Lambda表达式，如何在对象内部进行搜索？

Question

我开始喜欢Lambda表达式，但我努力通过这道墙：

public class CompanyWithEmployees { 
    public CompanyWithEmployees() { } 
    public Company CompanyInfo { get; set; } 
    public List<Person> Employees { get; set; } 
} 

我的搜索：

List<CompanyWithEmployees> companiesWithEmployees = ws.GetCompaniesWithEmployees(); 
CompanyWithEmployees ces = companiesWithEmployees 
     .Find(x => x.Employees 
     .Find(y => y.PersonID == person.PersonID)); 

所以，我想要得到的对象“CompanyWithEmployees”有那个人（员工），我正在寻找，但我得到“不能隐式转换'人'到'布尔'）”这是正确的，但如果我没有通过人对象，如何可以第一次查找执行？

Answer 1

因为要检查所有脑干，也许尝试：

ces = companiesWithEmployees 
     .Find(x => x.Employees 
     .Find(y => y.ParID == person.ParID) != null); 

这将检查与同ParID任何Person;如果你的意思是相同Person实例（参考），然后Contains应该足够了：

ces = companiesWithEmployees 
     .Find(x => x.Employees.Contains(person)); 

Answer 2

ces = companiesWithEmployees 
    .First(x => x.Employees.Any(p=>p.PersonID == person.PersonID)); 

Answer 3

那是因为你没有指定一个合法查找表达你的顶级查找。

我会在这里表现出来：

ces = companiesWithEmployees 
    .Find (x => x.Employees.Find(y => y.ParID == Person.ParID) /*condition is missing here*/); 

那么，什么是您的初始查找的条件？

Answer 4

Find()返回找到的对象。使用Any()来检查表达式对于任何元素是否为真。

var ces = companiesWithEmployees 
    .Find(x => x.Employees 
    .Any(y => y.PersonID == person.PersonID)); 

Answer 5

ces = companiesWithEmployees.Find(x => x.Employees.Find(...)); 

.Find返回只有一个对象，x.Employees.Find(..)返回Person。

.Find预计布尔参数（即条件的结果），这就是为什么有一个编译器错误，说Cannot implicit convert 'Person' To 'bool'

.Where可以返回多个对象，因此可以通过所有列表进行迭代。

在您的情况下使用.Where和.Any的组合。

下面的代码将说明.Where之间的差别，.Find和.Any：

public partial class Form2 : Form { 
    public Form2() { 
     InitializeComponent(); 
     var companiesWithEmployees = new List<CompanyWithEmployees>() {     
      new CompanyWithEmployees {     
       CompanyInfo = new Company { CompanyName = "Buen" }, 
       Employees = new List<Person>() { 
        new Person { PersonID = 1976, PersonName = "Michael" }, 
        new Person { PersonID = 1982, PersonName = "Mark" }, 
        new Person { PersonID = 1985, PersonName = "Matthew" },        
        new Person { PersonID = 1988, PersonName = "Morris" } 
       } 
      }, 
      new CompanyWithEmployees { 
       CompanyInfo = new Company { CompanyName = "Muhlach" }, 
       Employees = new List<Person>() { 
        new Person { PersonID = 1969, PersonName = "Aga" }, 
        new Person { PersonID = 1971, PersonName = "Nino" }, 
        new Person { PersonID = 1996, PersonName = "Mark" } 
       } 
      }, 
      new CompanyWithEmployees { 
       CompanyInfo = new Company { CompanyName = "Eigenmann" }, 
       Employees = new List<Person>() { 
        new Person { PersonID = 1956, PersonName = "Michael" },       
        new Person { PersonID = 1999, PersonName = "Gabby" } 
       } 
      } 
     }; 

     // just explicitly declared the types (instead of var) so the intent is more obvious 

     IEnumerable<CompanyWithEmployees> whereAreMichaels = companiesWithEmployees 
      .Where(cx => cx.Employees.Any(px => px.PersonName == "Michael")); 

     string michaelsCompanies = string.Join(", ", whereAreMichaels 
      .Select(cx => cx.CompanyInfo.CompanyName).ToArray()); 

     MessageBox.Show("Company(s) with employee Michael : " + michaelsCompanies); 

     Person findAga = companiesWithEmployees 
      .Find(company => company.CompanyInfo.CompanyName == "Muhlach") 
      .Employees.Find(person => person.PersonName == "Aga"); 

     if (findAga != null) 
      MessageBox.Show("Aga's ID : " + findAga.PersonID.ToString()); 
    } 
} 

class CompanyWithEmployees { 
    public Company CompanyInfo { get; set; } 
    public List<Person> Employees { get; set; } 
} 
class Company { 
    public string CompanyName { get; set; } 
} 
class Person { 
    public int PersonID { get; set; } 
    public string PersonName { get; set; } 
} 

Answer 6

最简单的一个是

ces = companiesWithEmployees.FirstOrDefault(x => 
      x.Employees.Any(y => y.PersonID == person.ParID)); 

没有任何空检查