Question

In a .NET 4.0 application (WPF) we’re using SHGetFileInfo to obtain shell icons for a directory tree. Since this takes quite some time in some cases (i.e. for a network drive that is unreachable or for a floppy drive), we wanted to do this in a thread and then update the icon when it has been read in.

The call is basically the same, it is now just executed within a thread. Because someone said that the thread must be STA for this to work, we used Thread rather than ThreadPool for testing, with the same results. Using ThreadPool also did not work.

SHGetFileInfo succeeds (returns 1), but the hIcon member in the structure is zero.

IntPtr GetIcon(string name)
{
    Shell32.SHFILEINFO shfi = new Shell32.SHFILEINFO();
    uint flags = Shell32.SHGFI_ICON | Shell32.SHGFI_USEFILEATTRIBUTES | Shell32.SHGFI_SMALLICON;

    Shell32.SHGetFileInfo(
        name, System.IO.Directory.Exists(name) ? Shell32.FILE_ATTRIBUTE_DIRECTORY : Shell32.FILE_ATTRIBUTE_NORMAL,
        ref shfi, 
        (uint) System.Runtime.InteropServices.Marshal.SizeOf(shfi), 
        flags );
    return shfi.hIcon;
}

The very same code works fine from the GUI thread. What has to be done to make the function work from a separate thread, or, however, make it work without blocking the GUI thread?

---

Update: The code around this is basically this:

var thread = new System.Threading.Thread(() => {
    var result = GetIcon("C:\\");
    // ... do something with the result
});
thread.SetApartmentState(System.Threading.ApartmentState.STA);
thread.Start();

if only the lines within the thread delegate are left in, it works fine (but on the GUI thread, of course).

---

Update: For now, we just Invoke the call to SHGetFileInfo to make it work. This has the advantage that the original problem (the page with the file view was not displayed until all the icons have been loaded) has been solved, though it means that the page hangs for each icon. But at least the user now sees that something is going on. We’re still looking for an actual solution to the problem.

Answer 1

I don’t think there’s any problem. You don’t need to use SetApartmentState. According to the documentation you do need to have called CoInitialize or OleInitialize, but I think this should have been called for you anyway by WPF.

I created a simple WPF application below. This works fine. SHGetFileInfo runs on a different thread to the UI thread and shfi.hIcon is not zero.

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        Debug.WriteLine(Thread.CurrentThread.ManagedThreadId);
        Task<IntPtr> task = Task.Factory.StartNew(() => GetIcon("C:\\"));
    }

    private IntPtr GetIcon(string name)
    {
        Debug.WriteLine(Thread.CurrentThread.ManagedThreadId);

        var shfi = new Shell32.SHFILEINFO();
        uint flags = Shell32.SHGFI_ICON | Shell32.SHGFI_USEFILEATTRIBUTES | Shell32.SHGFI_SMALLICON;

        Shell32.SHGetFileInfo(
            name,
            Directory.Exists(name) ? Shell32.FILE_ATTRIBUTE_DIRECTORY : Shell32.FILE_ATTRIBUTE_NORMAL,
            ref shfi,
            (uint) Marshal.SizeOf(shfi),
            flags);

        Debug.WriteLine(shfi.hIcon);

        return shfi.hIcon;
    }
}


public class Shell32
{
    public const int MAX_PATH = 256;

    // Browsing for directory.
    public const uint BIF_RETURNONLYFSDIRS = 0x0001;
    public const uint BIF_DONTGOBELOWDOMAIN = 0x0002;
    public const uint BIF_STATUSTEXT = 0x0004;
    public const uint BIF_RETURNFSANCESTORS = 0x0008;
    public const uint BIF_EDITBOX = 0x0010;
    public const uint BIF_VALIDATE = 0x0020;
    public const uint BIF_NEWDIALOGSTYLE = 0x0040;
    public const uint BIF_USENEWUI = (BIF_NEWDIALOGSTYLE | BIF_EDITBOX);
    public const uint BIF_BROWSEINCLUDEURLS = 0x0080;
    public const uint BIF_BROWSEFORCOMPUTER = 0x1000;
    public const uint BIF_BROWSEFORPRINTER = 0x2000;
    public const uint BIF_BROWSEINCLUDEFILES = 0x4000;
    public const uint BIF_SHAREABLE = 0x8000;

