如何与只读类成员共享构造函数代码？

Question

我有一个类，FooBarSet与一个“核心”块的初始化逻辑。

A FooBar由Foo和Bar组成。类别FooBarSet初始化为FooBar的列表。 FooBarSet可以也可以被分开的并行Foo和Bar列表初始化。

理想情况下，我可以像这样运行：

public class FooBarSet 
{ 
    private readonly List<FooBar> _list; 

    // Primary constructor. 
    public FooBarSet(List<FooBar> foobarList) 
    { 
     // Contracts and initialization... 
     _list = foobarList; 
    } 

    // Secondary constructor. 
    public FooBarSet(List<Foo> fooList, List<Bar> barList) 
    { 
     // Zip a new list of new FooBars 
     var zipped = fooList.Zip(barList, 
           (foo, bar) => new FooBar(foo, bar)); 

     // Call primary constructor with zipped list. 
     this(zipped); 
    } 
} 

这是C＃，而不是Java，所以this(zipped)是非法的。常见的解决办法，as in this answer，就是拉核心初始化成一个共同的私有方法：

public class FooBarSet 
{ 
    private readonly List<FooBar> _list; 

    // Common "constructor" called from actual constructors. 
    private Init(List<FooBar> foobarList) 
    { 
     // Contracts and initialization... 
     _list = foobarList; 
    } 

    public FooBarSet(List<FooBar> foobarList) 
    { 
     Init(foobarList); 
    } 

    public FooBarSet(List<Foo> fooList, List<Bar> barList) 
    { 
     var zipped = fooList.Zip(barList, 
           (foo, bar) => new FooBar(foo, bar)); 

     Init(zipped); 
    } 

} 

然而，这也不行，因为readonly _list领域。

假设_list必须是readonly，我怎样才能让这些构造函数共享初始化代码？

Answer 1

简单的答案是让Init方法返回值在构造函数中进行设置：

public class FooBarSet 
{ 
    private readonly List<FooBar> _list; 

    // Common "constructor" called from actual constructors. 
    private List<FooBar> Init(List<FooBar> foobarList) 
    { 
     // Contracts and initialization... 
     return whateverList; 
    } 

    public FooBarSet(List<FooBar> foobarList) 
    { 
     _list = Init(foobarList); 
    } 

    public FooBarSet(List<Foo> fooList, List<Bar> barList) 
    { 
     var zipped = fooList.Zip(barList, 
           (foo, bar) => new FooBar(foo, bar)); 

     _list = Init(zipped); 
    } 

} 

Answer 2

可以“拉链”的代码移动到一个静态的功能和使用：

public class FooBarSet 
{ 
    private readonly List<FooBar> _list; 

    private static List<FooBar> Prepare(List<Foo> fooList, List<Bar> barList) 
    { 
     return fooList.Zip(barList, (foo, bar) => new FooBar(foo, bar)); 
    } 

    public FooBarSet(List<Foo> fooList, List<Bar> barList) : 
     this(Prepare(fooList, barList)) 
    { 
    } 

    public FooBarSet(List<FooBar> zippedList) 
    { 
     _list = zippedList; 
    } 
}