C#开发-集合使用和技巧（三）集合扩展方法介绍_c# enumerable

C#开发-集合使用和技巧（三）

Enumerable 类扩展方法介绍

这些方法允许我们以声明式的方式处理集合数据。下面是 Enumerable 类中的一些常见方法及其用途：

扩展方法分类介绍

接下来，挑选一些常用的方法进行分类和介绍：

查询方法

1. Where
   定义

public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);
public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate);
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**功能**：过滤序列中的元素。
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示例：

var filtered = numbers.Where(n => n > 0); // 大于0的元素
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2. All
   定义

public static bool All<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);
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功能：检查序列中的所有元素是否满足指定条件。
示例：检查一个数字序列是否全部大于零。

bool allPositive = numbers.All(num => num > 0);
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3. Any
   定义

public static bool Any<TSource>(this IEnumerable<TSource> source);
public static bool Any<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);
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功能：检查序列中是否有任何一个元素满足指定条件。
示例：检查一个序列是否包含任何负数。

bool hasNegative = numbers.Any(num => num < 0);
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4. Contains
   定义：

public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value, IEqualityComparer<TSource>? comparer);
public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value);
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用途：检查序列中是否包含某个特定的元素。
示例：检查序列中是否包含某个数字。

bool containsFive = numbers.Contains(5);
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5. Count
   定义

public static int Count<TSource>(this IEnumerable<TSource> source);
public static int Count<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);
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功能：返回序列中元素的数量。
示例：获取序列中的元素总数。

int count = numbers.Count();
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6. First
   定义

public static TSource First<TSource>(this IEnumerable<TSource> source);
public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);
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功能：返回序列的第一个元素。
示例：获取序列中的第一个数字。

int firstNumber = numbers.First();
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数值聚合

1. Average
   定义

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, TSource> func);
public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func);
public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector);

public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector);
public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector);
public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector);
public static float Average<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector);
public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector);
public static float? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector);
public static double Average<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector);
public static double? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector);
public static decimal Average<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector);
public static decimal? Average<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector);

public static float? Average(this IEnumerable<float?> source);
public static double? Average(this IEnumerable<long?> source);
public static double? Average(this IEnumerable<int?> source);
public static double? Average(this IEnumerable<double?> source);
public static decimal? Average(this IEnumerable<decimal?> source);
public static double Average(this IEnumerable<long> source);
public static double Average(this IEnumerable<int> source);
public static double Average(this IEnumerable<double> source);
public static decimal Average(this IEnumerable<decimal> source);
public static float Average(this IEnumerable<float> source);

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功能：计算序列中所有元素的平均值。
示例：计算一组数字的平均值。

double average = numbers.Average();
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2. Max
   定义

public static long Max<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector);
public static decimal Max<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector);
public static double Max<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector);
public static int Max<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector);
public static decimal? Max<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector);
public static TSource? Max<TSource>(this IEnumerable<TSource> source, IComparer<TSource>? comparer);
public static int? Max<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector);
public static long? Max<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector);
public static float? Max<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector);
public static TResult? Max<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);
public static double? Max<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector);
public static TSource? Max<TSource>(this IEnumerable<TSource> source);
public static float Max<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector);
public static float Max(this IEnumerable<float> source);
public static float? Max(this IEnumerable<float?> source);
public static long? Max(this IEnumerable<long?> source);
public static int? Max(this IEnumerable<int?> source);
public static double? Max(this IEnumerable<double?> source);
public static decimal? Max(this IEnumerable<decimal?> source);
public static long Max(this IEnumerable<long> source);
public static int Max(this IEnumerable<int> source);
public static double Max(this IEnumerable<double> source);
public static decimal Max(this IEnumerable<decimal> source);

