Win Forms Designer not loading

I think I found what's wrong. 

When I create a new WinForms application, I can select `net9.0` for my framework. This fails to load in the IDE. When I look at the properties of my csproj, I notice the target framework is `net9.0-windows`. When I deselect this and select `net8.0-windows` then my designer will load properly. 

Interestingly if I select `net8.0-windows` and return to my csproj properties, `net9.0-windows` is no longer an option.

Hello,

Thank you for contacting Rider support.

Unfortunately, I couldn't reproduce the issue you mentioned.
.Net 9 is indeed supported in 2024.3. I suggest ensuring you have a stable release version of .Net 9 installed (not a release candidate version released earlier).

If the issue is relevant, I would appreciate it if you could share the additional information as described below:

• Go to Help | Diagnostic tools | Choose Trace Scenarios;
• Enable the trace scenario for WinForms;
• Reproduce the issue. If possible, make a recording that demonstrates the issue;
• Share a screenshot that demonstrates what is set in Settings | Build, Execution, Deployment | Toolset and Build | .NET CLI executable path and MSBuild version.
• Collect the full log bundle via Help | Collect Logs and Diagnostic Data;
• Disable the trace scenarios enabled;
• Attach the resulting log bundle to this ticket or upload it to our server and share the ID.

Thank you in advance and have a great day!

I have a similar problem; most times I can get the designer to load/show at launch, but will not reload after attempting to add an event to the form or any controls. The IDE switches to Code View and shows the added event e.g. Form_Load (…); switching back to Design View does… nothing. Per the above I have created the log file and screenshots, ID #2025_05_20_Mzvnur8NcoCGg8qX2Ap3id

Hello, 

I've checked the logs you uploaded. The issue is Winform Designer exited unexpectedly due to exception below:

Message = “BinaryFormatter serialization and deserialization have been removed. See https://aka.ms/binaryformatter for more information.”
StackTraceString = “
  at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph) in Serialize.il:line IL_000A mvid 2677
     at System.ComponentModel.Design.Serialization.CodeDomComponentSerializationService.CodeDomSerializationStore.Close() in Close.il:line IL_0129 mvid 6832
     at System.ComponentModel.Design.UndoEngine.UndoUnit.AddRemoveUndoEvent..ctor(UndoEngine engine, IComponent component, Boolean add)
...

Starting with .NET 9, BinaryFormatter is no longer supported due to its known security risks and its APIs always throw a PlatformNotSupportedException for all project types, including Windows Forms apps. 

Please follow BinaryFormatter migration guide: WinForms apps - .NET | Microsoft Learn and check if BinaryFormatter is implicitly used in your app, such as storing or loading resources. 

If still no clue, could you make a sample project and provide repro steps? I will investigate further in my environment. 

Regards,

Tom

I knew about the BinaryFormatter deprecation, but I didn't think I was serializing anything during form creation - until I added an icon to the form. That caused it to crash. Thank you for your help.

Thanks for the update. Would it be possible for you to create a sample project that we can test locally? You can upload it to the Upload Service and let me know the Upload ID. 

If that’s not convenient, could you kindly record a video demonstrating the steps to reproduce the issue? With these details, I’ll be able to escalate the matter to the development team for further investigation. Your assistance is greatly appreciated!

Regards,

Tom

I've uploaded a zipped file of screenshots to replicate the problem (ID 2025_05_22_2Brn3NV4jZiN7w8G2x2aEn).

Create a new .Net 9 Winform app, exclude the folders from MS Defender, and rebuild to fix missing references (ss1 - ss3).

Add any icon to the project folder for the application and form icon (ss4a - .b). In Project Properties, add the icon as the Application icon (ss5).

Add some events to the form, throw on a control or six (ss6). Now add the icon to the form (ss7a - .b).

Switch to Code view and see that the events created were added. Now switch back to Design view… (ss8 - ss9). You can restore Design view by exiting Rider, and relaunching - but you won't be able to switch between views without restarting.

Thank you! It's really a big help for us to diagnose this issue. I am able to reproduce this issue in my local environment. To better track this issue, I have created an issue in our issue tracker:RIDER-126146 Adding Icon in Winform Designer makes it to crash. Please feel free to upvote or subscribe to this ticket if you'd like to track its progress.

Thanks, 

Tom