如何在 C# 中为结构设置默认值? [英] How to make a default value for the struct in C#?

问题描述

我正在尝试为我的结构设置默认值.例如 Int 的默认值 - 0，DateTime 的默认值 - 1/1/0001 12:00:00 AM.众所周知，我们不能在结构中定义无参数构造函数.

I'm trying to make default value for my struct. For example default value for Int - 0, for DateTime - 1/1/0001 12:00:00 AM. As known we can't define parameterless constructor in structure.

struct Test
{
    int num;
    string str;
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(default(Test)); // shows namespace and name of struct test.Test
        Console.WriteLine(new Test()); // same

        Console.ReadKey(true);
    }
}

如何为 struct 设置默认值?

How can I make a default value for struct?

推荐答案

你不能.结构总是预先归零，并且不能保证构造函数会被调用(例如 new MyStruct[10]).如果您需要零以外的默认值，则需要使用类.这就是为什么您不能首先更改默认构造函数的原因(直到 C# 6) - 它永远不会执行.

You can't. Structures are always pre-zeroed, and there is no guarantee the constructor is ever called (e.g. new MyStruct[10]). If you need default values other than zero, you need to use a class. That's why you can't change the default constructor in the first place (until C# 6) - it never executes.

最接近的是使用 Nullable 字段，并通过属性将它们解释为具有一些默认值:

The closest you can get is by using Nullable fields, and interpreting them to have some default value if they are null through a property:

public struct MyStruct
{
  int? myInt;

  public int MyInt { get { return myInt ?? 42; } set { myInt = value; } }
}

myInt 仍然预先归零，但您将零"解释为您自己的默认值(在本例中为 42).当然，这可能完全是不必要的开销:)

myInt is still pre-zeroed, but you interpret the "zero" as your own default value (in this case, 42). Of course, this may be entirely unnecessary overhead :)

对于Console.WriteLine，它只是调用虚拟的ToString.您可以更改它以返回任何您想要的东西.

As for the Console.WriteLine, it simply calls the virtual ToString. You can change it to return it whatever you want.

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