How do I determine the Windows 'Download folder' path?

Windows does not define a CSIDL for the Downloads folder and it is not available through the Environment.SpecialFolder enumeration.

However, the new Vista Known Folder API does define it with the ID of FOLDERID_Downloads. Probably the easiest way to obtain the actual value is to P/invoke SHGetKnownFolderPath.

public static class KnownFolder
{
    public static readonly Guid Downloads = new Guid("374DE290-123F-4565-9164-39C4925E467B");
}

[DllImport("shell32.dll", CharSet=CharSet.Unicode)]
static extern int SHGetKnownFolderPath([MarshalAs(UnmanagedType.LPStruct)] Guid rfid, uint dwFlags, IntPtr hToken, out string pszPath);

static void Main(string[] args)
{
    string downloads;
    SHGetKnownFolderPath(KnownFolder.Downloads, 0, IntPtr.Zero, out downloads);
    Console.WriteLine(downloads);
}

Note that the P/invoke given on pinvoke.net is incorrect since it fails to use Unicode character set. Also I have taken advantage of the fact that this API returns memory allocated by the COM allocator. The default marshalling of the P/invoke above is to free the returned memory with CoTaskMemFree which is perfect for our needs.

Be careful that this is a Vista and up API and do not attempt to call it on XP/2003 or lower.

You can use the Windows API Code Pack for Microsoft .NET Framework.

Reference: Microsoft.WindowsAPICodePack.Shell.dll

Need the following namespace:

using Microsoft.WindowsAPICodePack.Shell;

Simple usage:

string downloadsPath = KnownFolders.Downloads.Path;