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功能：获取序列中的最大值。
示例：

int max = numbers.Max(); // 获取最大值
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3. Min
   定义

public static decimal Min(this IEnumerable<decimal> source);
public static TResult? Min<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);
public static float Min<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector);
public static float? Min<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector);
public static int? Min<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector);
public static double? Min<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector);
public static decimal? Min<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector);
public static long Min<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector);
public static int Min<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector);
public static decimal Min<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector);
public static TSource? Min<TSource>(this IEnumerable<TSource> source, IComparer<TSource>? comparer);
public static TSource? Min<TSource>(this IEnumerable<TSource> source);
public static long? Min<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector);
public static double Min<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector);
public static float Min(this IEnumerable<float> source);
public static float? Min(this IEnumerable<float?> source);
public static long? Min(this IEnumerable<long?> source);
public static int? Min(this IEnumerable<int?> source);
public static double? Min(this IEnumerable<double?> source);
public static decimal? Min(this IEnumerable<decimal?> source);
public static double Min(this IEnumerable<double> source);
public static long Min(this IEnumerable<long> source);
public static int Min(this IEnumerable<int> source);
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功能：获取序列中的最小值。
示例：

int min = numbers.Min(); // 获取最小值
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4. Sum
   定义

public static int Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int> selector);
public static long Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long> selector);
public static decimal? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal?> selector);
public static long? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, long?> selector);
public static int? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, int?> selector);
public static double Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double> selector);
public static float? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float?> selector);
public static float Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, float> selector);
public static double? Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, double?> selector);
public static decimal Sum<TSource>(this IEnumerable<TSource> source, Func<TSource, decimal> selector);
public static long? Sum(this IEnumerable<long?> source);
public static float? Sum(this IEnumerable<float?> source);
public static int? Sum(this IEnumerable<int?> source);
public static double? Sum(this IEnumerable<double?> source);
public static decimal? Sum(this IEnumerable<decimal?> source);
public static long Sum(this IEnumerable<long> source);
public static int Sum(this IEnumerable<int> source);
public static double Sum(this IEnumerable<double> source);
public static decimal Sum(this IEnumerable<decimal> source);
public static float Sum(this IEnumerable<float> source);
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功能：计算序列中所有元素的总和。
示例：

int total = numbers.Sum(); // 计算总和
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操作方法

1. Distinct
   定义

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source);
public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource>? comparer);
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功能：返回序列中不同的元素。
示例：去除重复的数字。

IEnumerable<int> unique = numbers.Distinct();
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2. Reverse
   定义

public static IEnumerable<TSource> Reverse<TSource>(this IEnumerable<TSource> source);
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功能：反转序列中的元素顺序。
示例：

IEnumerable<int> reversed = numbers.Reverse(); // 反转序列
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分组方法

1. GroupBy
   定义

public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey>? comparer);
public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector);
public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey>? comparer);
public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector);
public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector);
public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector);
public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer);
public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer);

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功能：根据指定的键将序列分割成多个组。
示例：根据长度将字符串分组。

var groups = strings.GroupBy(s => s.Length);
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转换方法

1. ToArray
   定义

public static TSource[] ToArray<TSource>(this IEnumerable<TSource> source);
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功能：将序列（集合或者数组）转换成数组。
示例：

int[] array = numbers.ToArray(); // 转换成数组
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2. ToList
   定义

public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source);
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功能：将序列（集合或者数组）转换成列表。
示例：

List<int> list = numbers.ToList(); // 转换成列表
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3. Select
   定义

public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, int, TResult> selector);
public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector);
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功能：投影每个元素到新的形式。
示例：

var squaredNumbers = numbers.Select(n => n * n); // 将每个元素平方
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其他方法

1. Skip
   定义

public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> source, int count);
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功能：跳过序列中的前N个元素。
示例：跳过前两个元素

var skipped = numbers.Skip(2); // 跳过前两个元素
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2. Take
   定义

public static IEnumerable<TSource> Take<TSource>(this IEnumerable<TSource> source, Range range);
public static IEnumerable<TSource> Take<TSource>(this IEnumerable<TSource> source, int count);
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功能：获取序列中的前N个元素。
示例：获取前三个元素

var taken = numbers.Take(3); // 获取前三个元素
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这些方法提供了强大的数据处理能力，可以根据需要灵活运用。在实际开发中，经常会将这些方法组合起来使用，以达到复杂的数据查询和处理需求。后续会对这些方法结合实例进行详细讲解和应用。