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layout: stdlib-reference
---

# attribute [open]

> #### Experimental Feature
> The feature described in this page is marked as experimental, and may be subject to change in future releases.
> Users are advised that any code that depend on this feature may not be compilable by future versions of the compiler.

## Description

Mark a type as open, allowing it from being inherited from or implemented by types defined in other modules.
This is the default behavior.

## Signature

<pre>
[<a href="open.html">open</a>]
</pre>

## See also

<span class='code'>[<a href="sealed.html">sealed</a>]</span>.