我有一个具有Nullable DateOfBirth属性的Person对象。是否有一种方法可以使用LINQ来查询Person对象列表中最早/最小的DateOfBirth值?
这是我的开场白:
var firstBornDate = People.Min(p => p.DateOfBirth.GetValueOrDefault(DateTime.MaxValue));
Null DateOfBirth值被设置为DateTime。MaxValue，以便将它们排除在Min考虑之外(假设至少有一个具有指定的DOB)。
但是所有这些对我来说都是将firstBornDate设置为DateTime值。我想要的是与之匹配的Person对象。我是否需要像这样写第二个查询:
var firstBorn = People.Single(p=> (p.DateOfBirth ?? DateTime.MaxValue) == firstBornDate);
或者有没有更精简的方法?
当前回答
另一种实现，可以使用可空的选择器键，如果没有找到合适的元素，则对于引用类型集合返回null。 例如，这对处理数据库结果很有帮助。
public static class IEnumerableExtensions
{
/// <summary>
/// Returns the element with the maximum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the maximum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the maximum value of a selector function.</returns>
public static TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, 1);
/// <summary>
/// Returns the element with the minimum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the minimum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the minimum value of a selector function.</returns>
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, -1);
private static TSource MaxOrMinBy<TSource, TKey>
(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, int sign)
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));
Comparer<TKey> comparer = Comparer<TKey>.Default;
TKey value = default(TKey);
TSource result = default(TSource);
bool hasValue = false;
foreach (TSource element in source)
{
TKey x = keySelector(element);
if (x != null)
{
if (!hasValue)
{
value = x;
result = element;
hasValue = true;
}
else if (sign * comparer.Compare(x, value) > 0)
{
value = x;
result = element;
}
}
}
if ((result != null) && !hasValue)
throw new InvalidOperationException("The source sequence is empty");
return result;
}
}
例子:
public class A
{
public int? a;
public A(int? a) { this.a = a; }
}
var b = a.MinBy(x => x.a);
var c = a.MaxBy(x => x.a);
其他回答
People.Aggregate((curMin, x) => (curMin == null || (x.DateOfBirth ?? DateTime.MaxValue) <
curMin.DateOfBirth ? x : curMin))
下面是更通用的解决方案。它本质上做相同的事情(以O(N)顺序)，但对任何IEnumerable类型，并且可以与属性选择器可以返回null的类型混合。
public static class LinqExtensions
{
public static T MinBy<T>(this IEnumerable<T> source, Func<T, IComparable> selector)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if (selector == null)
{
throw new ArgumentNullException(nameof(selector));
}
return source.Aggregate((min, cur) =>
{
if (min == null)
{
return cur;
}
var minComparer = selector(min);
if (minComparer == null)
{
return cur;
}
var curComparer = selector(cur);
if (curComparer == null)
{
return min;
}
return minComparer.CompareTo(curComparer) > 0 ? cur : min;
});
}
}
测试:
var nullableInts = new int?[] {5, null, 1, 4, 0, 3, null, 1};
Assert.AreEqual(0, nullableInts.MinBy(i => i));//should pass
另一种实现，可以使用可空的选择器键，如果没有找到合适的元素，则对于引用类型集合返回null。 例如，这对处理数据库结果很有帮助。
public static class IEnumerableExtensions
{
/// <summary>
/// Returns the element with the maximum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the maximum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the maximum value of a selector function.</returns>
public static TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, 1);
/// <summary>
/// Returns the element with the minimum value of a selector function.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
/// <param name="source">An IEnumerable collection values to determine the element with the minimum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
/// <exception cref="System.InvalidOperationException">source contains no elements.</exception>
/// <returns>The element in source with the minimum value of a selector function.</returns>
public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => MaxOrMinBy(source, keySelector, -1);
private static TSource MaxOrMinBy<TSource, TKey>
(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, int sign)
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));
Comparer<TKey> comparer = Comparer<TKey>.Default;
TKey value = default(TKey);
TSource result = default(TSource);
bool hasValue = false;
foreach (TSource element in source)
{
TKey x = keySelector(element);
if (x != null)
{
if (!hasValue)
{
value = x;
result = element;
hasValue = true;
}
else if (sign * comparer.Compare(x, value) > 0)
{
value = x;
result = element;
}
}
}
if ((result != null) && !hasValue)
throw new InvalidOperationException("The source sequence is empty");
return result;
}
}
例子:
public class A
{
public int? a;
public A(int? a) { this.a = a; }
}
var b = a.MinBy(x => x.a);
var c = a.MaxBy(x => x.a);
你可以使用现有的linq扩展，比如MoreLinq。但如果你只需要这些方法，那么你可以使用这里的简单代码:
public static IEnumerable<T> MinBys<T>(this IEnumerable<T> collection, Func<T, IComparable> selector)
{
var dict = collection.GroupBy(selector).ToDictionary(g => g.Key);
return dict[dict.Keys.Min()];
}
public static IEnumerable<T> MaxBys<T>(this IEnumerable<T> collection, Func<T, IComparable> selector)
{
var dict = collection.GroupBy(selector).ToDictionary(g => g.Key);
return dict[dict.Keys.Max()];
}
无需额外包装的解决方案:
var min = lst.OrderBy(i => i.StartDate).FirstOrDefault();
var max = lst.OrderBy(i => i.StartDate).LastOrDefault();
你也可以把它包装成扩展:
public static class LinqExtensions
{
public static T MinBy<T, TProp>(this IEnumerable<T> source, Func<T, TProp> propSelector)
{
return source.OrderBy(propSelector).FirstOrDefault();
}
public static T MaxBy<T, TProp>(this IEnumerable<T> source, Func<T, TProp> propSelector)
{
return source.OrderBy(propSelector).LastOrDefault();
}
}
在这种情况下:
var min = lst.MinBy(i => i.StartDate);
var max = lst.MaxBy(i => i.StartDate);
顺便说一下……O(n²)不是最佳解。保罗·贝茨给出的解决方案比我的。但我仍然是LINQ解决方案，它比这里的其他解决方案更简单，更简短。
