我的类的 C# 通用列表包含找不到我的实例的方法

Question

我们为我们的 CMS 设置了几个类，我正在尝试让平等工作，​​以便我可以检查通用列表是否包含项目。我们有一些继承层，我将在下面向您展示。在此之下，我将向您展示一些行为与我的预期相反的示例代码。如果您发现我做错了什么，请告诉我。我已经减少了下面的示例，只是为了向您展示相关的部分。我的实际课程要大得多，但我认为这是你需要看到的一切。

IBaseTemplate.cs

public interface IBaseTemplate {
  bool Equals(IBaseTemplate other);
  string GUID { get; }
}

BasePage.cs

public class BasePage : System.Web.UI.Page, IBaseTemplate, IEquatable<IBaseTemplate> {

  // code to define properties, including GUID

  // various constructors
  public BasePage(string GUID) { 
    this.GUID = GUID;
  }

  // interface methods
  public bool Equals(IBaseTemplate other) {
    return (this.GUID == other.GUID);
  }

}

LandingPage.cs

public class LandingPage : BasePage {
  // a bunch of extra properties and method specific to LandingPage
  // but NO definition for Equals since that's taken care of in BasePage

  public LandingPage(string GUID) : base(GUID) {}
}

SamplePage.aspx.cs

var p1 = new LandingPage("{3473AEF9-7382-43E2-B783-DB9B88B825C5}");
var p2 = new LandingPage("{3473AEF9-7382-43E2-B783-DB9B88B825C5}");
var p3 = new LandingPage("{3473AEF9-7382-43E2-B783-DB9B88B825C5}");
var p4 = new LandingPage("{3473AEF9-7382-43E2-B783-DB9B88B825C5}");

var coll = new List<LandingPage>();
coll.Add(p1);
coll.Add(p2);
coll.Add(p3);

p1.Equals(p4);     // True, as expected
coll.Contains(p4); // False, but I expect True here!

我希望coll.Contains(p4) 返回true，因为即使p1 到p4 是不同的实例，从BasePage 继承的Equals 方法会根据IBaseTemplate 的要求比较它们的GUID 属性。我错过了什么吗？

我查看了docs for List(T)'s Contains method，我正在实现IEquatable<T>.Equals，其中T 是IBaseTemplate。

Answer 1

您还需要覆盖Object.Equals(Object) - 请参阅this link。

Answer 2

我敢打赌，在幕后，这行代码：

p1.Equals(p4)

 is actually calling upon the Equals method of `Object` rather than yours.

尝试让你的接口实现IEqualityComparer，并在你的接口定义中去掉显式的Equals。

Answer 3

如果您希望Contains 方法使用IEquatable.Equals 方法，您的BasePage 类型需要实现IEquatable<BasePage>，而不是IEquatable<IBaseTemplate>。

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable<T>.Equals method for T (the type of values in the list).

因为你的类没有实现IEquatable<BasePage>，Contains 方法回退到使用虚拟的、非泛型的object.Equals 方法。在 BasePage 类中覆盖该方法应该可以解决问题

Answer 4

对List.Contains(T item) 的调用使用默认相等比较器确定相等性，该比较器由对象对 T（列表中值的类型）的 IEquatable.Equals 方法的实现定义。

因此，要在列表中定位的项目需要提供IEquatable 的实现，并且还需要重写Equals(Object obj) 方法以调用等价逻辑。

举个例子，假设你有一个这样的界面：

public interface IRole : INotifyPropertyChanged, INotifyPropertyChanging
{
    #region ConcurrencyToken
    /// <summary>
    /// Gets the unique binary concurrency token for the security role.
    /// </summary>
    /// <value>
    /// A <see cref="IList{Byte}"/> collection that contains the unique binary concurrency token for the security role.
    /// </value>
    IList<byte> ConcurrencyToken
    {
        get;
    }
    #endregion

    #region Description
    /// <summary>
    /// Gets or sets the description of the security role.
    /// </summary>
    /// <value>
    /// The human readable description of the security role.
    /// </value>
    string Description
    {
        get;
        set;
    }
    #endregion

    #region Id
    /// <summary>
    /// Gets or sets the unique identifier for the security role.
    /// </summary>
    /// <value>
    /// A <see cref="Int32"/> value that represents the unique identifier for the security role.
    /// </value>
    int Id
    {
        get;
        set;
    }
    #endregion

    #region LastModifiedBy
    /// <summary>
    /// Gets or sets the name of the user or process that last modified the security role.
    /// </summary>
    /// <value>
    /// The name of the user or process that last modified the security role.
    /// </value>
    string LastModifiedBy
    {
        get;
        set;
    }
    #endregion

    #region LastModifiedOn
    /// <summary>
    /// Gets or sets the date and time at which the security role was last modified.
    /// </summary>
    /// <value>
    /// A <see cref="Nullable{DateTime}"/> that represents the date and time at which the security role was last modified.
    /// </value>
    DateTime? LastModifiedOn
    {
        get;
        set;
    }
    #endregion

    #region Name
    /// <summary>
    /// Gets or sets the name of the security role.
    /// </summary>
    /// <value>
    /// The human readable name of the security role.
    /// </value>
    string Name
    {
        get;
        set;
    }
    #endregion
}

这个接口的默认实现可能是这样的，注意IEquatable是如何实现的：

public class Role : EntityBase, IRole, IEquatable<Role>
{
    //=======================================================================================================
    //  Constructors
    //=======================================================================================================
    #region Role()
    /// <summary>
    /// Initializes a new instance of <see cref="Role"/> class.
    /// </summary>
    public Role()
    {
    }
    #endregion

