One of web projects I'm working on needed a web file explorer. There was one specific requirement for this explorer, files should be displayed with icons they have on server. I decided that the best way will be dedicated HttpHandler.

So first I needed to P/Invoke SHGetFileInfo and DestroyIcon functions:

/// <summary>
/// Retrieves information about an object in the file system, such as a file, folder, directory, or drive root
/// </summary>
/// <param name="pszPath">Contains the path and file name. Both absolute and relative paths are valid</param>
/// <param name="dwFileAttributes">A combination of one or more file attribute flags</param>
/// <param name="psfi">SHFILEINFO structure to receive the file information</param>
/// <param name="cbSizeFileInfo">The size, in bytes, of the SHFILEINFO structure</param>
/// <param name="uFlags">The flags that specify the file information to retrieve</param>
/// <returns>Meaning depends on the uFlags parameter</returns>
[DllImport("shell32.dll")]
static extern IntPtr SHGetFileInfo(string pszPath, uint dwFileAttributes, ref SHFileInfo psfi, uint cbSizeFileInfo, uint uFlags);
/// <summary>
/// Destroys an icon and frees any memory the icon occupied
/// </summary>
/// <param name="handle">Handle to the icon to be destroyed</param>
/// <returns>If the function succeeds, the return value is nonzero</returns>
[DllImport("user32.dll", CharSet = CharSet.Auto)]
extern static bool DestroyIcon(IntPtr handle);

With those two functions getting icon for file looks like this:

Icon tempIcon;
SHFileInfo tempFileInfo = new SHFileInfo();
//Getting SHFileInfo for file (based on extension)
SHGetFileInfo(extension, 0, ref tempFileInfo, (uint)Marshal.SizeOf(tempFileInfo), SHGFI_ICON | SHGFI_USEFILEATTRIBUTES | (uint)iconSize);
//Creating Icon based on SHFileInfo
tempIcon = (Icon)Icon.FromHandle(tempFileInfo.hIcon);
//Making managed clone
Icon managedIcon = (Icon)tempIcon.Clone();
//Destroying unmanaged handle
DestroyIcon(tempIcon.Handle);
return managedIcon;

There is only one difference when getting icon for folder:

//Getting SHFileInfo for folder
SHGetFileInfo(null,
FILE_ATTRIBUTE_DIRECTORY, ref tempFileInfo, (uint)Marshal.SizeOf(tempFileInfo), SHGFI_ICON | SHGFI_USEFILEATTRIBUTES | (uint)iconSize | (uint)folderType );