What do you call a UI element which expands an area of fields in a window?

Question

I am trying to make UI element which expands/collapses an area of fields in a window. I need it specifically for a WPF project. As an example take the element in the link below.

https://i.sstatic.net/MDgNl.png

Pushing the down button expands the area to include more fields as in the example below

https://i.sstatic.net/jN6bO.png

Does this kind of UI element have a name? I don't really know what to google to see whether it has been done before.

Answer 1

I think you are looking for the Expander control:

<Expander>
    <Grid>
        <TextBlock>put your controls to be expanded/collapsed here...</TextBlock>
    </Grid>
</Expander>