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

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.