    #region Role(IEnumerable<byte> concurrencyToken)
    /// <summary>
    /// Initializes a new instance of <see cref="Role"/> class 
    /// using the specified unique binary concurrency token.
    /// </summary>
    /// <param name="concurrencyToken">The unique binary concurrency token for the security role.</param>
    /// <exception cref="ArgumentNullException">The <paramref name="concurrencyToken"/> is a <see langword="null"/> reference (Nothing in Visual Basic).</exception>
    public Role(IEnumerable<byte> concurrencyToken) : base(concurrencyToken)
    {

    }
    #endregion

    //=======================================================================================================
    //  Public Methods
    //=======================================================================================================
    #region ToString()
    /// <summary>
    /// Returns a <see cref="String"/> that represents the current <see cref="Role"/>.
    /// </summary>
    /// <returns>
    /// A <see cref="String"/> that represents the current <see cref="Role"/>.
    /// </returns>
    public override string ToString()
    {
        return this.Name;
    }
    #endregion

    //=======================================================================================================
    //  IEquatable<Role> Implementation
    //=======================================================================================================
    #region Equals(Role other)
    /// <summary>
    /// Indicates whether the current object is equal to another object of the same type.
    /// </summary>
    /// <param name="other">An object to compare with this object.</param>
    /// <returns><see langword="true"/> if the current object is equal to the other parameter; otherwise, <see langword="false"/>.</returns>
    public bool Equals(Role other)
    {
        if (other == null)
        {
            return false;
        }

        if (!String.Equals(this.Description, other.Description, StringComparison.Ordinal))
        {
            return false;
        }
        else if (!Int32.Equals(this.Id, other.Id))
        {
            return false;
        }
        else if (!String.Equals(this.Name, other.Name, StringComparison.Ordinal))
        {
            return false;
        }

        return true;
    }
    #endregion

    #region Equals(object obj)
    /// <summary>
    /// Determines whether the specified <see cref="Object"/> is equal to the current <see cref="Object"/>.
    /// </summary>
    /// <param name="obj">The <see cref="Object"/> to compare with the current <see cref="Object"/>.</param>
    /// <returns>
    /// <see langword="true"/> if the specified <see cref="Object"/> is equal to the current <see cref="Object"/>; otherwise, <see langword="false"/>.
    /// </returns>
    public override bool Equals(object obj)
    {
        return this.Equals(obj as Role);
    }
    #endregion

    #region GetHashCode()
    /// <summary>
    /// Returns the hash code for this instance.
    /// </summary>
    /// <returns>A 32-bit signed integer hash code.</returns>
    /// <a href="http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx"/>
    public override int GetHashCode()
    {
        int descriptionHashCode     = this.Description.GetHashCode();
        int idHashCode              = this.Id.GetHashCode();
        int nameHashCode            = this.Name.GetHashCode();

        /*
            * The 23 and 37 are arbitrary numbers which are co-prime.
            * 
            * The benefit of the below over the XOR (^) method is that if you have a type 
            * which has two values which are frequently the same, XORing those values 
            * will always give the same result (0) whereas the above will 
            * differentiate between them unless you're very unlucky.
        */
        int hashCode    = 23;
        hashCode        = hashCode * 37 + descriptionHashCode;
        hashCode        = hashCode * 37 + idHashCode;
        hashCode        = hashCode * 37 + nameHashCode;

        return hashCode;
    }
    #endregion

    //=======================================================================================================
    //  IRole Implementation
    //=======================================================================================================
    #region Description
    /// <summary>
    /// Gets or sets the description of this security role.
    /// </summary>
    /// <value>
    /// The human readable description of this security role. The default value is an <see cref="String.Empty"/> string.
    /// </value>
    [DataMember()]
    public string Description
    {
        get
        {
            return _roleDescription;
        }

        set
        {
            if (PropertyChangeNotifier.AreNotEqual(_roleDescription, value))
            {
                using (new PropertyChangeNotifier(OnPropertyChanging, OnPropertyChanged))
                {
                    _roleDescription = !String.IsNullOrEmpty(value) ? value : String.Empty;
                }
            }
        }
    }
    private string _roleDescription = String.Empty;
    #endregion

    #region Id
    /// <summary>
    /// Gets or sets the unique identifier for this security role.
    /// </summary>
    /// <value>
    /// A <see cref="Int32"/> value that represents the unique identifier for this security role. 
    /// The default value is <i>zero</i>.
    /// </value>
    [DataMember()]
    public int Id
    {
        get
        {
            return _roleId;
        }

        set
        {
            if (PropertyChangeNotifier.AreNotEqual(_roleId, value))
            {
                using (new PropertyChangeNotifier(OnPropertyChanging, OnPropertyChanged))
                {
                    _roleId = value;
                }
            }
        }
    }
    private int _roleId;
    #endregion

    #region Name
    /// <summary>
    /// Gets or sets the name of this security role.
    /// </summary>
    /// <value>
    /// The human readable name of this security role. The default value is an <see cref="String.Empty"/> string.
    /// </value>
    [DataMember()]
    public string Name
    {
        get
        {
            return _roleName;
        }

        set
        {
            if (PropertyChangeNotifier.AreNotEqual(_roleName, value))
            {
                using (new PropertyChangeNotifier(OnPropertyChanging, OnPropertyChanged))
                {
                    _roleName = !String.IsNullOrEmpty(value) ? value : String.Empty;
                }
            }
        }
    }
    private string _roleName = String.Empty;
    #endregion
}

Answer 5

您需要将 p4 实例实际添加到集合中。

coll.Add(p4);

编辑

为什么不只比较 GUID？

coll.Add(p1.GUID);
coll.Add(p2.GUID);
coll.Add(p3.GUID);
coll.Contains(p4.GUID);   //expect this to be true