No matching constructor found on type 'System.Activities.Activity' error 2025.10.12

Help Activities
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1

Hello,

I am getting the following error on a handful of xaml files:
Unable to create activity builder for [WorkflowName.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.'.

There are a handful of topics available that suggest solutions but none have worked. I have tried:

  • Updating UiPath Studio from 2024.10.14 to 2025.10.12
  • Reinstalling UiPath to 2025.10.12
  • Adjusting the UiPath.System.Activities package version to various available versions.
  • Deleting the System.Activities package from the .nuget file
  • Creating a new project

UiPath version is 2025.10.12 LTS, and the System.Activities package version is currently 25.10.5. I have tried ~10 versions of the same package to no success.

2

@william.coulson

You are getting this issue in the time development from starting same machine or did you exported or downloaded from other sources.
Did you tired by restarting the machine, sometimes that may work.Just give a try and see.

Happy Automation

3

I have just moved the process to another developer’s machine (using 2024.10.12 Studio) and they are getting the same issue on the Main xaml when compiling the process.

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The error comes up on workflows that are a part of the template I am working from and also workflows that I copied from other projects.

4

The XAML file template should come from the same Studio version and from the same project type compatibility.

Also, the XAML file should not be placed in a project root folder with reserved keywords such as Application, System, Process, and so on that can break the namespaces.

Can you provide the sample Studio project with the issue in a zip file so I can test it also?

5

Hello,

Sorted it now, we think there was a corrupted part of a custom package library. I did the following:

  1. Removed the package from the affected projects
  2. Deleted the .nuget file
  3. Reinstalled the package in the previously affected projects

This then didn’t throw errors in complilation.

6

Update as another developer in the team had the same problem. He removed the Workflows.Runtime package alongside the custom package.