ From: Glenn ***@***.***> Sent: Thursday, December 4, 2025 2:21 PM To: reactiveui/ReactiveUI.SourceGenerators ***@***.***> Cc: Matthew Hintzen ***@***.***>; Author ***@***.***> Subject: Re: [reactiveui/ReactiveUI.SourceGenerators] [Bug]: Unable to use the [Reactive] attribute with error that attribute is inaccessible due to its protection level (Issue #338) [https://avatars.githubusercontent.com/u/5834289?s=20&v=4]glennawatson left a comment (reactiveui/ReactiveUI.SourceGenerators#338)<#338 (comment)> Are you adding references to the source generator NuGet for each project that uses it? Eg any project you use the reactive attribute will require a reference. — Reply to this email directly, view it on GitHub<#338 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ACVGS2AOFKGM5FVYNNOUEMD4ACCSVAVCNFSM6AAAAACOBYCU3GVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTMMJTHE3TEOBYHE>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[glennawatson]

If you're getting the inaccessible I would be double checking your package references. Source generators inject their dependencies in as source you compile for the project.

[matthewacme]

Could you expand on that thought? What should I be looking for in my package references that would cause an issue with the usage of the ReactiveUI.SourceGenerators? Again, this is a three project solution. Only one of the projects has a reference to anything related to ReactiveUI nuget packages If I use ReactiveUI.Fody, it builds If I remove ReactiveUI.Fody and replace it with ReactiveUI.SourceGenerators, clean the solution and delete all generated file (Obj / Bin, etc folder), and update the using statements to reference ReactiveUI.SourceGenerators, it won't compile and generates a ReactiveAttribute that is internal to the namespace ReactiveUI.SourceGenerator (so can't be used in my code space) [cid:278379be-171f-42d7-be94-00dc3b21cd7a] Matthew C. Hintzen [he-him-his] | Windows Washer • w – ***@***.******@***.***> p – 321.234.8839 •

…

________________________________ From: Glenn ***@***.***> Sent: Thursday, December 4, 2025 2:29 PM To: reactiveui/ReactiveUI.SourceGenerators ***@***.***> Cc: Matthew Hintzen ***@***.***>; Author ***@***.***> Subject: Re: [reactiveui/ReactiveUI.SourceGenerators] [Bug]: Unable to use the [Reactive] attribute with error that attribute is inaccessible due to its protection level (Issue #338) [https://avatars.githubusercontent.com/u/5834289?s=20&v=4]glennawatson left a comment (reactiveui/ReactiveUI.SourceGenerators#338)<#338 (comment)> If you're getting the inaccessible I would be double checking your package references. Source generators inject their dependencies in as source you compile for the project. — Reply to this email directly, view it on GitHub<#338 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ACVGS2FLSIEEANTXBSLEGE34ACDSRAVCNFSM6AAAAACOBYCU3GVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTMMJUGAYDINBYGE>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[ChrisPulman]

This is the legacy way of creating a Reactive Property using ReactiveUI.Fody.

using ReactiveUI.Fody;

namespace MyNamespace;

public class MyClass
{
	[Reactive]
	public int MyProperty { get; set; }
}

---

This is the new way to use ReactiveUI.SourceGenerators with C# 13+ note the use of partial

using ReactiveUI.SourceGenerators;

namespace MyNamespace;

// NOTE : Added Partial
public partial class MyClass
{
	// NOTE : Added Partial
	[Reactive]
	public partial int MyProperty { get; set; }
}

[matthewacme]

@ChrisPulman

I have the partial on both the class and on the property as noted in the readme (https://github.com/reactiveui/ReactiveUI.SourceGenerators?tab=readme-ov-file#usage-reactive-property-from-partial-property-with-default-value)

Still seeing the error that the ReactiveAttribute class is being generated as "internal" to the ReactiveUI namespace

I see I forgot to include that need to add the partial modifiers in the "steps to reproduce". but I did make those necessary changes originally and have modified the "steps to reproduce" to indicate as such

GitHub

GitHub - reactiveui/ReactiveUI.SourceGenerators: Use source generators to generate objects.

Use source generators to generate objects. . Contribute to reactiveui/ReactiveUI.SourceGenerators development by creating an account on GitHub.

[ChrisPulman]

@matthewacme

Thank you for clarifying, something that we have experienced many missed.

The fact that you are saying that the ReactiveAttribute is internal which is by design, this should not be an issue.
Have you added ReactiveUI.SourceGenerators to each project that you are using the generators within?
If you expand the Analysers section, you should see some or all of the following generators depending upon what you have defined

If you are not seeing this then it could be that you have not defined your C# version such as <LangVersion>13.0</LangVersion>

[matthewacme]

@ChrisPulman

Hey Chris. So a bit of an update.

