Dear forum,
I have a problem that might be very trivial to some of you. I am about writing a Grasshopper component and in the same fashion I would declare a Plane or a Line (like so)
var testLine = new Line();
DA.GetData(0, ref testLine);
I am now trying to declare a Curve. That is however not working, because
‘Curve’ does not contain a constructor that takes 0 arguments
However looking at the RhinoCommon API Line doesn’t either. Can someome please explain to me why a Line can be declared like that and a Curve not, and also tell me how a Curve should be declared properly? That would be very helpful, thanks!
This is used to retrieve Data, when you have a compiled component. So you normally feed the curve into the curve input.
Otherwise just create a line or list or whatever collection of lines and add it to.
Or do I completly miss the point of what you are trying to do?
The solution provided will still cause an “object reference not set to an instance of an object” error while executing the script. I’m not sure what is the formal way to initialize a curve, but this one seems generic to me.
You don’t need to initialize any geometry, when you assign an instance over the DA object. The accepted solution is correct, but misleading and redundant. Curve crv; (declaration) is sufficient.Edit: The accepted solution is 100% correct
Note that not all curves-alike objects are Curve class instances. A curve is a base class to most, but not all curve objects. What you describe is a null reference error, you always need to check if after an assignment, the instance is not null. Input can be null in GH
And regarding initialization. In general a constructor is sometimes not sufficient to initialize a class correctly. In programming the solution to a complex or manifold initialization is the factory pattern. The lightweight solution are factory methods, static „CreateXY“ methods which offer different ways of initialization. Note that inheritance makes it even harder to understand, because you can init a Curve by calling a NurbsCurve factory method.
Oh sorry, I should have tested. I mixed the ‘out’ with the ‘ref’ keyword. For the out keyword declaration is sufficient. But again, the error you received is a null error. This can be anywhere. You need to carefully check where it throws. I think with the debugger you should see. I think GetData also assigns a null, it will only fail if it cannot handle the input correctly, e.g. if you pass in a float instead.
C# compiler requires local variable assignment before use, and it can not garantee that GetData is going to assign the variable. Its a way to force predictability and avoid unexpected behaviour.
