接口的可选参数

Question

使用c#4.0 - 构建一个接口和一个实现接口的类.我想在界面中声明一个可选参数,并让它反映在类中.所以,我有以下内容:

 public interface IFoo
 {
      void Bar(int i, int j=0);
 }

 public class Foo
 {
      void Bar(int i, int j=0) { // do stuff }
 }

编译,但看起来不正确.接口需要具有可选参数,否则它在接口方法签名中无法正确反映.

我应该跳过可选参数并使用可空类型吗？或者这是否按预期工作,没有副作用或后果？

Answer 1

真正奇怪的是,您为接口中的可选参数赋予的值实际上有所不同.我想你必须质疑值是接口细节还是实现细节.我会说后者,但事情就像前者一样.例如,以下代码输出1 0 2 5 3 7.

// Output:
// 1 0
// 2 5
// 3 7
namespace ScrapCSConsole
{
    using System;

    interface IMyTest
    {
        void MyTestMethod(int notOptional, int optional = 5);
    }

    interface IMyOtherTest
    {
        void MyTestMethod(int notOptional, int optional = 7);
    }

    class MyTest : IMyTest, IMyOtherTest
    {
        public void MyTestMethod(int notOptional, int optional = 0)
        {
            Console.WriteLine(string.Format("{0} {1}", notOptional, optional));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyTest myTest1 = new MyTest();
            myTest1.MyTestMethod(1);

            IMyTest myTest2 = myTest1;
            myTest2.MyTestMethod(2);

            IMyOtherTest myTest3 = myTest1;
            myTest3.MyTestMethod(3);
        }
    }
}

有趣的是,如果你的接口使一个参数成为可选的,那么实现它的类不必做同样的事情:

// Optput:
// 2 5
namespace ScrapCSConsole
{
    using System;

    interface IMyTest
    {
        void MyTestMethod(int notOptional, int optional = 5);
    }

    class MyTest : IMyTest
    {
        public void MyTestMethod(int notOptional, int optional)
        {
            Console.WriteLine(string.Format("{0} {1}", notOptional, optional));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyTest myTest1 = new MyTest();
            // The following line won't compile as it does not pass a required
            // parameter.
            //myTest1.MyTestMethod(1);

            IMyTest myTest2 = myTest1;
            myTest2.MyTestMethod(2);
        }
    }
}

然而,似乎是一个错误的是,如果你明确地实现了接口,那么你在类中为可选值赋予的值是没有意义的.在以下示例中,您如何使用值9？编译时甚至不会发出警告.

// Optput:
// 2 5
namespace ScrapCSConsole
{
    using System;

    interface IMyTest
    {
        void MyTestMethod(int notOptional, int optional = 5);
    }

    class MyTest : IMyTest
    {
        void IMyTest.MyTestMethod(int notOptional, int optional = 9)
        {
            Console.WriteLine(string.Format("{0} {1}", notOptional, optional));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyTest myTest1 = new MyTest();
            // The following line won't compile as MyTest method is not available
            // without first casting to IMyTest
            //myTest1.MyTestMethod(1);

            IMyTest myTest2 = new MyTest();            
            myTest2.MyTestMethod(2);
        }
    }
}

Answer 2

您可以考虑pre-optional-parameters备选方案:

public interface IFoo
{
    void Bar(int i, int j);
}

public static class FooOptionalExtensions
{
    public static void Bar(this IFoo foo, int i)
    {
        foo.Bar(i, 0);
    }
}

如果您不喜欢新语言功能的外观,则不必使用它.

Answer 3

You don't have to make the parameter optional in the implementation. Your code will make somewhat more sense then:

 public interface IFoo
 {
      void Bar(int i, int j = 0);
 }

 public class Foo
 {
      void Bar(int i, int j) { // do stuff }
 }

This way, it's unambiguous what the default value is. In fact, I'm pretty sure the default value in the implementation will have no effect, since the interface provides a default for it.