First some background; I am attempting to PORT over projects that were set to use ReactiveUI 4 years ago originally in a UWP Store App. ( MyRadar is the app if you want some reference) due to the upcoming sunset of the UWP framework, we ported the app over to WinUI.

I saw the deprecation of the FODY so I decided to start by isolating one of our base object definition libraries in it's own standalone project, to limit any dependencies, and attempt to port it in a single use-case. The project makes heavy usage of Reactive / ReactiveUI so it is / was extensively instrumented with lots of [Reactive] attributes.

Also note all of my projects have had the <LangVersion>preview</LangVersion> declared in them and since I have Visual Studio 2026 installed, that includes <LangVersion>13.0</LangVersion> by implication.

I very carefully followed the instructions (and my steps I listed above) to start the port to source generators.

The important points here are.

1. The project was isolated, so no other dependencies or projects were in the solution that used or depended on the old Reactive Fody nuget package.
2. I removed the reference to Reactive.Fody from the project (and it naturally couldn't build, with the missing reference)
3. I ran a clean solution
4. I closed out the solution
5. I deleted all generated code in the source directory (including all the bins and objs folders) using powershell to make sure there were no left over traces of the Fody.
6. I opened the isolated project solution
7. I attempted to build and confirmed the build failed with errors for all declarations of the [Reactive] attributes
8. I added in the partial modifier to all classes and properties that had the [Reactive] attribute on them
9. I then added the reference to the Reactive.SourceGenerators to the project
10. I ran a clean solution, unloaded and again cleaned the directory using powershell
11. Reloaded the single project solution and attempted a build
12. and got the error that [Reactive] attribute was inaccessible

14. and that's when I filed this bug report.

So yesterday I found the "work around" (warning it's REALLY PAINFUL).

I decide first to get the project building without the ReactiveUI support for INotifyProperty support (since this is basically a object library that didn't actually use / need the INotifyProperty support, as a stand alone project this seemed like a reasonable first step)

I commented out ALL instantances of the [Reactive] attribute. And then I tried building the project and that worked fine.

Then I picked one of the simplest classes and uncommented the /*[Reactive]*/ attribute on ONE property, and attempted the build again and this time Visual Studio was able to build with no errors and that was successfully.

I then started going thru and uncommenting the [Reactive] attribute class by class trying to find the one that was causing the issue... and it just started building.

and I'm up and running now, but I'm not exactly sure why commenting out all the [Reactive] attributes, building WITHOUT them, then uncommenting them would fix the build bug.

I'm going to investigate a bit more carefully by replicating the whole process from scratch and see if I can figure out where the "pothole" that is causing the simple port to fail comes into play.

Matthew

[matthewacme]

@ChrisPulman

side note and not related to this bug per-se

Personally I prefer the Fody approach, mostly because the Fody approach allows for breakpoints to be set on properties in my own code, and can break on set operations. The SourceGeneration breaks this capability and now I have to search and find the MyClasss.g.cs instantiation made by the SourceGeneration and set the Break point in that code. And if the Source Generators regenerate the code, then my breakpoints are no longer valid and I have to do the process of finding / setting the breakpoints all over again, so I'm not a fan. Having said that I do understand that the majority of people don't like the blackbox aspect of Fody, and I fully understand why you folks have deprecated the Fody system.

[ChrisPulman]

I am glad that you came right in the end and fully understand that the debugging capabilities are different with Source Generated code. The partial properties give the impression that you can breakpoint on the get and set but in reality, they are never called once the property has been generated. It would have been nice if they had picked up on the debug attachment and forwarded it to the generated code, maybe in the future that will be possible.

I am thinking that the issue you are / were experiencing is related to the code not building correctly therefore the generator is failing to generate the code, once the code can compile then all is well, if I recall correctly the Source Generators run as the second part of the compile process and the core elements need to be correct first. Pressing "Return" in your code triggers the generator to build so long as that still produces valid code.

[matthewacme]

@ChrisPulman

Found the problem

Removing the Fody reference and cleaning the solution did NOT remove the Fody support files from the project directory

• FodyWeaver.xml
• FodyWeavers.xsd

I followed again the steps to where I originally logged the bug, and then looked in the project and noticed those files were STILL in the directory.

I deleted them and did a rebuild and it built and worked with the SourceGenerator nuget reference.

May I suggest you add a note to the nuget's Readme for people upgrading to make sure that after removing the ReactiveUI.Fody, they also need to delete any files in the project related to FodyWeaver

Matthew

I guess we can mark this bug as Closed.