Adding to a list from another class - list

Question

I am trying to add the instance of an object that I click on to a list on my control object. However when I do so it says that the reference is not set to an instance of an object. The code I have to instantiate the list on the control object is:
public List<Transform> selected = new List<Transform>();
And I tried to add to it to that list using this code attached to the unit:
if (!selected)
{
// Set selected state
selected = true;
// Add to Selected List
control.GetComponent<ForwardCommandScript>().selected.Add(this.transform);
// Set material colour brighter
oldColour = gameObject.renderer.material.color;
newColour = oldColour + new Color(0.2f, 0.2f, 0.2f);
gameObject.renderer.material.color = newColour;
}
I have tried with transform as well. Later I will try to remove it by finding a reference id that was set when the unit is instantiated so should I try to add the script instead of the object if I need to find its variables and then delete the game object attached to the script. I have tried with the GameObject, transform and the class. I wanted to use the class so I can easily access the variables. I have posted this on unity answers and forums but no one replied in the week it was up and I don't like reposting the same stuff on the same site.
Cheers, Scobbo

Answer 1

Your error NullReferenceException: Object reference not set to an instance of an object states that something in the associated line is null. Since the error message doesn't state which part is null, you have to split your code up and check which part is failing.
I'm not sure how you split it up, but try it this way:
var script = control.GetComponent<ForwardCommandScript>();
if (script == null) Debug.Log("script not found");
if (script.selected == null) Debug.Log("selected is null");
script.selected.Add(this.transform);
Now you should get one of the two messages in your debug log before the exception raises. Either the script was not found and you have to check if it is correctly assigned to the game object and if control is the correct game object, or selected is null which should not happen if you initialized it like you posted...

Answer 2

Thanks for adding the complete Error Message :)
you need to replace
control.GetComponent<ForwardCommandScript>().selected.Add(this.transform);
with
control.GetComponent<ForwardCommandScript>().selected.Add(transform);
because
this
is a reference to the script and not the GameObject. you could also use gameObject.transform which transform is just an abbreviation for