VSTA (C#) workspace: how to disable the designer for a partial class file ?

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C#

Hello.

I would like
to hold some functions of a form-derived partial class in the n-th file,
that is inserted by me into the project, and it works fine...

...but the new item is presented by a form icon in the workspace tree,
that opens the designer for the n-th new form on its double click... :)

Please observe the picture below:


The MetaCollect.cs file was inserted as a functional part
of the form-derived partial class in the MetaExportWizard.cs
However, the designer will be open by the double click at the MetaCollect.cs item too
and the second form template will be given to be edited... :)

Is there a way, please,
to hold an code file item "as source" only in the VSTA2008 (C#) workspace ?

Thank you !