Because I have read a long time ago that the Interop libs are also displayed in Visual Studio and then also loaded in the output directory. I have now tried it as follows: Version 1.0.112 without properties: Interop libs not present. Version 1.0.112 with properties: Interop libs present. Version 1.0.113 without properties: Interop libs not present. Version 1.0.113 with Properties: Interop not libs present. Version 1.0.114 without Properties: Interop libs not present. Version 1.0.114 with Properties: Interop not libs present.