WinAPI 方法SHGetKnownFolderPath 是检索特殊文件夹（包括个人文件夹和下载文件夹）路径的唯一正确方法。
还有其他方法可以获得类似的结果，看起来很有希望，但最终在特定系统上会出现完全错误的路径（例如，组合或硬编码路径的部分或滥用旧的注册表项）。其背后的原因在my CodeProject article 中说明，其中还列出了完整的解决方案。它提供了一个包装类，支持检索所有已知的 94 个特殊文件夹，以及更多好东西。
这里有一个简单的例子，我只是粘贴了一个缩短版本的解决方案，只能检索个人特殊文件夹，例如下载：
using System;
using System.Runtime.InteropServices;
/// <summary>
/// Class containing methods to retrieve specific file system paths.
/// </summary>
public static class KnownFolders
{
private static string[] _knownFolderGuids = new string[]
{
"{56784854-C6CB-462B-8169-88E350ACB882}", // Contacts
"{B4BFCC3A-DB2C-424C-B029-7FE99A87C641}", // Desktop
"{FDD39AD0-238F-46AF-ADB4-6C85480369C7}", // Documents
"{374DE290-123F-4565-9164-39C4925E467B}", // Downloads
"{1777F761-68AD-4D8A-87BD-30B759FA33DD}", // Favorites
"{BFB9D5E0-C6A9-404C-B2B2-AE6DB6AF4968}", // Links
"{4BD8D571-6D19-48D3-BE97-422220080E43}", // Music
"{33E28130-4E1E-4676-835A-98395C3BC3BB}", // Pictures
"{4C5C32FF-BB9D-43B0-B5B4-2D72E54EAAA4}", // SavedGames
"{7D1D3A04-DEBB-4115-95CF-2F29DA2920DA}", // SavedSearches
"{18989B1D-99B5-455B-841C-AB7C74E4DDFC}", // Videos
};
/// <summary>
/// Gets the current path to the specified known folder as currently configured. This does
/// not require the folder to be existent.
/// </summary>
/// <param name="knownFolder">The known folder which current path will be returned.</param>
/// <returns>The default path of the known folder.</returns>
/// <exception cref="System.Runtime.InteropServices.ExternalException">Thrown if the path
/// could not be retrieved.</exception>
public static string GetPath(KnownFolder knownFolder)
{
return GetPath(knownFolder, false);
}
/// <summary>
/// Gets the current path to the specified known folder as currently configured. This does
/// not require the folder to be existent.
/// </summary>
/// <param name="knownFolder">The known folder which current path will be returned.</param>
/// <param name="defaultUser">Specifies if the paths of the default user (user profile
/// template) will be used. This requires administrative rights.</param>
/// <returns>The default path of the known folder.</returns>
/// <exception cref="System.Runtime.InteropServices.ExternalException">Thrown if the path
/// could not be retrieved.</exception>
public static string GetPath(KnownFolder knownFolder, bool defaultUser)
{
return GetPath(knownFolder, KnownFolderFlags.DontVerify, defaultUser);
}
private static string GetPath(KnownFolder knownFolder, KnownFolderFlags flags,
bool defaultUser)
{
int result = SHGetKnownFolderPath(new Guid(_knownFolderGuids[(int)knownFolder]),
(uint)flags, new IntPtr(defaultUser ? -1 : 0), out IntPtr outPath);
if (result >= 0)
{
string path = Marshal.PtrToStringUni(outPath);
Marshal.FreeCoTaskMem(outPath);
return path;
}
else
{
throw new ExternalException("Unable to retrieve the known folder path. It may not "
+ "be available on this system.", result);
}
}
[DllImport("Shell32.dll")]
private static extern int SHGetKnownFolderPath(
[MarshalAs(UnmanagedType.LPStruct)]Guid rfid, uint dwFlags, IntPtr hToken,
out IntPtr ppszPath);
[Flags]
private enum KnownFolderFlags : uint
{
SimpleIDList = 0x00000100,
NotParentRelative = 0x00000200,
DefaultPath = 0x00000400,
Init = 0x00000800,
NoAlias = 0x00001000,
DontUnexpand = 0x00002000,
DontVerify = 0x00004000,
Create = 0x00008000,
NoAppcontainerRedirection = 0x00010000,
AliasOnly = 0x80000000
}
}
/// <summary>
/// Standard folders registered with the system. These folders are installed with Windows Vista
/// and later operating systems, and a computer will have only folders appropriate to it
/// installed.
/// </summary>
public enum KnownFolder
{
Contacts,
Desktop,
Documents,
Downloads,
Favorites,
Links,
Music,
Pictures,
SavedGames,
SavedSearches,
Videos
}
（完整注释版本可在上面链接的 CodeProject 文章中找到。）
虽然这只是一堵令人讨厌的代码墙，但您必须处理的表面非常简单。这是一个控制台程序输出下载文件夹路径的示例。
private static void Main()
{
string downloadsPath = KnownFolders.GetPath(KnownFolder.Downloads);
Console.WriteLine("Downloads folder path: " + downloadsPath);
Console.ReadLine();
}
例如，只需调用 KnownFolders.GetPath() 并使用您要查询路径的文件夹的 KnownFolder 枚举值。
NuGet 包
如果您不想经历所有这些麻烦，只需安装我最近创建的 NuGet 包。这里是project site，这里是gallery link（注意用法不同，经过打磨，更多信息请查阅项目网站的Usage部分）。
PM> Install-Package Syroot.Windows.IO.KnownFolders
用法：
using System;
using Syroot.Windows.IO;
class Program
{
static void Main(string[] args)
{
string downloadsPath = new KnownFolder(KnownFolderType.Downloads).Path;
Console.WriteLine("Downloads folder path: " + downloadsPath);
Console.ReadLine();
}
}