C#: Passing address of public variable

Question

When i try to pass the address of a public variable like this:

ML.Register("Radius", &lBeacons[i].Radius, 0.0f, 200.0f, 10.0f);

I get this error:

error CS0212: You can only take the address of an unfixed expression inside of a fixed statement initializer

The Register function looks like this:

public unsafe void Register(string desc, float* val, float minimum, float maximum, float stepsize)

Beacons is a List. It holds a class with the Public Radius.

Answer 1

Why use pointers? You can just use the "ref" keyword to pass a reference of the variable.

example:

object blah = new object();

CallMethod(ref blah);


public void CallMethod(ref object param)
{

}

Answer 2

Would it not make sense just to pass the value by reference?

public void Register(string desc, ref float val, float minimum,
         float maximum, float stepsize) {...}

Of course, using public variables (fields) is a bad idea too... it would work like so:

ML.Register("Radius", ref lBeacons[i].Radius, 0.0f, 200.0f, 10.0f);

But it won't work if you make Radius a property - so don't do this. Consider passing the beacon (or similar) itself, or some other object-based (or maybe event-based) mechanism.

---

something like:

ML.Register("Radius", lBeacons[i], 0.0f, 200.0f, 10.0f);

with:

private Beacon beacon;
public void Register(string desc, Beacon beacon, float minimum,
         float maximum, float stepsize) {
    this.beacon = beacon;
}
void Foo() {
    beacon.Radius++; // etc
}

Here we have a reference to the Beacon object, which doesn't have the unsafe issues of pointers. If you don't want to expose the Beacon directly, consider using an interface.

Answer 3

You can try using a fixed block.

fixed(float *rad = &(lBeacons[i].Radius))) {
    ML.Register("Radius", rad, 0f, 200f, 10f);
}

I don't think you want to do this in any case. If ML uses rad outside of the fixed block, it is likely to fail. I think you're better off rethinking your API.

Answer 4

To get a pointer to a variable, use the fixed keyword to pin it in place, like:

fixed(float *radius=&lBeacons[i].Radius)
{
  ML.Register("Radius", radius, 0.0f, 200.0f, 10.0f);
}

The method this gets placed in needs to be declared as unsafe.