Why can I not make a boundary surface with these two curves?

I was trying to make a boundary surface of two closed curves, where one of them surrounds the other one. The surface should only be the space between them.
I have to done this in other occasions and it always worked fine, but not for therse two curves, maybe because I used both polylines and control point curves when creating the curves?

I am sure there is an easy solution to my problem but I can’t figure this out myself. So I am really hoping for your input.

Thank you
2021_11_27_QuestionBoundarySurface.3dm (31.2 KB)
2021_11_27_QuestionBoundarySurface.gh (4.8 KB)

Right click your ‘Edges’ input and choose ‘Flatten’

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Thank you so much.
I had the feeling the solution might be super simple. I just couldn’t get to it. thanks!

Feel like to explain a bit, since if you don’t understand why it didn’t work, you probably will run into similar situations and won’t be able so solute.
The boundary surface component creates a surface for each branch.
Your both curves lied on different branches,this happens since the offset component adds a 0 index at the beginning of the branch.

So the boundary surface component created one surface for each branch:

To get them on the same branch you will need to flatten the Offset output (or everyting, like @DanielPiker did) or use surify(since simplify wont get rid of the zero here).

If you are planning to work more with gh I would recommend getting more knowledge about the idea of datatrees. There are many resources, I can recommend this one from Andrew Heumann:

Hope it helped!
Happy grasshoppering and a nice weekend everyone!

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Thanks a lot for this explanation! This really helped to understand the problem. And thanks for the link. I will go and learn. Thank you