List<T> 二分查找返回错误值

Question

即使列表已排序，二分查找也会返回错误值。这是列表：

1707 ABCD
1707 XXXX
1725 DEFG
1725 HIJK
1725 LMNOP

我从按时间（第一列）预排序的文件中获取此列表，因此我不在我的代码中对其进行排序。当我对 1725 DEFG 进行二进制搜索时，它会在按位补码之前返回 1725 LMNOP。如果我进行按位补码，结果将是 1725 HIJK。

为什么会这样？

下面是实现：

 public class RecComparer: IComparer<MyData>
{
 public  int Compare(MyData x, MyData y)
{
    if (x.DateDetails == null)
    {
        if (y.DateDetails == null)
        {
                           return 0;
        }
        else
        {

            return -1;
        }
    }
    else
    {

        if (y.DateDetails == null)
                      {
            return 1;
        }
        else
        {
            int retval = x.DateDetails.Length.CompareTo(y.DateDetails.Length);

            if (retval != 0)
            {

                return retval;
            }
            else
            {

                return x.DateDetails.CompareTo(y.DateDetails);
            }
        }
    }
}
}

这是对 BinarySearch 的调用：

lookAhead = recordList.BinarySearch(lookAheadtime, (IComparer<MyData>)rc);

任何原因都是它的行为方式。

编辑：

public class MyData
{
    public string DateDetails { get; set; }
    public string TrackDetails { get; set; }
}

Answer 1

更新 #2：在包含重复项的列表中使用自定义比较器进行二分搜索。

假设在“1707 ABCD”中，“1707”是DateDetails，而“ABCD”是TrackDetails，那么您有一个包含两个字段的数据数组，这些字段已经按其中一个字段的顺序排序两个字段，即DateDetails。因此可能有多个条目具有相同的DateDetails。

如果您执行 BinarySearch，它将返回具有指定 DateDetails 的条目之一，不一定是第一个。然后您必须向后扫描才能找到第一个。

public static class ListHelper
{
    /// <summary>
    /// Return the zero-based index of the first matching item in the sorted List, 
    /// if a match for item is found; otherwise, a negative number that is the bitwise 
    /// complement of the index of the next element that is larger than item or, if there is
    /// no larger element, the bitwise complement of Count.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="list"></param>
    /// <param name="item"></param>
    /// <param name="comparer"></param>
    /// <returns>The zero-based index of the first matching item in the sorted List, 
    /// if item is found; otherwise, a negative number that is the bitwise complement 
    /// of the index of the next element that is larger than item or, if there is
    /// no larger element, the bitwise complement of Count.</returns>
    public static int BinarySearchFirst<T>(this List<T> list, T item, IComparer<T> comparer)
    {
        int start = list.BinarySearch(item, comparer);
        for (; start > 0 && comparer.Compare(list[start], list[start - 1]) == 0; start--)
            ;
        return start;
    }
    /// <summary>
    /// Find the zero-based indices of the first and last matching items in the sorted List, 
    /// if a match for item is found; otherwise, a negative number for both that is the bitwise 
    /// complement of the index of the next element that is larger than item or, if there is
    /// no larger element, the bitwise complement of Count.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="list"></param>
    /// <param name="item"></param>
    /// <param name="comparer"></param>
    /// <param name="start">The zero-based index of the first matching item in the List, 
    /// if item is found; otherwise, a negative number that is the bitwise complement 
    /// of the index of the next element that is larger than item or, if there is
    /// no larger element, the bitwise complement of Count.</param>
    /// <param name="end">The zero-based index of the first matching item in the sorted List, 
    /// if item is found; otherwise, a negative number that is the bitwise complement 
    /// of the index of the next element that is larger than item or, if there is
    /// no larger element, the bitwise complement of Count.</param>
    /// <returns>true if found, else false</returns>
    public static bool BinarySearchRange<T>(this List<T> list, T item, IComparer<T> comparer, out int start, out int end)
    {
        start = end = list.BinarySearch(item, comparer);
        if (start < 0)
            return false;
        for (; start > 0 && comparer.Compare(list[start], list[start - 1]) == 0; start--)
            ;
        for (int last = list.Count - 1; end < last && comparer.Compare(list[end], list[end + 1]) == 0; end++)
            ;
        return true;
    }
}

Answer 2

我不确定我是否理解这个问题，但你不能只使用 linq 搜索列表吗？

recordList.Where(x=>x.columnName == 1725).First();