什么是“索引超出范围"?异常，我该如何解决? [英] What is an "index out of range" exception, and how do I fix it?

问题描述

我收到以下错误之一:

• 索引超出范围.必须为非负且小于集合的大小"
• 插入索引超出范围.必须为非负且小于或等于大小."
• 索引超出数组范围."

这是什么意思，我该如何解决?

What does it mean, and how do I fix it?

另见
IndexOutOfRangeException
ArgumentOutOfRangeException

推荐答案

为什么会出现这个错误?

因为您尝试使用超出集合边界的数字索引访问集合中的元素.

Why does this error occur?

Because you tried to access an element in a collection, using a numeric index that exceeds the collection's boundaries.

集合中的第一个元素通常位于索引 0.最后一个元素位于索引 n-1 处，其中 n 是集合的 Size(它包含的元素数).如果您尝试使用负数作为索引，或者使用大于 Size-1 的数字，则会出现错误.

The first element in a collection is generally located at index 0. The last element is at index n-1, where n is the Size of the collection (the number of elements it contains). If you attempt to use a negative number as an index, or a number that is larger than Size-1, you're going to get an error.

当你像这样声明一个数组时:

When you declare an array like this:

var array = new int[6]

数组中的第一个和最后一个元素是

The first and last elements in the array are

var firstElement = array[0];
var lastElement = array[5];

所以当你写:

var element = array[5];

您正在检索数组中的第六个元素，而不是第五个.

you are retrieving the sixth element in the array, not the fifth one.

通常，你会像这样循环遍历一个数组:

Typically, you would loop over an array like this:

for (int index = 0; index < array.Length; index++)
{
    Console.WriteLine(array[index]);
}

这是可行的，因为循环从零开始，到 Length-1 结束，因为 index 不再小于 Length.

This works, because the loop starts at zero, and ends at Length-1 because index is no longer less than Length.

然而，这将引发异常:

for (int index = 0; index <= array.Length; index++)
{
    Console.WriteLine(array[index]);
}

注意到 <= 了吗?index 现在将在最后一次循环迭代中超出范围，因为循环认为 Length 是有效索引，但事实并非如此.

Notice the <= there? index will now be out of range in the last loop iteration, because the loop thinks that Length is a valid index, but it is not.

列表的工作方式相同，不同之处在于您通常使用 Count 而不是 Length.它们仍然从零开始，到 Count - 1 结束.

Lists work the same way, except that you generally use Count instead of Length. They still start at zero, and end at Count - 1.

for (int index = 0; i < list.Count; index++)
{
    Console.WriteLine(list[index]);
} 

但是，您也可以使用 foreach 遍历列表，从而完全避免索引的整个问题:

However, you can also iterate through a list using foreach, avoiding the whole problem of indexing entirely:

foreach (var element in list)
{
    Console.WriteLine(element.ToString());
}

您不能索引尚未添加到集合中的元素.

You cannot index an element that hasn't been added to a collection yet.

var list = new List<string>();
list.Add("Zero");
list.Add("One");
list.Add("Two");
Console.WriteLine(list[3]);  // Throws exception.

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