Use WinRT's FileOpenPicker and FolderPicker from WPF.

DialogHelper.cs

using System;
using System.Threading.Tasks;
using System.Windows.Interop;
using Windows.Storage.Pickers;

public static async Task<string?> OpenFileDialog(System.Windows.Window window)
{
    var picker = new FileOpenPicker()
    {
        ViewMode = PickerViewMode.Thumbnail
    };
    picker.FileTypeFilter.Add(".jpg");
    picker.FileTypeFilter.Add(".jpeg");
    picker.FileTypeFilter.Add(".png");

    IntPtr hwnd = new WindowInteropHelper(window).Handle;
    WinRT.Interop.InitializeWithWindow.Initialize(picker, hwnd);

    var file = await picker.PickSingleFileAsync();
    return file?.Path;
}

public static async Task<string?> OpenFolderDialog(System.Windows.Window window)
{
    var picker = new FolderPicker();
    picker.FileTypeFilter.Add("*");

    IntPtr hwnd = new WindowInteropHelper(window).Handle;
    WinRT.Interop.InitializeWithWindow.Initialize(picker, hwnd);

    var folder = await picker.PickSingleFolderAsync();
    return folder?.Path;
}