StoragePicker.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.UI.Xaml;
using System.Runtime.InteropServices;
using WinRT;
using Windows.Storage.Pickers;

namespace ImageOverlay
{




        public static class WindowsStoragePickersExtensions
        {
            [ComImport]
            [Guid("3E68D4BD-7135-4D10-8018-9FB6D9F33FA1")]
            [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
            public interface IInitializeWithWindow
            {
                void Initialize(IntPtr hwnd);
            }
            [ComImport]
            [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
            [Guid("EECDBF0E-BAE9-4CB6-A68E-9598E1CB57BB")]
            internal interface IWindowNative
            {
                IntPtr WindowHandle { get; }
            }

            /// <summary>
            /// Sets the owner window for this <see cref="FileOpenPicker"/>. This is required when running in WinUI for Desktop.
            /// </summary>
            public static void SetOwnerWindow(this FileOpenPicker picker, Window window)
            {
                SetOwnerWindow(picker.As<IInitializeWithWindow>(), window);
            }

            /// <summary>
            /// Sets the owner window for this <see cref="FileSavePicker"/>. This is required when running in WinUI for Desktop.
            /// </summary>
            public static void SetOwnerWindow(this FileSavePicker picker, Window window)
            {
                SetOwnerWindow(picker.As<IInitializeWithWindow>(), window);
            }

            /// <summary>
            /// Sets the owner window for this <see cref="FolderPicker"/>. This is required when running in WinUI for Desktop.
            /// </summary>
            public static void SetOwnerWindow(this FolderPicker picker, Window window)
            {
                SetOwnerWindow(picker.As<IInitializeWithWindow>(), window);
            }

            private static void SetOwnerWindow(IInitializeWithWindow picker, Window window)
            {
                // See https://github.com/microsoft/microsoft-ui-xaml/issues/4100#issuecomment-774346918
                picker.Initialize(window.As<IWindowNative>().WindowHandle);
            }
        }
    
}