c# - Dictionary.ContainsKey 总是返回 False

Question

好吧，我正在尝试创建一个小型应用程序来保存一些员工的姓名、年龄和薪水。所以我决定使用 Dictionary 来设置每个员工的薪水，我想出了那个代码

代码

var employeeSalaryDictionary = new Dictionary<Employee, int>();
employeeSalaryDictionary.Add(new Employee { Name = "Chuck", Age = 37 }, 1000);
employeeSalaryDictionary.Add(new Employee { Name = "Norris", Age = 37 }, 2000);
employeeSalaryDictionary.Add(new Employee { Name = "Rocks", Age = 44 }, 3000);

Employee employeeToFind = new Employee { Name = "Chuck", Age = 37 };
//or even
Employee employeeToFind = new Employee { Name = "Chuck"};

//Always False...
bool exists = employeeSalaryDictionary.ContainsKey(employeeToFind); 

员工类

public class Employee
{
    public string Name { get; set; }
    public int Age { get; set; }
}

但是 正如我注释掉的或主题标题所述，.ContainsKey 总是返回我 False 尽管我尝试了两种方式，如中所示代码。

Answer 1

您没有使用 Dictionary接受 IEqualityComparer<T> 的构造函数你也没有在 Employee 上实现自定义相等性类。

所以现在字典是通过引用来比较员工。当你new员工，你有一个不同的引用，即使例如名称可能相同。

这里最简单的方法可能是实现您自己的 IEqualityComparer<Employee> ，您可以在其中选择哪些成员将用于相等比较，并将其传递给字典的构造函数。

[编辑] 正如 promise 的那样，片段:

//ReSharper's courtesy
public sealed class NameAgeEqualityComparer : IEqualityComparer<Employee>
{
    public bool Equals(Employee x, Employee y)
    {
        if (ReferenceEquals(x, y)) return true;
        if (ReferenceEquals(x, null)) return false;
        if (ReferenceEquals(y, null)) return false;
        if (x.GetType() != y.GetType()) return false;
        return string.Equals(x.Name, y.Name) && x.Age == y.Age;
    }

    public int GetHashCode(Employee obj)
    {
        unchecked
        {
            return ((obj.Name != null ? obj.Name.GetHashCode() : 0) * 397) ^ obj.Age;
        }
    }
}

然后:

var employeeSalaryDictionary = new Dictionary<Employee, int>(new NameAgeEqualityComparer());
employeeSalaryDictionary.Add(new Employee { Name = "Chuck", Age = 37 }, 1000);
employeeSalaryDictionary.Add(new Employee { Name = "Norris", Age = 37 }, 2000);
employeeSalaryDictionary.Add(new Employee { Name = "Rocks", Age = 44 }, 3000);

Employee employeeToFind = new Employee { Name = "Chuck", Age = 37 };
bool exists = employeeSalaryDictionary.ContainsKey(employeeToFind); // true!

为了完整起见，这里是仅名称比较器(也是 ReSharper 提供的):

 public sealed class NameEqualityComparer : IEqualityComparer<Employee>
 {
        public bool Equals(Employee x, Employee y)
        {
            if (ReferenceEquals(x, y)) return true;
            if (ReferenceEquals(x, null)) return false;
            if (ReferenceEquals(y, null)) return false;
            if (x.GetType() != y.GetType()) return false;
            return string.Equals(x.Name, y.Name);
        }

        public int GetHashCode(Employee obj)
        {
            return (obj.Name != null ? obj.Name.GetHashCode() : 0);
        }
  }

但是，正如您所注意到的，您必须决定在创建字典时将使用哪个比较器进行键比较。以后无法更改...