Quite often you need to use a third party application in your code. This may be a workflow tool, a reporting service.
If you need to integrate with it (you need to call it or it needs to call you) keep these to a small well defined interface.
Build a simple assembly for calling it an use that for all access. For it to call you produce a second small assembly.
Do not let any of it’s implementation details leak beyond that. Those should be the only assemblies that have a reference to the third party code.
Careful use of this can allow you to replace a product (possibly with some custom code) at a later time with minimal effort.
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