下面是Windows管理规范(WMI)方法:

string _osVersion = "";
string _osServicePack = "";
string _osArchitecture = "";

ManagementObjectSearcher searcher = new ManagementObjectSearcher("select * from Win32_OperatingSystem");
ManagementObjectCollection collection = searcher.Get();

foreach (ManagementObject mbo in collection)
{
    _osVersion = mbo.GetPropertyValue("Caption").ToString();
    _osServicePack = string.Format("{0}.{1}", mbo.GetPropertyValue("ServicePackMajorVersion").ToString(), mbo.GetPropertyValue("ServicePackMinorVersion").ToString());

    try
    {
        _osArchitecture = mbo.GetPropertyValue("OSArchitecture").ToString();
    }
    catch
    {
        // OSArchitecture only supported on Windows 7/Windows Server 2008
    }
}

Console.WriteLine("osVersion     : " + _osVersion);
Console.WriteLine("osServicePack : " + _osServicePack);
Console.WriteLine("osArchitecture: " + _osArchitecture);

/////////////////////////////////////////
// Test on Windows 7 64-bit
//
// osVersion     : Microsoft Windows 7 Professional
// osservicePack : 1.0
// osArchitecture: 64-bit

/////////////////////////////////////////
// Test on Windows Server 2008 64-bit
//    --The extra r's come from the registered trademark
//
// osVersion     : Microsoftr Windows Serverr 2008 Standard
// osServicePack : 1.0
// osArchitecture: 64-bit

/////////////////////////////////////////
// Test on Windows Server 2003 32-bit
//    --OSArchitecture property not supported on W2K3
//
// osVersion     : Microsoft(R) Windows(R) Server 2003, Standard Edition
// osServicePack : 2.0
// osArchitecture:

最快的方式:

if(IntPtr.Size == 8) {
    // 64 bit machine
} else if(IntPtr.Size == 4)  {
    // 32 bit machine
} 

注意:这是非常直接的，只有当程序不强制作为32位进程执行时(例如，在项目设置中通过<Prefer32Bit>true</Prefer32Bit>)，才能在64位上正确工作。

只要看看“C:\Program Files (x86)”是否存在。如果不是，那么你使用的是32位操作系统。如果是这样，那么操作系统是64位(Windows Vista或Windows 7)。

使用此命令获取已安装的Windows体系结构:

string getOSArchitecture()
{
    string architectureStr;
    if (Directory.Exists(Environment.GetFolderPath(
                           Environment.SpecialFolder.ProgramFilesX86))) {
        architectureStr ="64-bit";
    }
    else {
        architectureStr = "32-bit";
    }
    return architectureStr;
}

来自Chriz Yuen的博客

介绍了两个新的环境属性 Environment.Is64BitOperatingSystem; Environment.Is64BitProcess;

当你使用这两种属性时请小心。 在Windows 7 64位机器上测试

//Workspace: Target Platform x86
Environment.Is64BitOperatingSystem True
Environment.Is64BitProcess False

//Workspace: Target Platform x64
Environment.Is64BitOperatingSystem True
Environment.Is64BitProcess True

//Workspace: Target Platform Any
Environment.Is64BitOperatingSystem True
Environment.Is64BitProcess True

完整的答案是这样的(摘自stefan-mg, ripper234和BobbyShaftoe的答案):

    [DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool IsWow64Process([In] IntPtr hProcess, [Out] out bool lpSystemInfo);

    private bool Is64Bit()
    {
        if (IntPtr.Size == 8 || (IntPtr.Size == 4 && Is32BitProcessOn64BitProcessor()))
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    private bool Is32BitProcessOn64BitProcessor()
    {
        bool retVal;

        IsWow64Process(Process.GetCurrentProcess().Handle, out retVal);

        return retVal;
    } 

首先检查您是否处于64位进程中。如果不是，检查32位进程是否是Wow64Process。