Calling Properties from Struct in c#

Question

i have a structure defined, which contains a public field and a public property named _one and One respectively, now i instantiate the struct in the main function (not creating new object), and called the Property from the struct, i am getting the compile time error saying use of unassigned local variable One, however when i called the field _one, it works pretty expected here what i am doing:

 public struct myStruct
    {
        public int _one;
        public int One
        {
            get { return _one; }
            set { _one = value; }
        }
        public void Display()
        {
            Console.WriteLine(One);
        }
    }



static void Main(string[] args)
        {

            myStruct _struct;
            _struct.One = 2; // Does not works
            _struct._one = 2; // Works fine
        }

can anyone explain whats the reason behind this, could not understand the concept.

score 2 · Accepted Answer · edited May 23 '17 at 12:18

2

You need to initialize the struct in order for the property to be accessible - _struct has a default value otherwise:

myStruct _struct = new myStruct();

By the way - mutable value types are evil.

edited May 23 '17 at 12:18

Community

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answered Nov 27 '12 at 16:48

Oded

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yes i checked with that when i initialize the struct properties are accessible, can you tell me whats actually the difference that creates, if without initialize it does not allows Properties then why Fields – Abbas Nov 27 '12 at 16:57
@Abbas - Because a value type cannot be null, when you reference one, it will be initialized to something. The fields will be initialized to default values. For an `int`, that will be `0`. – Oded Nov 27 '12 at 17:01

score 1 · Answer 2 · answered Nov 27 '12 at 17:46

This is unintuitive behavior, but it is permitted by the rules of Definite assignment checking. Described in excruciating detail in section 5.3 of the C# Language Specification. The key phrase, early in the chapter is:

In additional to the rules above, the following rules apply to struct-type variables and their instance variables:
- An instance variable is considered definitely assigned if its containing struct-type variable is considered definitely assigned.
- A struct-type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.

It is the latter rule that permits this. In other words, you can also initialize a struct by assigning all of its variables. You can see this by trying these snippets:

myStruct _struct = new myStruct();
_struct.Display();   // fine by the 1st bullet

myStruct _struct;
_struct.Display();   // bad

myStruct _struct;
_struct._one = 2;
_struct.Display();   // fine by the 2nd bullet

So you don't get CS0165 by assigning the field because that would disallow initializing the structure by assigning its variables.

score 0 · Answer 3 · answered Nov 27 '12 at 17:01

The reasons which would favor using read-write properties instead of exposed fields in class definitions do not apply to structures, since they can support neither inheritance nor update notifications, and the mutability of a struct's field depends upon the mutability of the struct instance, regardless of whether the field is exposed or not. If a struct is supposed to represent a group of related but freely-independently-modifiable variables, it should simply expose those variables as fields. If a property with a backing field is supposed to be read-only, the constructor should set the backing field directly, rather than via property setter.

Calling Properties from Struct in c#

3 Answers3