Compilation fails on any workflow: "No matching constructor found on type 'System.Activities.Activity'" [Studio 26.0.195 Preview]

Feedback Studio
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1

Summary

After updating to Studio 26.0.195-cloud preview (build 23460), every workflow fails to compile , including a brand-new empty workflow containing only a single Use Browser activity. Error appears in the Output panel; the canvas itself shows no validation errors.

This blocks both Run and Debug across all projects on this Studio install.

Environment

  • Studio version: 26.0.195-cloud.23460 (preview channel)
  • Build SHA: Branch.release-s195.Sha.0674d0d1744009884f5fcff296a185779ba69ff3
  • License: Community
  • Project framework: Windows
  • Project type: Process
  • OS: Windows, Build: 19045.7417
  • Package sources active: Official + Marketplace (default)

Steps to reproduce

  1. Create a brand new Process project (Windows framework).
  2. Add any modern UI Automation activity, e.g. Use Browser (NApplicationCard) into the default Sequence. Leave it empty.
  3. Save → click Run (or Debug File).

Expected

Workflow runs (or fails on actual UI target), with no compilation error.

Actual

Compilation aborts immediately. Output panel shows:

26.0.195-cloud.23460+Branch.release-s195.Sha.0674d0d1744009884f5fcff296a185779ba69ff3

Compilation failed or was cancelled. Check the Output panel for more information.

Unable to create activity builder for Workflows\Test.xaml. Reason was 'No matching constructor found on type 'System.Activities.Activity'. You can use the Arguments or FactoryMethod directives to construct this type. No default constructor found for type 'System.Activities.Activity'. You can use the Arguments or FactoryMethod directives to construct this type.'.

Things I already tried (no effect)

  • Updated all packages to the latest versions available in the dropdown
    • UiPath.System.Activities 26.6.0
    • UiPath.UIAutomation.Activities (latest available)
    • UiPath.WebAPI.Activities 2.4.0 (note: previously 2.5.0-preview was installed but the dropdown only shows up to 2.4.0 now , looks like 2.5.0-preview was pulled)
    • UiPath.GSuite.Activities 3.9.11
  • Confirmed both Official and Marketplace feeds are present and enabled
  • Created a fresh project with a single Use Browser activity → same error
  • Restarted Studio multiple times

Notes / suspicions

  • The fact that even an empty new project fails strongly suggests this is a Studio runtime / activity loader regression in the 26.0.195 preview build, not a project-specific issue.
  • The 2.5.0-preview of UiPath.WebAPI.Activities being unreachable may be related (was it pulled?).

Happy to provide additional logs (ProgramData\UiPath\Logs) or collect a ProcDump if it would help triage. Has anyone else on the cloud preview channel seen this in the past few days?

Thanks!

3

@balkis_anton

Can you check last response from the below thread,

Happy Automation

4

Thank you for replying this thread. Yes I’ve seen it early. It’s not a proper solution. There is 100k files over 18GB files there. I’m not sure it’s the best solution. I prefer downgrade app version and match it with current project. But the actual problem still exists.

5

Hi @balkis_anton , thank you for your feedback, I’ve raised an item on our side to further investigate this.