    public const uint SHGFI_ICON = 0x000000100; // get icon
    public const uint SHGFI_DISPLAYNAME = 0x000000200; // get display name
    public const uint SHGFI_TYPENAME = 0x000000400; // get type name
    public const uint SHGFI_ATTRIBUTES = 0x000000800; // get attributes
    public const uint SHGFI_ICONLOCATION = 0x000001000; // get icon location
    public const uint SHGFI_EXETYPE = 0x000002000; // return exe type
    public const uint SHGFI_SYSICONINDEX = 0x000004000; // get system icon index
    public const uint SHGFI_LINKOVERLAY = 0x000008000; // put a link overlay on icon
    public const uint SHGFI_SELECTED = 0x000010000; // show icon in selected state
    public const uint SHGFI_ATTR_SPECIFIED = 0x000020000; // get only specified attributes
    public const uint SHGFI_LARGEICON = 0x000000000; // get large icon
    public const uint SHGFI_SMALLICON = 0x000000001; // get small icon
    public const uint SHGFI_OPENICON = 0x000000002; // get open icon
    public const uint SHGFI_SHELLICONSIZE = 0x000000004; // get shell size icon
    public const uint SHGFI_PIDL = 0x000000008; // pszPath is a pidl
    public const uint SHGFI_USEFILEATTRIBUTES = 0x000000010; // use passed dwFileAttribute
    public const uint SHGFI_ADDOVERLAYS = 0x000000020; // apply the appropriate overlays
    public const uint SHGFI_OVERLAYINDEX = 0x000000040; // Get the index of the overlay

    public const uint FILE_ATTRIBUTE_DIRECTORY = 0x00000010;
    public const uint FILE_ATTRIBUTE_NORMAL = 0x00000080;

    [DllImport("Shell32.dll")]
    public static extern IntPtr SHGetFileInfo(
        string pszPath,
        uint dwFileAttributes,
        ref SHFILEINFO psfi,
        uint cbFileInfo,
        uint uFlags
        );

    #region Nested type: BROWSEINFO

    [StructLayout(LayoutKind.Sequential)]
    public struct BROWSEINFO
    {
        public IntPtr hwndOwner;
        public IntPtr pidlRoot;
        public IntPtr pszDisplayName;
        [MarshalAs(UnmanagedType.LPTStr)] public string lpszTitle;
        public uint ulFlags;
        public IntPtr lpfn;
        public int lParam;
        public IntPtr iImage;
    }

    #endregion

    #region Nested type: ITEMIDLIST

    [StructLayout(LayoutKind.Sequential)]
    public struct ITEMIDLIST
    {
        public SHITEMID mkid;
    }

    #endregion

    #region Nested type: SHFILEINFO

    [StructLayout(LayoutKind.Sequential)]
    public struct SHFILEINFO
    {
        public const int NAMESIZE = 80;
        public IntPtr hIcon;
        public int iIcon;
        public uint dwAttributes;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_PATH)] public string szDisplayName;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = NAMESIZE)] public string szTypeName;
    };

    #endregion

    #region Nested type: SHITEMID

    [StructLayout(LayoutKind.Sequential)]
    public struct SHITEMID
    {
        public ushort cb;
        [MarshalAs(UnmanagedType.LPArray)] public byte[] abID;
    }

    #endregion
}

/// <summary>
/// Wraps necessary functions imported from User32.dll. Code courtesy of MSDN Cold Rooster Consulting example.
/// </summary>
public class User32
{
    /// <summary>
    /// Provides access to function required to delete handle. This method is used internally
    /// and is not required to be called separately.
    /// </summary>
    /// <param name="hIcon">Pointer to icon handle.</param>
    /// <returns>N/A</returns>
    [DllImport("User32.dll")]
    public static extern int DestroyIcon(IntPtr hIcon);
}