How to make a default value for the struct in C#?

问题

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);
    }
}

How can I make a default value for struct?

回答1:

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.

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 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 :)

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

回答2:

Your problem is not with the behaviour of C#/.Net. The way you instantiate the struct effectively creates an instance with default values for all member fields.

The Console.WriteLine converts its argument to a string using the ToString() method. The default implementation (Object.ToString()) simply writes the fully qualified class name (namespace and name, as you call it).

If you want another visualisation, you should override the ToString method:

public struct Test
{
    int num;
    string str;
    public override string ToString()
    {
        return $"num: {num} - str: {str}";
    } 
}

回答3:

Printing out objects of the C# results with namespaces unless you override .ToString() for your objects. Can you define your struct like below and try it ?

public struct Test
{
    int num;
    string str;
    public override string ToString()
    {
        return "Some string representation of this struct";
    }
}

PS: default(Test) gives you a struct contains default(int) and default(string) which I mean Test.num is 0 and Test.str is null

Hope this helps

回答4:

What you probably want to do is to override ToString(), e.g.

struct Test
{
    int num;
    string str;

    public override string ToString ()
    {
        return string.Format ($"{str} | {num}");
    }
}

As you have mentioned, it is impossible to define default values for fields other than default values for their appropriate types. However, with an overriden ToString(), you will see better formatted information about your structure in the console and during debugging.

来源：https://stackoverflow.com/questions/39846708/how-to-make-a-default-value-for-the-struct-in-c

标签

struct