Please note the following important points:
- Include the correct latest links/references, for example, Dlubal.RFEM5.dll, under the following path:
C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Dlubal.RFEM5\
or, for example, Dlubal.STEEL_EC3.dll under the following path:
C:\Windows\assembly\GAC_MSIL\Dlubal.STEEL_EC3\ - In Visual Studio, the properties "Embed Interop Types" and "Copy Local" must be set to "False" (see the image).