A dialog the can select either a folder or a file. Only one selection is possible.

DirectoryDialog.cs

using System;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

public class DirectoryDialog
{
    /// <summary>
    /// The BROWSEINFO structure used by SHBrowseForFolder.
    /// See http://msdn.microsoft.com/en-us/library/windows/desktop/bb773205%28v=vs.85%29.aspx
    /// </summary>
    public struct BROWSEINFO
    {
        public IntPtr hWndOwner;
        public int pIDLRoot;
        public string pszDisplayName;
        public string lpszTitle;
        public int ulFlags;
        public int lpfnCallback;
        public int lParam;
        public int iImage;
    }
    
    /// <summary>
    /// What to show in the directory dialog
    /// See 'ulFlags' at http://msdn.microsoft.com/en-us/library/windows/desktop/bb773205%28v=vs.85%29.aspx
    /// </summary>
    public enum BrowseForTypes : int
    {
        Directories = 1, // BIF_RETURNONLYFSDIRS (0x00000001)
        FileSystemAncestors = 8, // BIF_RETURNFSANCESTORS (0x00000008)
        Computers = 4096, // BIF_BROWSEFORCOMPUTER (0x00001000)
        FilesAndDirectories = 16384 // BIF_BROWSEINCLUDEFILES (0x00004000)
    }

    /// <summary>
    /// The maximum path length.
    /// A value of 260 matches the value used in the Windows API.
    /// </summary>
    const int MAX_PATH = 260;

    private BrowseForTypes browseFor = BrowseForTypes.Directories;
    private string title = "";
    private string selected = "";

    /// <summary>
    /// The title of the dialog
    /// </summary>
    public string Title
    {
        get
        {
            return title;
        }
        set
        {
            if (object.ReferenceEquals(value, DBNull.Value))
            {
                throw new ArgumentNullException();
            }
            title = value;
        }
    }

    /// <summary>
    /// The selected item
    /// </summary>
    public string Selected
    {
        get
        {
            return selected;
        }
    }

    /// <summary>
    /// What the dialog is browsing for
    /// </summary>
    public BrowseForTypes BrowseFor
    {
        get
        {
            return browseFor;
        }
        set
        {
            browseFor = value;
        }
    }

    /// <summary>
    /// DirectoryDialog constructor
    /// </summary>
    public DirectoryDialog()
    {
    }

    [DllImport("ole32", EntryPoint = "CoTaskMemFree", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern int CoTaskMemFree(IntPtr hMem);
    [DllImport("kernel32", EntryPoint = "lstrcat", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern IntPtr lstrcat(string lpString1, string lpString2);
    [DllImport("shell32", EntryPoint = "SHBrowseForFolder", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern IntPtr SHBrowseForFolder(ref BROWSEINFO lpbi);
    [DllImport("shell32", EntryPoint = "SHGetPathFromIDList", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    public static extern int SHGetPathFromIDList(IntPtr pidList, StringBuilder lpBuffer);
    /// <summary>
    /// Show the directory dialog
    /// The PInvoke for SHBrowseForFolder and friends is required for this
    /// </summary>
    /// <param name="hMem"></param>
    /// <returns></returns>
    protected bool RunDialog(IntPtr hWndOwner)
    {
        BROWSEINFO udtBI = new BROWSEINFO();

        IntPtr lpIDList = default(IntPtr);
        GCHandle hTitle = GCHandle.Alloc(Title, GCHandleType.Pinned);
        udtBI.hWndOwner = hWndOwner;
        udtBI.lpszTitle = Title;
        udtBI.ulFlags = (int)BrowseFor;

        StringBuilder buffer = new StringBuilder(MAX_PATH);
        buffer.Length = MAX_PATH;
        udtBI.pszDisplayName = buffer.ToString();

        lpIDList = SHBrowseForFolder(ref udtBI);
        hTitle.Free();

        if (lpIDList.ToInt64() == 0)
        {
            return false;
        }

        if (BrowseFor == BrowseForTypes.Computers)
        {
            selected = udtBI.pszDisplayName.Trim();
        }
        else
        {
            StringBuilder path = new StringBuilder(MAX_PATH);
            SHGetPathFromIDList(lpIDList, path);
            selected = path.ToString();
        }
        CoTaskMemFree(lpIDList);

        return true;
    }

    /// <summary>
    /// Show the directory dialog box
    /// </summary>
    /// <returns>A DialogResult enum</returns>
    public DialogResult ShowDialog()
    {
        return ShowDialog(null);
    }

    /// <summary>
    /// Show the directory dialog box
    /// </summary>
    /// <param name="owner">The owner window handler</param>
    /// <returns>A DialogResult enum</returns>
    public DialogResult ShowDialog(IWin32Window owner)
    {
        IntPtr handle = default(IntPtr);
        if ((owner != null))
        {
            handle = owner.Handle;
        }
        else
        {
            handle = IntPtr.Zero;
        }

        if (RunDialog(handle))
        {
            return DialogResult.OK;
        }
        else
        {
            return DialogResult.Cancel;
        }
    }
}