泛型列表和 Findall 谓词搜索子列表

Question

从下面的代码中，我可以使用谓词搜索并找到所有 Id 大于 4 的商家，使用类似的方法我将如何返回 Voucher 具有 VoucherTypeID 的所有商家及其优惠券让我们说 3 个。

提前致谢

class Program
{
    static void Main(string[] args)
    {
        List<Merchant> merchants = new List<Merchant>(10);
        for (int i = 0; i < 10; i++)
        {
             List<Voucher> vcs = new List<Voucher>();
            for (int j = 0; j < 3; j++)
            { 

                vcs.Add(new Voucher(j,"Voucher" + j.ToString(),i, j, "Type_" +j.ToString()));

            }
              merchants.Add(new Merchant(i, i.ToString() + "_Merchant",vcs));
        }

        //This will return all the merchant's where the ID is greater than 4

        Predicate<Merchant> filterByID;
        MerchantFilter filter = new MerchantFilter(4);
        filterByID = new Predicate<Merchant>(filter.FilterByMerchantGT4);
        List<Merchant> filteredMerchants = merchants.FindAll(filterByID);  

    }

    public class MerchantFilter 
    {
        private int idValue;
        public bool FilterByMerchantGT4(Merchant merch) 
        {

            return merch.MerchantID > idValue;
        }
        public MerchantFilter(int value)
        {
            idValue = value; 
        }
    } 

public class Merchant
{
    public int MerchantID { get; set; }
    public string MerchantName { get; set; }
    public List<Voucher> MerchantVouchers { get; set; }
    public Merchant(int id, string Name, List<Voucher> vouchers)
    {
        MerchantID = id;
        MerchantName = Name;
        MerchantVouchers = vouchers;

    }
}
public class Voucher
{
    public int VoucherID { get; set; }
    public string VoucherName { get; set; }
    public int MerchantVoucherID { get; set; }
    public int VoucherTypeID { get; set; }
    public string VoucherTypeName { get; set; }
    public Voucher(int ID, string Name, int merchantID, int typeID, string TypeName)
    {
        VoucherID = ID;
        VoucherName = Name;
        MerchantVoucherID = merchantID;
        VoucherTypeID = typeID;
        VoucherTypeName = TypeName;
    }
}

Answer 1

使用 linq，您可以这样做：

var query = from merchant in merchants
            let filteredVouchers = from voucher in merchant.MerchantVouchers
                                   where voucher.VoucherTypeID == 3
                                   select voucher
            select new { merchant, filteredVouchers };

或者，等效地，这个：

var query =
    merchants.Select(
        merchant =>
        new
        {
            merchant,
            filteredVouchers = merchant.MerchantVouchers.Where(voucher => voucher.VoucherTypeID == 3)
        })
        .Select(t => new { t.merchant, t.filteredVouchers });

或者，避免 linq：

foreach(var merchant in merchants)
{
    var filteredVouchers = merchant.MerchantVouchers.FindAll(v => v.VoucherTypeID == 3);
    //...some useful code here
}

注意您不需要定义 MerchantFilter 类。您可以使用 lambda 表达式来定义 FindAll 方法的参数。