如何最好地实现自定义类型的等于？

Question

比如说Point2类,以及以下等于:

public override bool Equals ( object obj )

public bool Equals ( Point2 obj )

这是有效C#3中显示的那个:

public override bool Equals ( object obj )
{
    // STEP 1: Check for null
    if ( obj == null )
    {
        return false;
    }

    // STEP 3: equivalent data types
    if ( this.GetType ( ) != obj.GetType ( ) )
    {
        return false;
    }
    return Equals ( ( Point2 ) obj );
}

public bool Equals ( Point2 obj )
{
    // STEP 1: Check for null if nullable (e.g., a reference type)
    if ( obj == null )
    {
        return false;
    }
    // STEP 2: Check for ReferenceEquals if this is a reference type
    if ( ReferenceEquals ( this, obj ) )
    {
        return true;
    }
    // STEP 4: Possibly check for equivalent hash codes
    if ( this.GetHashCode ( ) != obj.GetHashCode ( ) )
    {
        return false;
    }
    // STEP 5: Check base.Equals if base overrides Equals()
    System.Diagnostics.Debug.Assert (
        base.GetType ( ) != typeof ( object ) );

    if ( !base.Equals ( obj ) )
    {
        return false;
    }

    // STEP 6: Compare identifying fields for equality.
    return ( ( this.X.Equals ( obj.X ) ) && ( this.Y.Equals ( obj.Y ) ) );
}

Answer 1

在接受obj的那个中,如果obj的类型是Point2,则调用特定类型的Equals.在特定类型的Equals中,确保所有成员具有相同的值.

public override bool Equals ( object obj )
{
   return Equals(obj as Point2);
}

public bool Equals ( Point2 obj )
{
   return obj != null && obj.X == this.X && obj.Y == this.Y ... 
   // Or whatever you think qualifies as the objects being equal.
}

您可能应该重写GetHashCode以确保"相等"的对象具有相同的哈希码.

Answer 2

MSDN上也有一整套指南.你应该好好读一读,这既棘手又重要.

我发现最有帮助的几点:

• 值类型没有标识,所以在a中struct Point你通常会按成员比较来做成员.

• 引用类型通常具有标识,因此Equals测试通常在ReferenceEquals处停止(默认情况下,无需覆盖).但是有一些例外,例如string和your class Point2,其中一个对象没有有用的标识,然后你覆盖Equality成员来提供你自己的语义.在这种情况下,请遵循指南以首先完成null和其他类型的情况.

• 而且有充分的理由保持GethashCode()和operator==同步为好.

Answer 3

我用过的技术对我有用,如下所示.注意,我只是基于单个属性(Id)而不是两个值进行比较.根据需要调整

using System;
namespace MyNameSpace
{
    public class DomainEntity
    {
        public virtual int Id { get; set; }

        public override bool Equals(object other)
        {
            return Equals(other as DomainEntity);
        }

        public virtual bool Equals(DomainEntity other)
        {
            if (other == null) { return false; }
            if (object.ReferenceEquals(this, other)) { return true; }
            return this.Id == other.Id;
        }

        public override int GetHashCode()
        {
            return this.Id;
        }

        public static bool operator ==(DomainEntity item1, DomainEntity item2)
        {
            if (object.ReferenceEquals(item1, item2)) { return true; }
            if ((object)item1 == null || (object)item2 == null) { return false; }
            return item1.Id == item2.Id;
        }

        public static bool operator !=(DomainEntity item1, DomainEntity item2)
        {
            return !(item1 == item2);
        }
    }
}

Answer 4

使用 C# 7 和is type varname模式匹配语法提供了一种使用以下任一方法Equals(object)处理null和类型检查的清理：

// using strongly-typed overload of Equals
public override bool Equals(object obj) => (obj is Point2 other) && Equals(other);

public bool Equals(Point2 other);

// using the == operator (requires != to also be defined)
public override bool Equals(object obj) => (obj is Point2 other) && this == other;

public static bool operator ==(Point2 lhs, Point2 rhs);
public static bool operator !=(Point2 lhs, Point2 rhs);

（显然您需要提供相关方法存根的实现。）