How to use a FolderBrowserDialog from a WPF application

And here's my final version.

public static class MyWpfExtensions
{
    public static System.Windows.Forms.IWin32Window GetIWin32Window(this System.Windows.Media.Visual visual)
    {
        var source = System.Windows.PresentationSource.FromVisual(visual) as System.Windows.Interop.HwndSource;
        System.Windows.Forms.IWin32Window win = new OldWindow(source.Handle);
        return win;
    }

    private class OldWindow : System.Windows.Forms.IWin32Window
    {
        private readonly System.IntPtr _handle;
        public OldWindow(System.IntPtr handle)
        {
            _handle = handle;
        }

        #region IWin32Window Members
        System.IntPtr System.Windows.Forms.IWin32Window.Handle
        {
            get { return _handle; }
        }
        #endregion
    }
}

And to actually use it:

var dlg = new FolderBrowserDialog();
System.Windows.Forms.DialogResult result = dlg.ShowDialog(this.GetIWin32Window());

If you specify Owner, you will get a Modal dialog over the specified WPF window.

To get WinForms compatible Win32 window create a class implements IWin32Window like this

 public class OldWindow : System.Windows.Forms.IWin32Window
{
    IntPtr _handle;

    public OldWindow(IntPtr handle)
    {
        _handle = handle;
    }

    #region IWin32Window Members

    IntPtr System.Windows.Forms.IWin32Window.Handle
    {
        get { return _handle; }
    }

    #endregion
}

And use an instance of this class at your WinForms

        IntPtr mainWindowPtr = new WindowInteropHelper(this).Handle; // 'this' means WPF Window
        folderBrowserDialog.ShowDialog(new OldWindow(mainWindowPtr));

I realize this is an old question, but here is an approach which might be slightly more elegant (and may or may not have been available before)...

using System;
using System.Windows;
using System.Windows.Forms;

// ...

/// <summary>
///     Utilities for easier integration with WinForms.
/// </summary>
public static class WinFormsCompatibility {

    /// <summary>
    ///     Gets a handle of the given <paramref name="window"/> and wraps it into <see cref="IWin32Window"/>,
    ///     so it can be consumed by WinForms code, such as <see cref="FolderBrowserDialog"/>.
    /// </summary>
    /// <param name="window">
    ///     The WPF window whose handle to get.
    /// </param>
    /// <returns>
    ///     The handle of <paramref name="window"/> is returned as <see cref="IWin32Window.Handle"/>.
    /// </returns>
    public static IWin32Window GetIWin32Window(this Window window) {
        return new Win32Window(new System.Windows.Interop.WindowInteropHelper(window).Handle);
    }

    /// <summary>
    ///     Implementation detail of <see cref="GetIWin32Window"/>.
    /// </summary>
    class Win32Window : IWin32Window { // NOTE: This is System.Windows.Forms.IWin32Window, not System.Windows.Interop.IWin32Window!

        public Win32Window(IntPtr handle) {
            Handle = handle; // C# 6 "read-only" automatic property.
        }

        public IntPtr Handle { get; }

    }

}

Then, from your WPF window, you can simply...

public partial class MainWindow : Window {

    void Button_Click(object sender, RoutedEventArgs e) {
        using (var dialog = new FolderBrowserDialog()) {
            if (dialog.ShowDialog(this.GetIWin32Window()) == System.Windows.Forms.DialogResult.OK) {
                // Use dialog.SelectedPath.
            }
        }
    }

}

Actually, does it matter?

I'm not sure if it matters in this case, but generally, you should tell Windows what is your window hierarchy, so if a parent window is clicked while child window is modal, Windows can provide a visual (and possibly audible) clue to the user.

Also, it ensures the "right" window is on top when there are multiple modal windows (not that I'm advocating such UI design). I've seen UIs designed by a certain multi-billion dollar corporation (which shell remain unnamed), that hanged simply because one modal dialog got "stuck" underneath another, and user had no clue it was even there, let alone how to close it.