Could not determine a valid designer type for 'Syncfusion.Windows.Forms.Tools.Design.ToolStripExDesigner, Syncfusion.Tools.Windows'.

[Ahmed Hakan Okumus]

Could not determine a valid designer type for 'Syncfusion.Windows.Forms.Tools.Design.ToolStripExDesigner, Syncfusion.Tools.Windows'.

[Sekar Sivalingam]

Hi Ahmed Hakan Okumus,

Thanks for reaching out.

Microsoft’s ToolStrip control is available in both .NET Framework and .NET Core/.NET via the System.Windows.Forms assembly, which allows it to function normally in both platforms. However, the designer support for ToolStrip, provided by the ToolStripDesigner class, depends on the System.Design assembly. This assembly is included only in the .NET Framework and is not available in .NET Core.

Because the designer class ToolStripDesigner cannot be loaded in the .NET Core/.NET environment, the reflection process fails during design-time initialization of ToolStripExDesigner, causing the error message.

We have already reported this issue to the .NET WinForms team. we will keep monitoring the progress and update you as soon as there is a solution or workaround available. Thank you for your understanding and patience. 

Link: WinForms Designer Error: Could Not Determine a Valid Designer Type for Custom ToolStrip in .NET Core · Issue #13507 · dotnet/winforms

Regards,

Sekar Sivalingam

[Karthick Arjunan]

Hi Ahmed Hakan Okumus,

We are glad to announce that our Essential Studio® 2025 Volume 4 Main Release v32.1.19 is rolled out in which we have included the fix for the reported issue "Could not determine a valid designer type for 'Syncfusion.Windows.Forms.Tools.Design.ToolStripExDesigner" and is available for download under the following link.

Essential Studio® 2025 Volume ... | Announcements Forums | Syncfusion®

We thank you for your support and appreciate your patience in waiting for this release. Please get in touch with us if you require any further assistance.

Regards,
Karthick Arjunan