A custom add-ins project needs to contain all the references as the related system add-in project. Otherwise, when in a custom add-in you paste a code from a system add-in, it will complain that some references are missing.

Sync custom add-in references

1.In Visual Studio Express, open both projects: system add-in and custom add-in, and then click References.
On the following screenshot, we opened FredSystemAddInExpress and FredCustomAddInExpress, and tiled them horizontally.

2.Revise the list of references in the system add-in, and make the custom add-in list the same, by adding missing references (if any). To add a reference, click the Add button, select from .NET, COM, and Projects and then click OK.

3.Repeat above for all custom projects.

See also

Adding a new reference to an add-in project