Generic List as IEnumerable<object>

问题:

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?

回答1:

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) { ... }

回答2:

List implements IEnumerable so you don't need to cast them, you just need to make it so your method accepted a generic parameter, like so:

 public static void VerifyNotNullOrEmpty<T>(this IEnumerable<T> 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);

    }
}

You should just be able to call it with:

var myList = new List<string>
{
    "Test1",
    "Test2"
};

myList.VerifyNotNullOrEmpty("myList", "My position");

You could also improve the implementation slightly:

 public static void VerifyNotNullOrEmpty<T>(this IEnumerable<T> items,
                                    string name,
                                    string verifyingPosition)
{
    if (items== null)
    {
        Debug.Assert(false);
        throw new NullReferenceException(string.Format("{0} {1} is null.", verifyingPosition, name));
    }
    else if ( !items.Any() )
    {
        Debug.Assert(false);
        // you probably want to use a better (custom?) exception than this - EmptyEnumerableException or similar?
        throw new ApplicationException(string.Format("{0} {1} is empty.", verifyingPosition, name));

    }
}

回答3:

Supposing you're targeting at least framework 3.0:

Cast to a generic IEnumerable<object> using extension:

var myEnumerable = myList.Cast<object>();

EDIT: anyway I'd suggest you to change your method to get a pure IEnumerable like:

public static void VerifyNotNullOrEmpty(IEnumerable theIEnumerable,
                                        string theIEnumerableName,
                                        string theVerifyingPosition)

and inside the method check if empty using foreach or theIEnumerable.Cast<object>().Count()

In this way you don't have to cast every time to IEnumerable<object>