C＃中局部变量和字段的数组声明和数组内存分配的差异

Question

I have the question listed above. difference in array declaration and array memory allocation for local variables and fields in C#

I am working on a WindowsForm application but I would like to declare some arrays.

In the public partial class Form1 : Form , I cannot declare an array and do the memory allocation separately ( if I do int[] numero = new int[5] it works ) but separately (in two steps) as shown in the code below it does not.

When I hoop over the variable numero VS labels it as a field . I did the same in the derivative class public Form1() and it works fine in one step (int[] numero = new int[4]) and in two steps as shown below in the code. When I hoop over the variable casinumero , it labels it as a local variable . So the question is:

WHY THE ARRAY DECLARATION AND MEMORY ALLOCATION CANNOT BE DONE IN ONE STEP FOR FIELDS AS IT IS FOR LOCAL VARIABLES?

public partial class Form1 : Form
    {
        public int [] numero;
        numero = new int[5];

        public Form1()
       {
            InitializeComponent();
            int[] casinumero;
            casinumero = new int[4];
       }
    }

Answer 1

This is declaration:

public int [] numero;

Below is instantiation. Another way to word it: This is using the variable. . You are using the variable by assigning something to it or initializing it explicitly.

numero = new int[5];

You cannot use a variable at the class level.

From C# Specs:

An instance variable of a class comes into existence when a new instance of that class is created,

Therefore, you cannot use it before the instance is created.

You can declare it. If you do not initialize it, it will have a default initial value. Depending on the type, the default values vary. For reference type, as is the case with arrays, it will be initialized to null . This is called definite assignment and it is one of the C# rules.

You can do this, however (Please note inline comments):

public class Example
{
    // Declared here and it will be defaulted to null since we never
    // initialized it explicitly.
    public int[] numero;

    // Or we can initialize it and declare it in one step:
    // public int[] numero = new int[5];

    public Example()
    {
        // Now we are in the constructor, we can use the variable and 
        // explicitly initialize it.
        numero = new int[5];

        int[] casinumero;
        casinumero = new int[4];
    }

    public void SomeMethod()
    {
        // Or we can use it in a method
        numero = new int[5];
    }
}