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
public class Foo {
public int bar;
public int stuff;
};
void Main()
{
List<Foo> fooList = new List<Foo>(){
new Foo(){bar=1,stuff=2},
new Foo(){bar=3,stuff=4},
new Foo(){bar=2,stuff=3}};
Foo result = fooList.Aggregate((u,v) => u.bar < v.bar ? u: v);
result.Dump();
}
如果要选择具有最小或最大属性值的对象。另一种方法是使用实现IComparable。
public struct Money : IComparable<Money>
{
public Money(decimal value) : this() { Value = value; }
public decimal Value { get; private set; }
public int CompareTo(Money other) { return Value.CompareTo(other.Value); }
}
最大执行将是。
var amounts = new List<Money> { new Money(20), new Money(10) };
Money maxAmount = amounts.Max();
最小实现将是。
var amounts = new List<Money> { new Money(20), new Money(10) };
Money maxAmount = amounts.Min();
通过这种方式，您可以比较任何对象并在返回对象类型的同时获得Max和Min。
希望这能帮助到一些人。
编辑:
对不起。除了错过了可空值之外，我看错了函数，
Min<(Of <(TSource, TResult>)>)(IEnumerable<(Of <(TSource>)>)， Func<(Of <(TSource, TResult>)>))确实返回你所说的结果类型。
我想说一个可能的解决方案是实现IComparable，并使用Min<(Of <(TSource>)>)(IEnumerable<(Of <(TSource>)>))，它确实从IEnumerable中返回一个元素。当然，如果不能修改元素，这也没有帮助。我觉得微软的设计有点奇怪。
当然，如果你需要的话，你总是可以做一个for循环，或者使用Jon Skeet给出的MoreLINQ实现。
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);
这是一个获取最小值和最大值的简单方法:
`dbcontext.tableName.Select(x=>x.Feild1).Min()`
