GridRegionAdapter.cs

using Avalonia.Controls;
using Prism.Navigation.Regions;

namespace SampleApp.Main.Core.RegionAdapters
{
  public class GridRegionAdapter : RegionAdapterBase<Grid>
  {
    public GridRegionAdapter(IRegionBehaviorFactory regionBehaviorFactory)
      : base(regionBehaviorFactory)
    {
    }

    protected override void Adapt(IRegion region, Grid regionTarget)
    {
      region.Views.CollectionChanged += (sender, e) =>
      {
        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
        {
          foreach (Control item in e.NewItems)
          {
            regionTarget.Children.Add(item);
          }
        }

        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Remove)
        {
          foreach (Control item in e.OldItems)
          {
            regionTarget.Children.Remove(item);
          }
        }
      };
    }

    protected override IRegion CreateRegion() => new SingleActiveRegion() { };
  }
}