# Unity CreateInstance can only be called from the main thread.

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Hi
New to Unity and c# I get:

CreateInstance can only be called from the main thread.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

How to rewrite and/or put into the main cs?
private BSplineMath bmath = ScriptableObject.CreateInstance("BSplineMath") as BSplineMath;

If I put that in the main class I get a reference error:

error CS0103: The name bmath' does not exist in the current context

No wonder, how to use an instance in another class. In c++ I would only

Edited by FGFS

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Not familiar with Unity, but I do know C#. Based on the text in the error message you posted you want:

private BSplineMath bmath;

in your class, and then in some kind of Awake or Start function (defined by Unity most likely?) put your initialization code:

bmath = ScriptableObject.CreateInstance("BSplineMath") as BSplineMath;

Basically, you're splitting up the definition of the variable and the initialization since (apparently) Unity doesn't like you calling CreateInstance when an instance of your class is constructed.

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You need to post some code. Show the code where you are initialising bmath and the code where you want to use it.

From the error message you've posted I suspect you have something like this:

// Spline.cs

public class Spline
{
private BSplineMath bmath = ScriptableObject.CreateInstance("BSplineMath") as BSplineMath;
}

// program.cs
public class Program
{
static void Main(string[] args)
{
bmath.Something();
}
}


If you want to use bmath in your Main (probably a bad idea, btw) you need to get a reference to it somehow.

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So, the "main thread" is a completely different thing than "main cs". Explaining what a thread is...unfortunately a pretty complex topic and is beyond the scope of this discussion.

The correct solution in this case looks like this (like what SmkViper said):

public class YourClass : MonoBehaviour
{
private BSplineMath bmath;

void Awake() // NOTE: Unity always calls Awake on its main thread.
{
bmath = ScriptableObject.CreateInstance("BSplineMath") as BSplineMath;
}
}
`
Edited by Nypyren

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Thanks, I didn't know about awake nor didn't have that at all. Now it compiles but at runtime I get:

NullReferenceException: Object reference not set to an instance of an object
BSplineSurface.Init () (at Assets/BSplineSurface.cs:64)

The last line below is 64. Many thanks again in advance.

public class BSplineSurface : ScriptableObject {

/*
* THESE VALUES MUST BE SET BEFORE CALCULATING
*/
//size of the control net (e.g. 3x4 net)
public int NI = 4; //setting these to high can crash unity
public int NJ = 4;
//Grid of control points
public Vector3[,] controlGrid;

//The degree in each direction
public int TI = 3;
public int TJ = 3;
//output GRID resolution (e.g. 30x40)
public int RESOLUTIONI = 150; //setting these to high can crash unity
public int RESOLUTIONJ = 150;
//the output Grid
public Vector3[,] outputGrid;
public BSplineMath bmath;
/*
* INTERNAL VALUES
*/
//internal knots in each direction
private int[] knotsI;
private int[] knotsJ;
//internal variables
private int i, j, ki, kj;
private float intervalI, incrementI, intervalJ, incrementJ, bi, bj;

//FUNCTIONS

void Awake() // NOTE: Unity always calls Awake on its main thread.
{
bmath = ScriptableObject.CreateInstance("BSplineMath") as BSplineMath;
}

//constructor

public BSplineSurface()
{
Init();
}

//MUST BE CALLED FIRST
public void Init () {

controlGrid = new Vector3[NI+1,NJ+1];
outputGrid = new Vector3[RESOLUTIONI, RESOLUTIONJ];

//init step size (the increment steps)
incrementI = (NI-TI+2) / ((float)RESOLUTIONI - 1);
incrementJ = (NJ-TJ+2) / ((float)RESOLUTIONJ - 1);

//Calculate KNOTS
knotsI = bmath.SplineKnots(NI, TI);

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You Init() method runs on constructions, and makes use of the "bmath" field. Since that field isn't populated until Awake() is called (which is necessarily after construction), you are getting a NullReferenceException.

Given the constraints that BSplineMath has about which thread it is called from, you'll probably need to defer calculating the spline knots until Awake() is called. At this point, your constructor / Init() method aren't really giving you a lot of value - it might make sense to put all your initialisation code in the Awake() method.

I'd also mention that some of your fields, in particular "i, j, ki, kj" look like they should probably be local variables.

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