Array of Rectangles declaration problem

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Hey everyone,
Im trying to implement a tiling algorithm in C#. I'm having a couple of issues,
my approach is to take the size of a grid m x m as input, and then draw the grid using DrawRectangles method on my graphics object. the user then selects a tile around which the grid gets tiled.
My issues:
one : the OnPaint eventhandler only shows the graphics i want to generate when called on the form. I tried calling it on a picturebox control and a panel control but the graphics dont show up?
two: im trying to put my rectangles in an array so i can identify which rectangle is selected and apply a tiling algo based on which tile(rectangle) is selected. But its not working. NullExceptionReference is what is says when i run the code. Im an amateur, so please let me know if i'm declaring it incorrectly? or Am i using the wrong data structure?

public Rectangle [] rect;

C#

rect[(m*m)]=new Rectangle();
           k=1;
           for(i=0;i<m;i++)
           {
               for(j=0;j<m;j++)
               {
                   rect [k]=new Rectangle((x+(j*offset)),(y+i*offset)),offset,offset);
                   g.DrawRectangle(myPen,rect[k]);
                   
                   k++;
               }
           }

Posted 6-Mar-12 3:22am

ssiidd

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Pete O'Hanlon · Accepted Answer · 2012-03-06T03:35:00

As you are instantiating a rectangle and then drawing it immediately, there doesn't seem any reason for your code to be actually storing an array of rectangles. The simple way for you to draw the rectangle there is to simply replace the inner part of your loop with:

C#

g.DrawRectangle(myPen, new Rectangle(x+(j*offset), (y+i*offset), offset, offset));

Is there processing elsewhere that requires this list of rectangles, or is it sufficient that you are simply drawing it. BTW, by doing this you can get rid of the rectangle array declaration altogether.

[Edit]Following clarification from the OP as to what he was after:

I would separate the creation of the rectangle away from the drawing of the rectangle. One problem you currently face is that you are instantiating and populating the array in every OnPaint event. Instead, create it prior to drawing like this:

C#

private List<Rectangle> rectangles = new List<Rectangle();
private void SetupRectangles(int m, int offset)
{
  for (int i = 0; i < m; i++)
  {
    for (int j = 0; j < m; j++)
    {
      rectangles.Add(new Rectangle(x+(j*offset), (y+i*offset), offset, offset));
    }
  }
}

Then in your OnPaint handler, all you need do is this:

C#

if (rectangles.Count == 0)
{
  // This will only happen once. This could be moved elsewhere, but I don't know   
  // where you are setting up offset and m.
  SetupRectangles(m, offset); 
}
foreach (Rectangle rect in rectangles)
{
  g.DrawRectangle(myPen, rect);
}

It's as simple as that.