asp.net SHGetFileInfo[winapi call]

Question

Hello. I need tree file at my asp.net site. For getting icons I try use SHGetFileInfo api function. At non asp.net application it works well, returns corrects Icon. When I conusme it at asp.net context I got:

Attempted to read or write protected memory.

What's wrong? Can I get fiel icon at asp.net context?

Code:

 public class ExtractIcon
{
    [DllImport("Shell32.dll")]
    private static extern int SHGetFileInfo(
    string pszPath,
    uint dwFileAttributes,
    out SHFILEINFO psfi,
    uint cbfileInfo,
    uint uFlags
    );

    [StructLayout(LayoutKind.Sequential)]
    private struct SHFILEINFO
    {
        public SHFILEINFO(bool b)
        {
            hIcon = IntPtr.Zero;
            iIcon = 0;
            dwAttributes = 0;
            szDisplayName = String.Empty;
            szTypeName = String.Empty;
        }

        public IntPtr hIcon;
        public int iIcon;
        public uint dwAttributes;

        [MarshalAs(UnmanagedType.LPStr, SizeConst = 260)]
        public string szDisplayName;

        [MarshalAs(UnmanagedType.LPStr, SizeConst = 80)]
        public string szTypeName;
    };

    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;
    public const uint SHGFI_OVERLAYINDEX = 0x000000040;

    private static Icon GetIcon(string strPath, uint flags)
    {
        SHFILEINFO info = new SHFILEINFO(true);
        int cbFileInfo = Marshal.SizeOf(info);

        SHGetFileInfo(strPath, 256, out info, (uint)cbFileInfo, flags);
        return Icon.FromHandle(info.hIcon);
    }

    /// <summary>
    /// Get the associated Icon for a file or application, this method always returns
    /// an icon. If the strPath is invalid or there is no idonc the default icon is returned
    /// </summary>
    /// <param name="strPath">full path to the file</param>
    /// <param name="bSmall">if true, the 16x16 icon is returned otherwise the 32x32</param>
    /// <returns></returns>
    public static Icon GetIcon(string strPath, bool bSmall)
    {
        uint flags = SHGFI_ICON | SHGFI_USEFILEATTRIBUTES;
        flags |= bSmall ? SHGFI_SMALLICON : SHGFI_LARGEICON;
        return GetIcon(strPath, flags);
    }

    public static Icon GetIcon(string strPath, bool bSmall, bool bOpened)
    {
        uint flags = SHGFI_ICON | SHGFI_USEFILEATTRIBUTES;
        flags |= bSmall ? SHGFI_SMALLICON : SHGFI_LARGEICON;
        flags |= bOpened ? SHGFI_OPENICON : 0;
        return GetIcon(strPath, flags);
    }
} 

Answer 1

The P/Invoke signature you are using for SHGetFileInfo is wrong - use the one at PInvoke.net.

SHGetFileInfo returns a value that you need to check for success before you access the returned data. If the API call fails the results of accessing the Icon you expected are unpredictable (and usually bad).

Once you know what the error is, you can work on fixing that as a separate problem. You should never omit error checking code when P/Invoke-ing the Win32 APIs from managed code. Read the MSDN docs for what indicates success and what errors to expect.

In your case:

    SHGetFileInfo(strPath, 256, out info, (uint)cbFileInfo, flags);
    return Icon.FromHandle(info.hIcon);

should look like this

    IntPtr result = SHGetFileInfo(strPath, 256, out info, (uint)cbFileInfo, flags);
    if (result != IntPtr.Zero)
    {
      return Icon.FromHandle(info.hIcon);
    }
    else
    {
      // add error handling here
    }