对List.Contains(T item)的调用 通过使用默认相等比较器来确定相等性，该比较器由对象对 T（列表中的值的类型）的 IEquatable.Equals 方法的实现所定义。
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。
例如，假设您有一个这样的界面：
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
}