Collections types for non-public API methods
We define a non-public API method as:
- Any non-
public method in a class library solution
- Any method that is not a controller action / endpoint in a WCF / WebApi project
Return types for non-public API methods
Do not return IEnumerable<T>, except if one of the following is true:
- The items must be generated on-demand, for instance an infinite mathematical sequences (eg. Fibonacci).
- The length of the sequence is otherwise unknown.
- The items must be streamed from another source, for instance from a DB cursor.
- Performance is critical and can benefit by exposing lazy enumeration to the caller.
- There is no richer interface that better describes the semantics of the return value.
When none of the above are true:
- Expose a more specific interface, or the concrete type if no suitable interface exists. There is little advantage of returning an
IEnumerable<T> when you have a List<T>, as you will likely to need to .ToList() it as some point anyway. Expose IList<T> instead.
Don’t
// unnecessary downcast on private method hides a bunch of functionality
private IEnumerable<string> GetStrings()
{
return new List<string>{ ... };
}
Do
// expose the original functionality, but through an interface to make it
// easier to change the implementation later
private IList<string> GetStrings()
{
return new List<string>{ ... };
}