通用列表和Findall謂詞搜索子列表

Question

從下面的代碼中，我可以使用謂詞搜索並使用類似的方法找到所有商家的Id大於4，我將如何去返回所有商家和憑證的憑證類型ID爲3的憑證可以說。

由於提前

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，你可以這樣做：

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

，或者等價地，這樣的：

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 
} 

請注意您不需要定義MerchantFilter類。您可以使用lambda表達式來定義FindAll方法的參數。