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，但我正在努力越过这堵墙：

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，我想获取对象“CompanyWithEmployees”，其中包含我正在寻找的人员（员工），但我得到“无法将 'Person' 隐式转换为 'bool')”，这是正确的，但如果我不传递 Person 对象，第一个 Find 如何执行？

Answer 1

因为您想检查是否存在，也许可以尝试：

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

这将检查具有相同 ParID 的任何 Person； 如果您指的是同一个 Person 实例（参考），那么 Contains 就足够了：

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

Answer 2

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

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

Answer 3

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

Answer 4

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

.Find 返回仅一个对象，x.Employees.Find(..) 返回Person< /代码>。

.Find 需要布尔参数（即条件的结果），这就是为什么会出现编译器错误，提示 Cannotimplicit conversion 'Person' To 'bool'

.Where 可以返回多个对象，因此可以迭代所有列表。

根据您的情况使用 .Where 和 .Any 的组合。

下面的代码将说明 .Where、.Find 和 .Any 之间的区别：

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 5

这是因为您没有为顶级查找指定合法的查找表达式。

我在这里展示一下：

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

那么你最初发现的条件是什么？

Answer 6

最简单的一种是

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

没有任何空检查