Does DA.GetData make a copy?

Replies to This Discussion

Permalink Reply by Benjamin Felbrich on May 18, 2017 at 1:18am

Thanks a lot Tom!

I see, that was my assumption. I was thinking about passing pointers as well, but since myClass is a reference type I can not use pointers in C# (apparently C# only supports pointers for value types and structs of such).

Your idea of making a sort of custom pointer is very good, I'll try that.

Thanks again! Cheers

Permalink Reply by David Rutten on May 18, 2017 at 1:40am

If you ask for the IGH_Goo type, then you always get the original data:

GH_Brep brep = null;

if (!DA.GetData(0, ref brep)) return;

No copying. If you ask for a derived type, then it depends on how the conversion is defined. For almost all (I'd say all, but I can't remember for sure) native types this is also done without copying. I.e.:

Brep brep = null;

if (!DA.GetData(0, ref brep)) return;

also gives you a reference to the actual brep that is inside the GH_Brep instance, again no copying.

When you use VB/C# script components, then you always get a safe instance you can change. I did this because I figured scripters would be less savvy than VS developers about making sure data is copied whenever necessary.

When making a GHA, and you want to modify the data in the input, you must always make a copy yourself. It's the only way to be sure.

Permalink Reply by Benjamin Felbrich on May 18, 2017 at 2:14am

Hello David,

thanks a lot for the reply. And for custom classes I'd use GH_ObjectWrapper??
I'm thinking of something like:

GH_ObjectWrapper obj= null;

if (!DA.GetData(0, ref obj)) return;

MyClass mc = (MyClass)obj.Value;

...to point me to the original instance?


Best
Ben

Edit:
"When making a GHA, and you want to modify the data in the input, you must always make a copy yourself. It's the only way to be sure."

I don't want to change the data of myClass downstream, I just want to retrieve information from it. Is that save?

Permalink Reply by David Rutten on May 18, 2017 at 2:20am

GH_ObjectWrapper is a last resort type which is used to encapsulate data for which no suitable IGH_Goo implementation can be found. GH_ObjectWrappers do not know how to do anything with the data they contain (read it, write it, copy it, transform it, ...) so it will definitely not copy the values.

The major downside of using GH_ObjectWrapper is that it will probably fail to be written to a gh file in case it is internalised in any parameter.

Permalink Reply by Benjamin Felbrich on May 18, 2017 at 2:38am

Okay so for custom classes my best bet would be Tom's suggestion to link and refer to my instance in the global document scope, correct?

Permalink Reply by Benjamin Felbrich on May 18, 2017 at 2:56am

Sorry I made a different mistake here:

My problem was that in the second component I initialized another instance of myClass before assigning it. Like this:

MyClass instance = new MyClass();

DA.GetData(0, ref instance);

..this instance created internal problems, since i shouldn't initialize two instances.
It works when I set it to null instead:

MyClass instance = null;
DA.GetData(0, ref instance);

That was wrong thinking from my side. Maybe someone will find this text useful some day.

Thanks
Ben