确定IQueryable中元素的位置

Question

我有一个IQueryable,它按某种条件排序.现在我想知道IQueryable中特定元素的位置.是否有linq表达式来实现.比如说IQueryable中有10个元素,第6个元素与条件匹配,我想得到数字6.

Answer 1

首先用索引选择每个项目,然后过滤项目,最后提取原始索引:

var result = orderedList
    .Select((x, i) => new { Item = x, Index = i })
    .Where(itemWithIndex => itemWithIndex.Item.StartsWith("g"))
    .FirstOrDefault();

int index= -1;
if (result != null)
    index = result.Index;

试验台:

class Program
{
    static void Main(string[] args)
    {
        var orderedList = new List<string>
        {
            "foo", "bar", "baz", "qux", "quux",
            "corge", "grault", "garply", "waldo",
            "fred", "plugh", "xyzzy", "thud"
        }.OrderBy(x => x);

        // bar, baz, corge, foo, fred, garply, grault,
        // plugh, quux, qux, thud, waldo, xyzzy
        // Find the index of the first element beginning with 'g'.

        var result = orderedList
            .Select((x, i) => new { Item = x, Index = i })
            .Where(itemWithIndex => itemWithIndex.Item.StartsWith("g"))
            .FirstOrDefault();

        int index= -1;
        if (result != null)
            index = result.Index;

        Console.WriteLine("Index: " + index);
    }
}

输出:

Index: 5