[Mono-list] Matrix of Objects

[A Rafael D Teixeira]

Another way is for Celda to be an struct. Arrays of value-types like int
or your structs are preinitialized. objects like String and other
classes are reference types and must be created as Jonathan pointed out.

public struct Celda
{
    public int Valor ;
    public Celda(int V) {
        Valor = V;   
    }
    public override string ToString(){
        return "("+Valor+")";
    }   
}

Now your original class Terreno should work as it was...

HIH,

On Thu, 2005-03-03 at 09:07, Jonathan Pryor wrote:
> On Wed, 2005-03-02 at 22:30 +0000, Phillip Neumann wrote:
> > Hello...
> > 
> > I wonder how to work with arrays of objects.
> 
> <snip/>
> 
> > when executing i get:
> > System.NullReferenceException: Object reference not set to an instance 
> > of an object
> > 
> > how shall i create my matrix?
> 
> Your example is equivalent to this:
> 
> 	string[] array = new string[10];
> 	array[0].Length;	// throws NullReferenceException
> 
> The above will also generate a NullReferenceException.  The reason is
> because the array is separate from the objects it contains; you created
> an array, but never put anything in it.
> 
> The correct approach would be this:
> 
> 	// create array
> 	string[] array = new string[10];
> 
> 	// populate
> 	for (int i = 0; i < array.Length; ++i)
> 		array[i] = "some string" + i.ToString();
> 
> 	// access array
> 	int total_length = 0;
> 	for (int i = 0; i < array.Length; ++i)
> 		total_length += array[i].Length;
> 
> Or for your matrix code in the Terreno constructor:
> 
> 	// create matrix
> 	_Matrix = new Celda[px,py];
> 
> 	// populate matrix
> 	for (int i = 0; i < px; ++i)
> 		for (int j = 0; j < py; ++j)
> 			_Matrix[i, j] = new Celda ();
> 
>  - Jon
> 
> 
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