Minimalist example for opening a modern UI folder browser dialog in Windows Forms.

FolderBrowserDialog.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace CommonDialogs
{
    class FolderBrowserDialog : CommonDialog
    {
        const uint FOS_PICKFOLDERS = 0x20;
        const uint FOS_FORCEFILESYSTEM = 0x40;
        const uint SIGDN_FILESYSPATH = 0x80058000;
        const uint S_OK = 0;

        public string SelectedPath { get; set; }

        public override void Reset()
        {
            SelectedPath = null;
        }

        protected override bool RunDialog(IntPtr hwndOwner)
        {
            uint options;
            var dialog = (IFileDialog)new FileOpenDialogRCW();
            dialog.GetOptions(out options);
            options |= FOS_PICKFOLDERS | FOS_FORCEFILESYSTEM;
            dialog.SetOptions(options);

            if (dialog.Show(hwndOwner) == S_OK)
            {
                IShellItem shellItem;
                if (dialog.GetResult(out shellItem) == S_OK)
                {
                    IntPtr pszName;
                    if (shellItem.GetDisplayName(SIGDN_FILESYSPATH, out pszName) == S_OK &&
                        pszName != IntPtr.Zero)
                    {
                        try
                        {
                            SelectedPath = Marshal.PtrToStringAuto(pszName);
                            return true;
                        }
                        finally { Marshal.FreeCoTaskMem(pszName); }
                    }
                }
            }

            return false;
        }

        #region Com Interop

        [ComImport]
        [ClassInterface(ClassInterfaceType.None)]
        [TypeLibType(TypeLibTypeFlags.FCanCreate)]
        [Guid("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7")]
        class FileOpenDialogRCW
        {
        }

        [ComImport]
        [Guid("42F85136-DB7E-439C-85F1-E4075D135FC8")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        interface IFileDialog
        {
            [PreserveSig]
            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint Show([In, Optional] IntPtr hwndOwner); //IModalWindow

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFileTypes([In] uint cFileTypes, [In, MarshalAs(UnmanagedType.LPArray)] IntPtr rgFilterSpec);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFileTypeIndex([In] uint iFileType);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetFileTypeIndex(out uint piFileType);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint Advise([In, MarshalAs(UnmanagedType.Interface)] IntPtr pfde, out uint pdwCookie);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint Unadvise([In] uint dwCookie);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetOptions([In] uint fos);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetOptions(out uint fos);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void SetDefaultFolder([In, MarshalAs(UnmanagedType.Interface)] IShellItem psi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFolder([In, MarshalAs(UnmanagedType.Interface)] IShellItem psi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetFolder([MarshalAs(UnmanagedType.Interface)] out IShellItem ppsi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetCurrentSelection([MarshalAs(UnmanagedType.Interface)] out IShellItem ppsi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFileName([In, MarshalAs(UnmanagedType.LPWStr)] string pszName);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetFileName([MarshalAs(UnmanagedType.LPWStr)] out string pszName);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetTitle([In, MarshalAs(UnmanagedType.LPWStr)] string pszTitle);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetOkButtonLabel([In, MarshalAs(UnmanagedType.LPWStr)] string pszText);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFileNameLabel([In, MarshalAs(UnmanagedType.LPWStr)] string pszLabel);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetResult([MarshalAs(UnmanagedType.Interface)] out IShellItem ppsi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint AddPlace([In, MarshalAs(UnmanagedType.Interface)] IShellItem psi, uint fdap);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetDefaultExtension([In, MarshalAs(UnmanagedType.LPWStr)] string pszDefaultExtension);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint Close([MarshalAs(UnmanagedType.Error)] uint hr);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetClientGuid([In] ref Guid guid);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint ClearClientData();

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint SetFilter([MarshalAs(UnmanagedType.Interface)] IntPtr pFilter);
        }

        [ComImport]
        [Guid("43826D1E-E718-42EE-BC55-A1E261C37BFE")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        interface IShellItem
        {
            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint BindToHandler([In] IntPtr pbc, [In] ref Guid rbhid, [In] ref Guid riid, [Out, MarshalAs(UnmanagedType.Interface)] out IntPtr ppvOut);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetParent([MarshalAs(UnmanagedType.Interface)] out IShellItem ppsi);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetDisplayName([In] uint sigdnName, out IntPtr ppszName);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint GetAttributes([In] uint sfgaoMask, out uint psfgaoAttribs);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            uint Compare([In, MarshalAs(UnmanagedType.Interface)] IShellItem psi, [In] uint hint, out int piOrder);
        }

        #endregion
    }
}