C# IEqualityComparer 和 Contains 方法

IEqualityComparer and Contains method

提问by Pablo Rodríguez

I have this simple class with those 2 enum fields, I'm trying to find one item of this object in a collection (List<T>) but the Contains methods doesn't works correctly

我有一个带有这 2 个枚举字段的简单类，我试图在集合 ( List<T>) 中找到此对象的一项，但包含方法无法正常工作

public class Calculator : IEqualityComparer<Calculator>
{
    public DashboardsComputationMode ComputationMode { get; set; }
    public Modes Mode { get; set; }

    public Calculator(DashboardsComputationMode dashboardsComputationMode, Modes mode)
    {
        ComputationMode = dashboardsComputationMode;
        Mode = mode;
    }

    public bool Equals(Calculator x, Calculator y)
    {
        return (x.ComputationMode.Equals(y.ComputationMode) && x.Mode.Equals(y.Mode));
    }

    public int GetHashCode(Calculator obj)
    {
        return obj.ComputationMode.GetHashCode() ^ obj.Mode.GetHashCode();
    }
}

public enum DashboardsComputationMode
{
    Weighted = 0,
    Aggregated = 1,
    PR = 2,
    CurrentValue = 3,
    EquivalentHours = 4,
    AggregatedCorrected = 5,
    PRCorrected = 6
}

public enum Modes
{
    InstantaneousMode = 0,
    DailyMode = 1,
    MonthlyMode = 2,
    YearlyMode = 5,
    Undefined = 4,
}

Why could be that this test doesn't works

为什么这个测试不起作用

[TestMethod]
public void TestMethod1()
{
  var list = new List<Calculator>()
  {
    new Calculator(DashboardsComputationMode.PR, Modes.DailyMode),
    new Calculator(DashboardsComputationMode.CurrentValue, Modes.YearlyMode),
    new Calculator(DashboardsComputationMode.PRCorrected, Modes.MonthlyMode)
  };

  var item = new Calculator(DashboardsComputationMode.CurrentValue, Modes.YearlyMode);
  Assert.IsTrue(list[1].Equals(item));
  Assert.IsTrue(list.Contains(item));
}

The first assert works fine

第一个断言工作正常

Assert.IsTrue(list[1].Equals(item)) 

but the second doesn't

但第二个没有

Assert.IsTrue(list.Contains(item));

采纳答案by Nikolay Khil

List<T>.Containsdetermines equality by using the default equality comparer (the one returned by the EqualityComparer<T>.Default).

List<T>.Contains使用默认的相等比较器（由 返回的比较器）确定相等EqualityComparer<T>.Default。

Here's the MSDN explanation on how EqualityComparer<T>.Defaultworks:

这是关于如何EqualityComparer<T>.Default工作的 MSDN 解释：

The Default property checks whether type T implements the System.IEquatableinterface and, if so, returns an EqualityComparerthat uses that implementation. Otherwise, it returns an EqualityComparerthat uses the overrides of Object.Equalsand Object.GetHashCodeprovided by T.

Default 属性检查类型 T 是否实现 System.IEquatable接口，如果是，则返回使用该实现的 EqualityComparer。否则，它返回一个EqualityComparer，它使用T 提供的Object.Equals和Object.GetHashCode的覆盖 。

In other words, your Calculatorclass should either implement the System.IEquatable(notthe System.IEqualityComparer!) interface or override the Object.Equalsand Object.GetHashCodemethods.

换句话说，你的Calculator类要么实现System.IEquatable（没有的System.IEqualityComparer！）接口或重写的Object.Equals和Object.GetHashCode方法。

回答by D J

You are not using IEqualityComparer<Calculator>in Equals and Contains both. EqualityComparerhas a different significance. I corrected the code for you.

您没有IEqualityComparer<Calculator>在 Equals 和 Contains 中使用。EqualityComparer有着不同的意义。我为你修正了代码。

  public class CalculatorComparer : IEqualityComparer<Calculator>
{

    public bool Equals(Calculator x, Calculator y)
    {
        return (x.ComputationMode.Equals(y.ComputationMode) && x.Mode.Equals(y.Mode));
    }

    public int GetHashCode(Calculator obj)
    {
        return obj.ComputationMode.GetHashCode() ^ obj.Mode.GetHashCode();
    }
}
public class Calculator
{
    public DashboardsComputationMode ComputationMode { get; set; }
    public Modes Mode { get; set; }

    public Calculator(DashboardsComputationMode dashboardsComputationMode, Modes mode)
    {
        ComputationMode = dashboardsComputationMode;
        Mode = mode;
    }


    public override bool Equals(object obj)
    {
        Calculator y = obj as Calculator;

        return (this.ComputationMode.Equals(y.ComputationMode) && this.Mode.Equals(y.Mode));
    }
}

public enum DashboardsComputationMode
{
    Weighted = 0,
    Aggregated = 1,
    PR = 2,
    CurrentValue = 3,
    EquivalentHours = 4,
    AggregatedCorrected = 5,
    PRCorrected = 6
}

public enum Modes
{
    InstantaneousMode = 0,
    DailyMode = 1,
    MonthlyMode = 2,
    YearlyMode = 5,
    Undefined = 4,
}

Now both should return true.

现在两者都应该返回 true。