From the documentation:
Unlike classes, structs can be instantiated without using a new operator.
So why am I getting this error:
Use of unassigned local variable 'x'
When I try to do this?
Vec2 x;
x.X = det * (a22 * b.X - a12 * b.Y);
x.Y = det * (a11 * b.Y - a21 * b.X);
Where Vec2 x is a struct?
Well, are X and Y properties (rather than fields)? If so, that's the problem. Until all the fields within x are definitely assigned, you can't call any methods or properties.
For instance:
public struct Foo
{
public int x;
public int X { get { return x; } set { x = value; } }
}
class Program
{
static void Main(string[] args)
{
Foo a;
a.x = 10; // Valid
Foo b;
b.X = 10; // Invalid
}
}
Is Vec2 your own type? Do you have access to the fields involved, or only the properties?
If it's your own type, I would strongly urge you to try to stick to immutable structs. I know managed DirectX has some mutable structs for getting as close to optimal performance as possible, but that's at the cost of strange situations like this - and much worse, to be honest.
I would personally give the struct a constructor taking X and Y:
Vec2 x = new Vec2(det * (a22 * b.X - a12 * b.Y),
det * (a11 * b.Y - a21 * b.X));
It is still uninitialized. You need to initialize it before using it. You can use default operator for that if you don't want to create a static Vec.Empty value and happy with the defaults for the structs members:
Vec2 x = default(Vec2);
Mitch Wheat:
This, however doesn't:
public struct Vec2
{
int x;
int y;
public float X { get { return x; } set { x = value; } }
public float Y { get { return y; } set { y = value; } }
}
static void Main(string[] args)
{
Vec2 x;
x.X = 1;
x.Y = 2;
}
The compiler prevents you from calling propertis on types before all of it's members have been initialized, even though a property might just set one of the values. The solution, as Jon Skeet proposed, is to have an initializing constructor and preferably no setters.
