Interface Uses

One-to-one misuse

Often in .net one-to-one interfaces are introduced for every class. Arguments are typically:

They ease tests/mocking
They decouple
To assist dependency injection

For tests/mocking just use the original class unless it has dependencies like SQL, HTTP etc. If so create an interface to be able to mock it. For decoupling - if interface and class are the same and there is no abstraction the coupling is just moved to the interface instead. For dependency injection just inject the concrete class

Guidelines (just my own dynamic list)

Anything that is shared in api's or platform packages should probably use interfaces to allow changes easier without breaking dependents you don't know about
In general only create interfaces that actually abstract or where multiple implementations exist
It's easy to switch to an interface down the line if more implementations are needed by:
- Rename the class to the new interface name with the built in refactoring tool
- Rename the class back (or to a new name since you've learned that there are two implementations and how they differ) without using the refactoring tool
- Create the interface - its name is used everywhere

Interface Uses

One-to-one misuse

Guidelines (just my own dynamic list)

See also