定义

Aggregate扩展方法旨在对序列应用进行累加，它一共包含了3个重载方法，如下：

Aggregate(IEnumerable, Func)

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, TSource> func)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }
    if (func == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
    }
    //获取泛型版本的迭代器
    using (IEnumerator<TSource> e = source.GetEnumerator())
    {
        //判断第一个值是否存在
        if (!e.MoveNext())
        {
            ThrowHelper.ThrowNoElementsException();
        }
        //将第一个值赋予result
        TSource result = e.Current;
        //循环处理
        while (e.MoveNext())
        {
            //第一次调用func时: 第一个参数是集合的第一个值,第二个参数是集合的第二个值
            //第二次调用func时: 第一个参数是上一次func返回的值,第二个参数是集合的第三个值
            //......
            //第n次调用func时: 第一个参数依然是上一次func返回的值,第二个参数是集合的第n+1个值
            result = func(result, e.Current);
        }
        //如果集合只有一个元素,直接返回第一个值
        //如果集合超过一个元素,按上面的逻辑依次调用func后返回的最终值
        return result;
    }
}

//示例1
string[] strs = new string[] { "I","am","happy","to","share" };
string sentence = strs.Aggregate((str1, str2) => str1 + " " + str2);
//output: I am happy to share
//示例2
int[] nums = new int[] { 1,2,3,4,5 };
int count = nums.Aggregate((num1, num2) => num1 + num2);
//output: 15

Aggregate(IEnumerable, TAccumulate, Func)

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }
    if (func == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
    }
    TAccumulate result = seed;
    foreach (TSource element in source)
    {
        //第一次调用func时: 第一个参数是seed参数,第二个参数是集合的第一个值
        //第二次调用func时: 第一个参数是上一次func返回的值,第二个参数是集合的第二个值
        //......
        //第n次调用func时: 第一个参数依然是上一次func返回的值,第二个参数是集合的第n个值
        result = func(result, element);
    }
    return result;
}

/// <summary>
/// 定义一个类，包含花费的日期和购物花费、房租
/// </summary>
public class Expenses
{
    /// <summary>
    /// 日期
    /// </summary>
    public DateOnly Date { get; set; }
    /// <summary>
    /// 购物花费
    /// </summary>
    public decimal Shopping { get; set; }
    /// <summary>
    /// 房租
    /// </summary>
    public decimal Rent { get; set; }
}
//初始化一些数据
List<Expenses> expenses = new List<Expenses>()
{
    new Expenses(){ Date = DateOnly.Parse("2023-03-29"), Shopping = 50, Rent = 0 },
    new Expenses(){ Date = DateOnly.Parse("2023-03-30"), Shopping = 60, Rent = 0 },
    new Expenses(){ Date = DateOnly.Parse("2023-03-31"), Shopping = 70, Rent = 1500 },
};
//此时我们想求出这三天每样花费的总计
{
    //申明一个匿名变量,因为第一个传入的参数类型也是func的返回类型,所以我们都用decimal
    var expenseCount = new { shopping = (decimal)0, rent = (decimal)0  };
    //累加Shopping和Rent
    expenseCount = expenses.Aggregate(expenseCount, 
        (expense1, expense2) =>
        new {
            shopping = expense1.shopping + expense2.Shopping,
            rent = expense1.rent + expense2.Rent
        }
    );
    //expenseCount：{"shopping":180,"rent":1500}
}
//当然,我们也可以实例化一个Expenses,循环执行方法的时候在里面属性直接叠加
//这样我们都不需要返回值了,因为返回值就是expenseCount本身
{
    var expenseCount = new Expenses();
    expenseCount = expenses.Aggregate(expenseCount, 
        (expense1, expense2) =>
        {
            expense1.Shopping += expense2.Shopping;
            expense1.Rent += expense2.Rent;
            return expense1;
        }
    );
    //expenseCount: {"Date":"0001-01-01","Shopping":180,"Rent":1500}
}

Aggregate(IEnumerable, TAccumulate, Func, Func)

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }
    if (func == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
    }
    if (resultSelector == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.resultSelector);
    }
    TAccumulate result = seed;
    foreach (TSource element in source)
    {
        //第一次调用func时: 第一个参数是seed参数,第二个参数是集合的第一个值
        //第二次调用func时: 第一个参数是上一次func返回的值,第二个参数是集合的第二个值
        //......
        //第n次调用func时: 第一个参数依然是上一次func返回的值,第二个参数是集合的第n个值
        result = func(result, element);
    }
    //其实就是再对结果进行处理
    return resultSelector(result);
}

//上一个示例的基础上我们想得到这三天总计的花费
var expenseCount = new Expenses();
decimal count = expenses.Aggregate(expenseCount, 
    (expense1, expense2) =>
    {
        expense1.Shopping += expense2.Shopping;
        expense1.Rent += expense2.Rent;
        return expense1;
    },
    ec => ec.Shopping+ec.Rent
);
//ec其实就代表了叠加到最后的expenseCount
//count: 1680