CODEWITHSUNDEEP
c#
roamcel
roamcel

Reputation: 645

Define a default array value from a class instance

I have the following c# class

public class smallclass 
{
    public Vector3 var1 = new Vector3(1,2,3);
    public string var2 = "defaultval";

    public smallclass() 
    {
      var1 = new Vector3(11,22,33);
      var2 = "constructor";
    }
}

And I have another c# class

public class bigclass  
{
    public smallclass[] smallclasses;
}

Then somewhere else in my program, I increment smallclasses size with

smallclasses = new smallclass[3];

but I realize that the three elements are null... is this correct behavior?

Allow me to clarify that I need that, whenever I resize the smallclasses array, its elements get automatically 'constructed'... I just don't know if such a thing can be done.

Differently than what I'd have expected, setting default values has no effect. Immediately after I resize the smallclasses array, the elements are null...

Any advice appreciated.

Thanks.

Upvotes: 1

Views: 1424

Answers (4)

phoog
phoog

Reputation: 43056

smallclasses = new smallclass[3]; is not resizing the array. It is creating a new array.

Your array declaration (public smallclass[] smallclasses;) creates a field in the class bigclass whose initial value is null. The statement smallclasses = new smallclass[3]; creates a new array object and assigns a reference to that object to the field smallclasses.

Whenever you create a new array of a reference type (i.e., a class), all elements of the array are null. This is expected behavior. See section 1.8 of the C# specification, which says in part: "The new operator automatically initializes the elements of an array to their default value, which, for example, is zero for all numeric types and null for all reference types."

Upvotes: 2

David Heffernan
David Heffernan

Reputation: 613461

You are asking for your array of objects to be automatically created for you. This is not how the language works. You need to construct each object individually, or in a loop.

If you do not initialize an array when you declare it, the members are initialized to the default initial value for the array type. For a reference type this is null.

Upvotes: 1

Michael Goldshteyn
Michael Goldshteyn

Reputation: 74450

If I understand your question correctly, check out the Array.Resize method. It will help you keep the values of the objects from the original array, when the newly sized array is created. From the linked article:

This method allocates a new array with the specified size, copies elements from the old array to the new one, and then replaces the old array with the new one.

Update based on comment:

If you want the references in the array to refer to valid objects, you must allocate these objects and store them in the array. With a C# array you just have a reference to an array object. You still have to allocate the objects themselves and store their references into the array, if you want it to contain actual objects.

Upvotes: 1

luqi
luqi

Reputation: 2909

Try using List of Smallclass instead of the smallclass[]. You can also do Array.Resize on arrays. But the list solution is better for performance.

Upvotes: 0

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