2

Here's a piece of code coming from a C# project using a 2d array. For a reason I don't understand my program compiles perfectly but during run-time it crashes.

public class Tile_Info
{
  public int id;

  public Tile_Info(int _id)
  {
    id = _id;
  }
}

class Program
{

  public static void Main(string[] args)
  {
    int width = 20;
    int height = 30;

    Tile_Info[,] my_tile;
    my_tile = new Tile_Info[width, height];

    for(int y = 0; y < height; y++)
    {
      for(int x = 0; x < width; x++)
      {
        my_tile[x, y].id = 0;
      }
    }
  }
}

According to the debugger it's because "Object reference not set to an instance of an object", but I'm pretty sure it's what I'm doing here: my_tile = new Tile_Info[width, height];.

Anyone can tell what's wrong? Thank you for your support!

Improve this question
1
  • btw: try to use debugger to find such errors! Commented Jan 25, 2013 at 7:24

2 Answers 2

Reset to default
7

The creation of the array does not create the objects themselves, just like a creation of a parking lot does not create the cars that park there.

You still need to create the objects yourself. Change

my_tile[x, y].id = 0;

to

my_tile[x, y] = new Tile_Info(0);

This only happens when reference types (class) are used because the thing that is stored in the array is a reference to an instance, instead of the instance itself. On a lower level this (more or less) means that the memory for the instance is not yet allocated, just the memory for its reference, so you must new up an instance to initialize it. On the other hand if Tile_Info is a value type (struct) then the array will contain the actual instance, and the new Tile_Info[width, height] would have initialized the allocated memory to valid start state (all zeroes), which is exactly what the default parameterless constructor of a value type does.

So, if you had defined Tile_Info like this:

public struct Tile_Info
{
  public int id; // this should be a property, public fields are baaad
  public Tile_Info(int _id){ id = _id;}
}

both my_tile[x, y].id = 0 and my_tile[x, y] = new Tile_Info(0) would have been legal.

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Thank you very much you solved my problem, I was certain the object was automatically created. At least when I used the 'int' or 'string' type the object seemed to be already created.
I'll edit the answer to show why that is. Basically it's a "reference type vs value type" thing.
1

You have created a new 2D Tile_Info array by the first new keyword.

You have only created an empty array that can hold items of type Tile_Info. Its your job to create individual items and put them in the array.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.