ReSharper 2017.2 Help

Code Inspection: Return type can be IEnumerable<T> (non-private accessibility)

If a method returns a more generic type, it allows for more flexibility. On the one hand, caller methods can convert, if necessary, a value of IEnumerable<T> type into many other collection types or use it with LINQ. On the other hand, developers are likely to be able to change the method’s implementation without the need to change the return type.

Also, returning a more general type may help in the future if you decide to change the return value to a more specific type, for example, List<T>: if callers expect IEnumerable<T>, they will be able to accept List<T>, but not vice versa.

Below, ReSharper suggests changing the return type of GetNumbers() from List<String> to IEnumerable<String>:

Note that such a replacement is not always possible, though. If methods of the derived type are used on the returned object anywhere in the solution, ReSharper will not issue this suggestion.

Suboptimal codeAfter the quick-fix
class EnumerableTDemo { public static void Main() { string output = string.Join(" ", GetNumbers()); Console.WriteLine(output); } public static List<string> GetNumbers() { var Numbers = new List<string> {"1", "2", "3"}; return Numbers; } }
class EnumerableTDemo { public static void Main() { string output = string.Join(" ", GetNumbers()); Console.WriteLine(output); } public static IEnumerable<string> GetNumbers() { var Numbers = new List<string> {"1", "2", "3"}; return Numbers; } }

In some cases, however, returning a more specific type might be better. If a method is intended to work only with a specific type like an Array, and its implementation is unlikely to be changed in the future, you can suppress this inspection for that method.

Last modified: 11 December 2017

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