覆盖 Equals() 和 GetHashCode() 的正确方法 [重复]

Question

我以前从来没有真正这样做过，所以我希望有人能告诉我正确的方法是为我的班级实现 except() 和 GetHashCode() 的覆盖。

我正在尝试修改类，以便可以使用 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;
//}}

我查看了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。

任何帮助将不胜感激。

谢谢

Answer 1

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有效。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。

如果任何一个对象 ID 为零，我会使用 base.Equals()，但可能有更稳健的方法。

Answer 2

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();
}

Answer 3

您可以像这样在您的类上覆盖 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();
}