如何與只讀類成員共享構造函數代碼？

Question

我有一個類，FooBarSet與一個「核心」塊的初始化邏輯。

A FooBar由Foo和Bar組成。類別FooBarSet初始化爲FooBar的列表。 FooBarSet可以也可以被分開的並行Foo和Bar列表初始化。

理想情況下，我可以像這樣運行：

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); 
    } 
} 

這是C＃，而不是Java，所以this(zipped)是非法的。常見的解決辦法，as in this answer，就是拉核心初始化成一個共同的私有方法：

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，這也不行，因爲readonly _list領域。

假設_list必須是readonly，我怎樣才能讓這些構造函數共享初始化代碼？

Answer 1

簡單的答案是讓Init方法返回值在構造函數中進行設置：

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「拉鍊」的代碼移動到一個靜態的功能和使用：

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; 
    } 
}