如何在 C# 中为结构设置默认值？

Question

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?

Answer 1

You can't. Structures are always pre-zeroed, and there is no guarantee the constructor is ever called (eg 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.

Answer 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}";
    } 
}

Answer 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

Answer 4

What you probably want to do is to override ToString() , eg

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.

Answer 5

This is my take on this in case somebody finds it useful.

public struct MyStruct
{
    public int item1;
    public float item2;
    public float item3;

    public static MyStruct Null => new MyStruct(-1, 0, 0);
}

I have a static method inside my struct so that I can do this:

var data = MyStruct.Null;

instead of

var data = new MyStruct();
data.item1 = -1;
...

Or create a custom constructor to pass the data.

Answer 6

You can also do this:

public struct MyStruct
{
    public static readonly Default = new MyStruct(42);

    public int i;

    public MyStruct(int i)
    {
        this.i = i;
    }
}

And then when you create a default struct of this type do this:

public MyStruct newStruct = MyStruct.Default;

But of course, this won't override default and other programmers will bump their heads a few times. Really consider if a struct is the way to go, from the microsoft docs :

"A structure type (or struct type) is a value type that can encapsulate data and related functionality. Typically, you use structure types to design small data-centric types that provide little or no behavior."

Consider this: if you had 2 values in your struct and you wanted to make constructors, would 2 or less constructors suffice? If the answer is no, then the answer is: don't use a struct.