通用列表< T>作为IEnumerable< object> [英] Generic List<T> as IEnumerable<object>

问题描述

我正在尝试将列表强制转换为IEnumerable，因此我可以验证不同的列表不为null或为空：

I'm trying to do cast a List to an IEnumerable, so I can verify that different lists are not null or empty:

假设myList是一个列表< ; T>。然后在调用方代码中，我想要：

Suppose myList is a List < T > . Then in the caller code I wanted:

       Validator.VerifyNotNullOrEmpty(myList as IEnumerable<object>,
                                     @"myList",
                                     @"ClassName.MethodName");

验证代码为：

     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);

        }
    }

但是，这不起作用。它可以编译，但是IEnumerable为空！为什么？

However, this doens't work. It compiles, but theIEnumerable is null! Why?

推荐答案

IEnumerable< object> 不是 IEnumerable< T> ，因此它也不是 List< T> 的超类型。请参阅问题2575363 ，以简要了解这是为什么情况（关于Java，但概念相同）。顺便说一下，此问题已在C＃4.0中解决，该问题支持协变泛型。

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.

未找到此错误的原因是因为您将 x用作T ，应该使用常规强制转换（（T）x ），请参见问题2139798 。结果 InvalidCastException 会指出您的错误。 （实际上，如果类型关系正确（即 IEnumerable< object> 是 List< T> ），您根本不需要强制转换。）

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.)

要解决您的问题，请使您的方法通用，使其接受 IEnumerable< T> 而不是 IEnumerable< object> ，并完全跳过转换。

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) { ... }

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