Generic List<T> as IEnumerable<object>
c#, casting, generic-list, ienumerable
Solution
`IEnumerable<object>` is not a supertype of `IEnumerable<T>`, so it is not a supertype of `List<T>` either. See question 2575363 for a brief overview of why this is the case (it's about Java, but the concepts are the same). This problem has been solved in C# 4.0, by the way, which supports covariant generics.
The reason why you didn't find this error is because you used `x as T`, where you should have been using a normal cast (`(T)x`), see question 2139798. The resulting `InvalidCastException` would have pointed you at your error. (In fact, if the type relationship were correct (i.e. if `IEnumerable<object>` were a supertype of `List<T>`), you wouldn't need a cast at all.)
To solve your problem, make your method generic, so that it accepts an `IEnumerable<T>` instead of an `IEnumerable<object>`, and skip the cast completely.
public static void VerifyNotNullOrEmpty<T>(IEnumerable<T> theIEnumerable,
string theIEnumerableName,
string theVerifyingPosition) { ... }
Problem
I'm trying to do cast a List to an IEnumerable, so I can verify that different lists are not null or empty: Suppose myList is a List < T > . Then in the caller code I wanted: ``` Validator.VerifyNotNullOrEmpty(myList as IEnumerable<object>, @"myList", @"ClassName.MethodName"); ``` The valdiating code would be: ``` public static void VerifyNotNullOrEmpty(IEnumerable<object> theIEnumerable, string theIEnumerableName, string theVerifyingPosition) { string errMsg = theVerifyingPosition + " " + theIEnumerableName; if (theIEnumerable == null) { errMsg += @" is null"; Debug.Assert(false); throw new ApplicationException(errMsg); } else if (theIEnumerable.Count() == 0) { errMsg += @" is empty"; Debug.Assert(false); throw new ApplicationException(errMsg); } } ``` However, this doens't work. It compiles, but theIEnumerable is null! Why?