检查空或空的List< string> [英] Checking for empty or null List<string>

问题描述

我有一个列表，其中有时为空或为null.我希望能够检查它是否包含任何列表项，如果不包含，则将一个对象添加到列表中.

I have a List where sometimes it is empty or null. I want to be able to check if it contains any List-item and if not then add an object to the List.

 // I have a list, sometimes it doesn't have any data added to it
    var myList = new List<object>(); 
 // Expression is always false
    if (myList == null) 
        Console.WriteLine("List is never null"); 
    if (myList[0] == null) 
        myList.Add("new item"); 
    //Errors encountered:  Index was out of range. Must be non-negative and less than the size of the collection.
    // Inner Exception says "null"

推荐答案

尝试以下代码:

 if ( (myList!= null) && (!myList.Any()) )
 {
     // Add new item
     myList.Add("new item"); 
 }

后期编辑，因为对于这些检查，我现在想使用以下解决方案. 首先，添加一个称为Safe()的小型可重用扩展方法:

A late EDIT because for these checks I now like to use the following solution. First, add a small reusable extension method called Safe():

public static class IEnumerableExtension
{       
    public static IEnumerable<T> Safe<T>(this IEnumerable<T> source)
    {
        if (source == null)
        {
            yield break;
        }

        foreach (var item in source)
        {
            yield return item;
        }
    }
}

然后，您可以像执行以下操作:

And then, you can do the same like:

 if (!myList.Safe().Any())
 {
      // Add new item
      myList.Add("new item"); 
 }

我个人觉得这不太冗长，更易于阅读.现在，您可以安全地访问任何集合，而无需进行空检查.

I personally find this less verbose and easier to read. You can now safely access any collection without the need for a null check.

然后使用另一个编辑? (空条件)运算符(C#6.0):

And another EDIT, using ? (Null-conditional) operator (C# 6.0):

if (!myList?.Any() ?? false)
{
    // Add new item
    myList.Add("new item"); 
}

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