C# 如何一步获取列表中项目的索引？

How to get the index of an item in a list in a single step?

提问by Daniel Robinson

How can I find the index of an item in a list without looping through it?

如何在不循环的情况下找到列表中项目的索引？

Currently this doesn't look very nice - searching through the list for the same item twice, just to get the index:

目前这看起来不太好 - 在列表中搜索相同的项目两次，只是为了获取索引：

var oProp = something;

int theThingIActuallyAmInterestedIn = myList.IndexOf(myList.Single(i => i.Prop == oProp));

采纳答案by Alex Filipovici

How about the List.FindIndex Method:

List.FindIndex 方法如何：

int index = myList.FindIndex(a => a.Prop == oProp);

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

此方法执行线性搜索；因此，此方法是一个 O(n) 操作，其中 n 是 Count。

If the item is not found, it will return -1

如果未找到该项目，则返回 -1

回答by Jon Skeet

EDIT: If you're onlyusing a List<>and you onlyneed the index, then List.FindIndexis indeed the best approach. I'll leave this answer here for those who need anything different (e.g. on top of any IEnumerable<>).

编辑：如果您只使用 aList<>并且您只需要索引，那么List.FindIndex确实是最好的方法。我会把这个答案留在这里给那些需要任何不同的人（例如在 any 之上IEnumerable<>）。

Use the overload of Selectwhich takes an index in the predicate, so you transform your list into an (index, value) pair:

使用Selectwhich 在谓词中使用索引的重载，因此您可以将列表转换为 (index, value) 对：

var pair = myList.Select((Value, Index) => new { Value, Index })
                 .Single(p => p.Value.Prop == oProp);

Then:

然后：

Console.WriteLine("Index:{0}; Value: {1}", pair.Index, pair.Value);

Or if you onlywant the index and you're using this in multiple places, you could easily write your own extension method which was like Where, but instead of returning the original items, it returned the indexes of those items which matched the predicate.

或者，如果您只想要索引并且您在多个地方使用它，您可以轻松编写自己的扩展方法，就像Where，但不是返回原始项目，而是返回与谓词匹配的项目的索引。

回答by gzaxx

If you don't want to use LINQ, then:

如果您不想使用 LINQ，则：

int index;
for (int i = 0; i < myList.Count; i++)
{
    if (myList[i].Prop == oProp)
    {
       index = i;
       break;
    }
}

this way you are iterating list only once.

这样你只迭代列表一次。

回答by Jose Manuel Abarca Rodríguez

For simple types you can use "IndexOf" :

对于简单类型，您可以使用 "IndexOf" ：

List<string> arr = new List<string>();
arr.Add("aaa");
arr.Add("bbb");
arr.Add("ccc");
int i = arr.IndexOf("bbb"); // RETURNS 1.

回答by Jose Manuel Abarca Rodríguez

1. Simple solution to find index for any string value in the List.

1. 查找列表中任何字符串值的索引的简单解决方案。

Here is code for List Of String:

这是字符串列表的代码：

int indexOfValue = myList.FindIndex(a => a.Contains("insert value from list"));

1. Simple solution to find index for any Integer value in the List.

2. 查找列表中任何整数值的索引的简单解决方案。

Here is Code for List Of Integer:

这是整数列表的代码：

    int indexOfNumber = myList.IndexOf(/*insert number from list*/);

回答by Sn?bj?rn

Here's a copy/paste-able extension method for IEnumerable

这是 IEnumerable 的可复制/粘贴扩展方法

public static class EnumerableExtensions
{
    /// <summary>
    /// Searches for an element that matches the conditions defined by the specified predicate,
    /// and returns the zero-based index of the first occurrence within the entire <see cref="IEnumerable{T}"/>.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="list">The list.</param>
    /// <param name="predicate">The predicate.</param>
    /// <returns>
    /// The zero-based index of the first occurrence of an element that matches the conditions defined by <paramref name="predicate"/>, if found; otherwise it'll throw.
    /// </returns>
    public static int FindIndex<T>(this IEnumerable<T> list, Func<T, bool> predicate)
    {
        var idx = list.Select((value, index) => new {value, index}).Where(x => predicate(x.value)).Select(x => x.index).First();
        return idx;
    }
}

Enjoy.

享受。

回答by Ali Bordbar

If anyone wonders for the Arrayversion, it goes like this:

如果有人想知道这个Array版本，它是这样的：

int i = Array.FindIndex(yourArray, x => x == itemYouWant);