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;
}

如果适用，如何使用IEnumerable.Aggregate() 重写它？

Answer 1

有时，老式的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，否则你要么会多次枚举这些部分，要么会有一些非常不可读的 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，但它可能是枚举起来非常昂贵的东西，或者枚举之间不会产生一致值的东西。 IEnumerable 有时会让你大吃一惊。

Answer 2

我的解决方案与 Codesleuth 的解决方案非常接近，但会首先定义一个 Summary Struct

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：

好的，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。