实例化：将列表移动到参数中

Question

我是 C# 的新手，一直在努力寻找一种惯用的方法来在构造函数中初始化列表。

没有完全解决问题的相关问题：

How to initialize a list with constructor?
Initializing lists in a constructor.

这有效，但有一个缺陷：

class Datapoint
{
    public bool Debug { get; set; }
    public string Pattern { get; private set; }

    // I would prefer to initialize this list in the constructor
    public List<Dictionary<string, dynamic>> operations = 
            new List<Dictionary<string, dynamic>>();

    // constructor
    public Datapoint(bool debug = false, 
                string pattern = ""
                // I would prefer operations to go here 
                // but the following doesn't work:
                // List<Dictionary<string, dynamic>> operations = 
                //     new List<Dictionary<string, dynamic>>()
                ) 
    {
        Debug = debug;
        Pattern = pattern;
    }
}


// Let's define some Datapoints
class Definitions 
{
    public static Datapoint turtles = new Datapoint
    (
        pattern: @"turtle pattern",
        // I would prefer operations to go here
    )
    {
        operations =
                { new Dictionary<string, dynamic>
                    {
                        ["func"] = "stitch_lines"
                    }
                }
    };
}

缺陷是我不能将操作设置为私有，否则在创建海龟时会出错。

理想情况下，我希望 operations 成为构造函数的参数，但我遗漏了一些东西，因为我尝试的每个组合都会产生此错误：

Default parameter value for operations must be a compile-time constant.

提前感谢您的任何见解。

Answer 1

如错误所示，默认值必须是编译时常量。我会将其作为两个重载来执行：

// constructor
public Datapoint(bool debug = false, 
            string pattern = "")
{
    Debug = debug;
    Pattern = pattern;

    operations = new List<Dictionary<string, dynamic>>();
}

public Datapoint(List<Dictionary<string, dynamic>> operations,
            bool debug = false, 
            string pattern = "")
{
    Debug = debug;
    Pattern = pattern;

    this.operations = operations;
}

请注意，您需要在第二次重载中对参数重新排序，因为可选参数必须位于参数列表的末尾。

Answer 2

你可以接受一个 null 然后在你的构造函数中检查它：

public Datapoint(
    bool debug = false, 
    string pattern = "",
    List<Dictionary<string, dynamic>> operations = null
)
{
    Debug = debug;
    Pattern = pattern;
    this.operations = operations ?? new List<Dictionary<string, dynamic>>();
}

但是，请参阅 D Stanley 的回答的评论，以讨论一般情况下的缺点。

Answer 3

避免以这种方式使用可选参数

//constructor
public Datapoint(
    bool debug = false, //dangerous
    string pattern = "",//dangerous
    List<Dictionary<string, dynamic>> operations = null
)

改为使用：

//constructor
public Datapoint(
    bool? debug = null, 
    string pattern = null,
    List<Dictionary<string, dynamic>> operations = null
)
{
    Debug = debug.HasValue && debug.Value;
    Pattern = pattern;
    this.operations = (operations == null) ? 
              new List<Dictionary<string, dynamic>>() 
              : operations ;
}

来源： C# In Depth – Optional Parameters and Named Arguments

实例化：将列表移动到参数中

Instantiation: Moving a List into the Arguments

c#

constructor

instantiation