How can I import original dll?

Help
1

I created original dll named [Test.Uipath.Common.dll].
It’s not custom activity. I want to use original method in formula.

And I set this dll in [C:\Program Files (x86)\UiPath Studio].

But I can’t select it in namaspace combo box.

And, when I use the validation function in Assign activity as follows,
[var = Test.UiPath.Common.Validation.StringValidator.IsNumeric(“1111”)]

I got following error.
[‘Common’ is not a member of ‘UiPath’]

I also installed original nuget package that include
・Test.Uipath.Activities.Log
・Test.Uipath.Activities.Configuration
・Test.Uipath.Activities.Exception
And ,they work well.

How can I solce this problem…