How to I create a UserControl that accepts inner XAML controls?

Question

Similar to the Stackpanel, where I can have controls such as textboxes and buttons as inner-XAML:

<StackPanel>
    <Button Content="OK">
    <Button Content="Cancel">
    <Button Content="Apply">
</StackPanel>

What I need is:

<myControls:myOptionsList>
    <Button Content="OK">
    <Button Content="Cancel">
    <Button Content="Apply">
</myControls:myOptionsList>

Answer 1

Assuming your UserControl has an inner Panel element where the additional elements should be placed, i.e. like this:

<UserControl x:Class="YourNamespace.OptionsControl" ...>
    <Grid>
        <GroupBox Header="Options">
            <StackPanel x:Name="optionsPanel"/>
        </GroupBox>   
    </Grid>
</UserControl>

you may simply expose that Panel's Children property as a UIElementCollection property that is marked to be the ContentProperty of the UserControl:

[ContentProperty(nameof(Options))]
public partial class OptionsControl : UserControl
{
    public UIElementCollection Options
    {
        get { return optionsPanel.Children; }
    }

    ...
}