Code inspection: NUnit. Test case source must be non-abstract and implement IEnumerable.

NUnit's TestCaseSource and ValueSource attributes identify sources of arguments for parameterized tests. If TestCaseSource specifies a source class without a member name, the class needs to be non-abstract, have a default constructor, and implement IEnumerable. If either attribute specifies a field, property, or method, that member needs to provide IEnumerable or a type that implements it. JetBrains Rider issues a warning if the source does not meet the enumerable requirement. A string is an enumerable sequence of characters and satisfies this requirement.

Note that NUnit allows different ways of specifying test case sources. For more information, refer to the TestCaseSourceAttribute and ValueSourceAttribute documentation.