显式接口实现教程

教程
实现接口的类可以显式实现该接口的成员。当显式实现某成员时，不能通过类实例访问该成员，而只能通过该接口的实例访问该成员。本教程包含两个示例。第一个示例阐释如何显式实现和访问接口成员。第二个示例展示如何实现具有相同成员名的两个接口。
示例 1
本示例声明一个 IDimensions 接口和一个 Box 类，该类显式实现接口成员 Length 和 Width。通过接口实例 myDimensions 访问这些成员。
// explicit1.cs
interface IDimensions
{
   float Length();
   float Width();
}

class Box : IDimensions
{
   float lengthInches;
   float widthInches;

   public Box(float length, float width)
   {
      lengthInches = length;
      widthInches = width;
   }
   // Explicit interface member implementation:
   float IDimensions.Length()
   {
      return lengthInches;
   }
   // Explicit interface member implementation:
   float IDimensions.Width()
   {
      return widthInches;     
   }

   public static void Main()
   {
      // Declare a class instance "myBox":
      Box myBox = new Box(30.0f, 20.0f);
      // Declare an interface instance "myDimensions":
      IDimensions myDimensions = (IDimensions) myBox;
      // Print out the dimensions of the box:
      /* The following commented lines would produce compilation
         errors because they try to access an explicitly implemented
         interface member from a class instance:                   */
      //System.Console.WriteLine("Length: {0}", myBox.Length());
      //System.Console.WriteLine("Width: {0}", myBox.Width());
      /* Print out the dimensions of the box by calling the methods
         from an instance of the interface:                         */
      System.Console.WriteLine("Length: {0}", myDimensions.Length());
      System.Console.WriteLine("Width: {0}", myDimensions.Width());
   }
}
输出
Length: 30
Width: 20
代码讨论
请注意 Main 方法中下列代码行被注释掉，因为它们将产生编译错误。显式实现的接口成员不能从类实例访问：
//System.Console.WriteLine("Length: {0}", myBox.Length());
//System.Console.WriteLine("Width: {0}", myBox.Width());
还请注意，Main 方法中的下列代码行成功输出框的尺寸，因为这些方法是从接口实例调用的：
System.Console.WriteLine("Length: {0}", myDimensions.Length());
System.Console.WriteLine("Width: {0}", myDimensions.Width());
示例 2
显式接口实现还允许程序员继承共享相同成员名的两个接口，并为每个接口成员提供一个单独的实现。本示例同时以公制单位和英制单位显示框的尺寸。Box 类继承 IEnglishDimensions 和 IMetricDimensions 两个接口，它们表示不同的度量衡系统。两个接口有相同的成员名 Length 和 Width。
// explicit2.cs
// Declare the English units interface:
interface IEnglishDimensions
{
   float Length();
   float Width();
}
// Declare the metric units interface:
interface IMetricDimensions
{
   float Length();
   float Width();
}
// Declare the "Box" class that implements the two interfaces:
// IEnglishDimensions and IMetricDimensions:
class Box : IEnglishDimensions, IMetricDimensions
{
   float lengthInches;
   float widthInches;
   public Box(float length, float width)
   {
      lengthInches = length;
      widthInches = width;
   }
// Explicitly implement the members of IEnglishDimensions:
   float IEnglishDimensions.Length()
   {
      return lengthInches;
   }
   float IEnglishDimensions.Width()
   {
      return widthInches;     
   }
// Explicitly implement the members of IMetricDimensions:
   float IMetricDimensions.Length()
   {
      return lengthInches * 2.54f;
   }
   float IMetricDimensions.Width()
   {
      return widthInches * 2.54f;
   }
   public static void Main()
   {
      // Declare a class instance "myBox":
      Box myBox = new Box(30.0f, 20.0f);
      // Declare an instance of the English units interface:
      IEnglishDimensions eDimensions = (IEnglishDimensions) myBox;
      // Declare an instance of the metric units interface:
      IMetricDimensions mDimensions = (IMetricDimensions) myBox;
      // Print dimensions in English units:
      System.Console.WriteLine("Length(in): {0}", eDimensions.Length());
      System.Console.WriteLine("Width (in): {0}", eDimensions.Width());
      // Print dimensions in metric units:
      System.Console.WriteLine("Length(cm): {0}", mDimensions.Length());
      System.Console.WriteLine("Width (cm): {0}", mDimensions.Width());
   }
}
输出
Length(in): 30
Width (in): 20
Length(cm): 76.2
Width (cm): 50.8
代码讨论
如果希望默认度量采用英制单位，请正常实现 Length 和 Width 这两个方法，并从 IMetricDimensions 接口显式实现 Length 和 Width 方法：
// Normal implementation:
public float Length()
{
   return lengthInches;
}
public float Width()
{
   return widthInches;
}
// Explicit implementation:
float IMetricDimensions.Length()
{
   return lengthInches * 2.54f;
}
float IMetricDimensions.Width()
{
   return widthInches * 2.54f;
}
这种情况下，可以从类实例访问英制单位，而从接口实例访问公制单位：
System.Console.WriteLine("Length(in): {0}", myBox.Length());
System.Console.WriteLine("Width (in): {0}", myBox.Width());  
System.Console.WriteLine("Length(cm): {0}", mDimensions.Length());
System.Console.WriteLine("Width (cm): {0}", mDimensions.Width());