C# 重写 Equals() 和 GetHashCode() 的正确方法

Correct way to override Equals() and GetHashCode()

提问by Nugs

I have never really done this before so i was hoping that someone could show me the correct what of implementing a override of Except() and GetHashCode() for my class.

我以前从未真正这样做过，所以我希望有人可以向我展示如何为我的班级实现覆盖 Except() 和 GetHashCode() 的正确方法。

I'm trying to modify the class so that i can use the LINQ Except() method.

我正在尝试修改该类，以便我可以使用 LINQ Except() 方法。

public class RecommendationDTO{public Guid RecommendationId { get; set; }
public Guid ProfileId { get; set; }
public Guid ReferenceId { get; set; }
public int TypeId { get; set; }
public IList<TagDTO> Tags { get; set; }
public DateTime CreatedOn { get; set; }
public DateTime? ModifiedOn { get; set; }
public bool IsActive { get; set; }
public object ReferencedObject { get; set; }
public bool IsSystemRecommendation { get; set; }
public int VisibilityScore { get; set; }

public RecommendationDTO()
{
}

public RecommendationDTO(Guid recommendationid,
                            Guid profileid,
                            Guid referenceid,
                            int typeid,
                            IList<TagDTO> tags,
                            DateTime createdon,
                            DateTime modifiedon, 
                            bool isactive,
                            object referencedobject)
{
    RecommendationId = recommendationid;
    ProfileId = profileid;
    ReferenceId = referenceid;
    TypeId = typeid;
    Tags = tags;
    CreatedOn = createdon;
    ModifiedOn = modifiedon;
    ReferencedObject = referencedobject;
    IsActive = isactive;
}

public override bool Equals(System.Object obj)
{
    // If parameter is null return false.
    if (obj == null)
    {
        return false;
    }

    // If parameter cannot be cast to Point return false.
    RecommendationDTO p = obj as RecommendationDTO;
    if ((System.Object)p == null)
    {
        return false;
    }

    // Return true if the fields match:
    return (ReferenceId == p.ReferenceId);// && (y == p.y);
}

public bool Equals(RecommendationDTO p)
{
    // If parameter is null return false:
    if ((object)p == null)
    {
        return false;
    }

    // Return true if the fields match:
    return (ReferenceId == p.ReferenceId);// && (y == p.y);
}

//public override int GetHashCode()
//{
//    return ReferenceId;// ^ y;
//}}

I have taken a look at http://msdn.microsoft.com/en-us/library/ms173147.aspxbut i was hoping someone could show me within my own example.

我已经查看了http://msdn.microsoft.com/en-us/library/ms173147.aspx，但我希望有人可以在我自己的示例中向我展示。

Any help would be appreciated.

任何帮助，将不胜感激。

Thank you

谢谢

采纳答案by Craig

You can override Equals() and GetHashCode() on your class like this:

你可以像这样在你的类上覆盖 Equals() 和 GetHashCode() ：

public override bool Equals(object obj)
{
    var item = obj as RecommendationDTO;

    if (item == null)
    {
        return false;
    }

    return this.RecommendationId.Equals(item.RecommendationId);
}

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

回答by Trevor Pilley

public override bool Equals(System.Object obj)
{
    // Check if the object is a RecommendationDTO.
    // The initial null check is unnecessary as the cast will result in null
    // if obj is null to start with.
    var recommendationDTO = obj as RecommendationDTO;

    if (recommendationDTO == null)
    {
        // If it is null then it is not equal to this instance.
        return false;
    }

    // Instances are considered equal if the ReferenceId matches.
    return this.ReferenceId == recommendationDTO.ReferenceId;
}

public override int GetHashCode()
{
    // Returning the hashcode of the Guid used for the reference id will be 
    // sufficient and would only cause a problem if RecommendationDTO objects
    // were stored in a non-generic hash set along side other guid instances
    // which is very unlikely!
    return this.ReferenceId.GetHashCode();
}

回答by Eric Nelson

Be careful when using a primary key as your test for equality in overriding Equals() because it only works AFTER the object has been persisted. Prior to that your objects don't have primary keys yet and the IDs of the ones in memory are all zero.

在覆盖 Equals() 时使用主键作为相等性测试时要小心，因为它仅在对象被持久化后才起作用。在此之前，您的对象还没有主键，并且内存中的对象的 ID 都为零。

I use base.Equals() if either of the object IDs is zero but there probably is a more robust way.

如果任何一个对象 ID 为零，我使用 base.Equals() 但可能有更健壮的方法。