如何使用 Aggregate() 将多个独立的 LINQ 查询重写为单个查询？

Question

我有下一个非常未优化的代码：

void Summarize(IEnumerable<Section> sections)
{
    this.DateBegin = sections.Select(s => s.Date).Min();
    this.DateEnd = sections.Select(s => s.Date).Max();
    this.Income = sections.Where(s => s.IsIncome).Sum(r => r.Amount);
    this.ExpenditureBank = sections.Where(s => s.IsExpenditureBank).Sum(r => r.Amount);
    this.ExpenditureClient = sections.Where(s => s.IsExpenditureClient).Sum(r => r.Amount);
    this.Expenditure = this.ExpenditureBank + this.ExpenditureClient;
}

如何使用 IEnumerable.Aggregate() 重写它（如果适用）？

Answer 1

有时，一个好的老式 foreach 循环是完成这项工作的最佳工具。我认为这里就是这种情况，否则您要么会多次枚举这些部分，要么会出现一些非常不可读的 LINQ 代码。

尽管您可能提前知道sections参数将是一个数组或集合或其他什么，但它可能是枚举非常昂贵的东西或者在枚举之间不能产生一致值的东西。 IEnumerable 有时会让您感到惊讶。

Answer 2

我的解决方案与 Codesleuth 的解决方案非常接近，但首先定义一个摘要结构，

struct Summary 
{
    public DateTime DateBegin {get; set;}
    public DateTime DateEnd {get; set;}
    public int Income  {get; set;}
    public int ExpenditureBank {get; set;}
    public int ExpenditureClient {get; set;}
    public int Expenditure {get {return ExpenditureBank+ExpenditureClient; }}
}

void Summarize(IEnumerable<Section> sections)
{
    var result = sections.Aggregate(new Summary
    {
        DateBegin = DateTime.MaxValue,
        DateEnd = DateTime.MinValue,
        Income = 0,
        ExpenditureBank =0,
        ExpenditureClient =0,
    },
    (agg, next) =>
    new Summary
    {
        DateBegin = next.Date < agg.DateBegin ? next.Date : agg.DateBegin,
        DateEnd = next.Date > agg.DateEnd ? next.Date : agg.DateEnd,
        Income = agg.Income + (next.IsIncome ? next.Amount : 0),
        ExpenditureBank = next.IsExpenditureBank ? next.Amount: 0,
        ExpenditureClient = next.IsExpenditureClient ? next.Amount : 0
    });
}

请注意，理想情况下该结构是不可变的。

Answer 3

void Summarize(IEnumerable<Section> sections)
{
    this.DateBegin = sections.Select(s => s.Date).Min();
    this.DateEnd = sections.Select(s => s.Date).Max();
    this.Income = sections.Where(s => s.IsIncome).Sum(r => r.Amount);
    this.Expenditure = sections.Aggregate(0, (agg, next) => 
        agg += (next.IsExpenditureBank ? next.Amount : 0) +
            (next.IsExpenditureClient ? next.Amount : 0));
}

怎么样？

编辑：

好吧，我重新考虑了一下，看看：

void Summarize(IEnumerable<Section> sections)
{
    var result = sections.Aggregate(new
    {
        DateBegin = DateTime.MaxValue,
        DateEnd = DateTime.MinValue,
        Income = 0,
        Expenditure = 0
    },
        (agg, next) =>
        new
        {
            DateBegin = next.Date < agg.DateBegin ? next.Date : agg.DateBegin,
            DateEnd = next.Date > agg.DateEnd ? next.Date : agg.DateEnd,
            Income = agg.Income + (next.IsIncome ? next.Amount : 0),
            Expenditure = agg.Expenditure + (next.IsExpenditureBank ? next.Amount : 0) +
                (next.IsExpenditureClient ? next.Amount : 0)
        }
    );
}

如果sections为空，请注意错误。