C#遍历超大List的foreach性能极差，求优化方案

我有一个遍历response的foreach循环，response是最多包含23万条记录的List。每次遍历response时，会过滤并求和另一个最多3万条记录的_MaterialIssued列表；仅当Issued>0时（约15%的情况）才进入后续分支。接着从最多1.7万条记录的_OnHand列表中获取itemOnhand，约85%的情况会进入该分支，而此分支内的代码导致性能急剧下降——需要再次过滤response获取子项并修改_OnHand和response列表。

该foreach循环的执行时间因机器而异，耗时45至75分钟，我无法定位瓶颈所在，也不知道如何优化此代码块。我曾尝试将_OnHand列表改为HashSet，但性能无改善。

核心循环代码 #

foreach (var b in response)
{
    var Issued = _MaterialIssued.Where(x => x.ItemId == b.ItemId && x.Job == b.Job).Sum(c => c.Qty);

    if (Issued > 0)
    {
        var prctIssued = Issued / b.QtyDemand;
        var childItems = response.Where(x => x.Job == b.Job && x.BomPath.StartsWith(b.BomPath));

        foreach (var child in childItems)
        {
            child.QtyIssued = child.QtyDemand * prctIssued;
        }
    }
    var itemOnhand = _OnHand.Where(x => x.ItemId == b.ItemId).FirstOrDefault();

    if (itemOnhand.Onhand > 0)
    {
        decimal prctOnhand = 1;
        var childItems = response.Where(x => x.Job == b.Job && x.BomPath.StartsWith(b.BomPath) && x.ItemId != b.ItemId && x.SiteRef == b.SiteRef && x.QtyIssued < x.QtyDemand);
        var DemandWithIssued = b.QtyDemand - b.QtyIssued;
        if (itemOnhand.Onhand < DemandWithIssued)
        {
            prctOnhand = itemOnhand.Onhand / DemandWithIssued;
        }

        itemOnhand.Onhand -= DemandWithIssued * prctOnhand;
        foreach (var child in childItems)
        {
            child.QtyParentAvailable = (child.QtyDemand - child.QtyIssued) * prctOnhand;
        }
    }
}

相关模型定义 #

_OnHand 模型

private class ItemOnhand
{
    public string ItemId { get; set; }
    public string SiteRef { get; set; }
    public decimal Onhand { get; set; }
}

_MaterialIssued 模型

public class FiniteDemandBase
{
    public string ItemId { get; set; }
    public string Item { get; set; }
    public string JobId { get; set; }
    public string Job { get; set; }
    public DateTime StartDate { get; set; }
    public string SiteRef { get; set; }
    public decimal Qty { get; set; }
}

response 模型

public class FiniteDemand
{
    public int Id { get; set; }
    public DateTime StartDate { get; set; }
    public string BomPath { get; set; }
    public string SiteRef { get; set; }
    public string Job { get; set; }
    public string ItemId { get; set; }
    public string JobId { get; set; }
    public string ParentItemId { get; set; }
    public decimal QtyPerUnit { get; set; }
    public decimal QtyDemand { get; set; }
    public decimal QtyOnhand { get; set; }
    public decimal QtyWip { get; set; }
    public decimal QtyOnhandRunning { get; set; }
    public decimal QtyInSchedule { get; set; }
    public decimal QtyIssued { get; set; }
    public decimal QtyParentAvailable { get; set; }
    public decimal QtyDemandNeeded { get; set; }
    public decimal QtyDemandNeededRunning { get; set; }
}

内容的提问来源于stack exchange，提问作者Abraham Friessen