如何与只读类成员共享构造函数代码？

Question

我有一个类，FooBarSet，它有一个“核心”初始化逻辑块。

FooBar 由Foo 和Bar 组成。类FooBarSet 使用FooBar 列表初始化。 FooBarSet 可以也通过单独的并行 Foo 和 Bar 列表进行初始化。

理想情况下，我可以这样运行它：

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#，不是 Java，所以this(zipped) 是非法的。常见的解决方案as in this answer，就是把核心初始化拉到一个常用的私有方法中：

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);
    }

}

但是，这也不起作用，因为 readonly _list 字段。

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方法返回要在构造函数中设置的值：

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

您可以将“拉链”代码移动到静态函数并使用它：

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;
    }
}