c# - 调用 `Environment.GetEnvironmentVariable`影响后续调用

Question

在 .NET 4.5.1 或 .NET 4.5.2 上执行的以下代码示例(相同结果)中，当代码查询“不存在”变量时会发生一些奇怪的事情。另一个完全存在的值为空字符串的变量“myvar”将不再通过调用 GetEnvironmentVariable 看到，但通过整个环境的迭代仍然可以看到。

此行为可能无法仅使用 .NET API 重新创建，因为它们不允许将环境变量设置为空字符串；但 native API 允许这样做。

调用 Environment.GetEnvironmentVariable 会使另一个变量从环境中消失，或半消失，感觉很奇怪。

目标框架设置为 .NET 2.0 时的行为略有不同。直接获取 myvar 与迭代之间的不匹配发生在对 SetEnvironmentVariable 的 native 调用之后 - 无需查询另一个变量即可查看它。

编辑:添加 Charset=CharSet.Auto，正如 Hans Passant 的善意建议(谢谢!)将 .NET 4.5.x 的疯狂级别降低到 .NET 4.5.x 的疯狂级别。 NET 2.0，除了修复 Unicode 处理之外，完全按照上一段中的描述。 DllImport 也缺少一个 SetLastError 参数，但这只是一个人为的例子，我们知道这个 native 调用在 Win32 级别上是成功的。所以，到目前为止没有任何解释。我知道解决该问题的几种方法，但我想更好地理解我所看到的。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("Kernel32.dll")]
        public static extern int SetEnvironmentVariable(string name, string value);

        static void Main(string[] args)
        {
            ShowMyVar();

            Environment.SetEnvironmentVariable("myvar", "somevalue");
            ShowMyVar();

            Environment.SetEnvironmentVariable("myvar", String.Empty);
            ShowMyVar();

            SetEnvironmentVariable("myvar", String.Empty);
            ShowMyVar();
            // once again, for good measure.
            ShowMyVar();

            Console.WriteLine("\nOkay, sane results so far.  Now let's query an unrelated non-existent variable.");

            Environment.GetEnvironmentVariable("nonexistent");
            ShowMyVar(); // Here we get weird results.

            Console.WriteLine("\nNow again, but purely through .NET APIs.");

            Environment.SetEnvironmentVariable("myvar", "somevalue");
            ShowMyVar();

            Environment.SetEnvironmentVariable("myvar", String.Empty);
            ShowMyVar();

            Environment.GetEnvironmentVariable("nonexistent");
            ShowMyVar();


        }

        private static void ShowMyVar()
        {
            if (Environment.GetEnvironmentVariable("myvar") != null)
            {
                Console.WriteLine("myvar is set to \"{0}\"", Environment.GetEnvironmentVariable("myvar"));
            }
            else
            {
                Console.WriteLine("myvar is not set");
            }
            foreach (var x in Environment.GetEnvironmentVariables().Keys)
            {
                if (x.ToString() == "myvar")
                {
                    Console.WriteLine("iteration gives value of myvar as \"{0}\"", Environment.GetEnvironmentVariable("myvar"));
                    return;
                }
            }
            Console.WriteLine("iteration over environment does not yield myvar");
        }
    }
}

Answer 1

这样做的原因:

Okay, sane results so far.  Now let's query an unrelated non-existent variable.

myvar is not set

.NET 调用 kernel32!GetEnvironmentVariableW ( "myvar", <pointer>, 128 )并返回 0。 GetLastError()设置为 203 = 系统找不到输入的环境选项。

此 API 记录在此处 http://msdn.microsoft.com/en-us/library/windows/desktop/ms683188(v=vs.85).aspx

iteration gives value of myvar as ""

第 2 步调用 GetEnvironmentStringsW()它返回指向环境字符串 block 的指针。MyVar 出现在这里。

此 API 记录在此处 http://msdn.microsoft.com/en-us/library/windows/desktop/ms683187(v=vs.85).aspx

此信息是使用 Rohitab API 监视器获得的 http://www.rohitab.com/apimonitor将 API 监视器用于 .NET EXE 时，您可能需要在应用程序开始时暂停，然后再附加 API 监视器。

在这种情况下启用监控系统服务 -> 进程和线程 -> 进程 -> Kernel32.dll 和未勾选的 GetCurrentProcess()

调用的 .NET 源代码在这里 http://referencesource.microsoft.com/#mscorlib/system/environment.cs

Win32Native.ERROR_ENVVAR_NOT_FOUND = 203，这会触发返回 null。

[System.Security.SecuritySafeCritical]  // auto-generated
        [ResourceExposure(ResourceScope.Machine)]
        [ResourceConsumption(ResourceScope.Machine)]
        public static String GetEnvironmentVariable(String variable)
        {
            if (variable == null)
                throw new ArgumentNullException("variable");
            Contract.EndContractBlock();

#if !FEATURE_CORECLR
            (new EnvironmentPermission(EnvironmentPermissionAccess.Read, variable)).Demand();
#endif //!FEATURE_CORECLR

            StringBuilder blob = new StringBuilder(128); // A somewhat reasonable default size
            int requiredSize = Win32Native.GetEnvironmentVariable(variable, blob, blob.Capacity); 

            if( requiredSize == 0) {  //  GetEnvironmentVariable failed
                if( Marshal.GetLastWin32Error() == Win32Native.ERROR_ENVVAR_NOT_FOUND)
                    return null;
            }

            while (requiredSize > blob.Capacity) { // need to retry since the environment variable might be changed 
                blob.Capacity = requiredSize;
                blob.Length = 0;
                requiredSize = Win32Native.GetEnvironmentVariable(variable, blob, blob.Capacity); 
            }
            return blob.ToString();
        }

与 .NET 不同，在 Win32 C++ 中，使用以下代码可以正常工作:

DWORD dwResult;
wchar_t buffer[128];

if (!SetEnvironmentVariableW(L"myvar", L""))
{
    dwResult = GetLastError();
    std::cout << "ERROR #" << dwResult << std::endl;

}

if (!GetEnvironmentVariableW(L"myvar", buffer,128))
{
    dwResult = GetLastError();
    std::cout << "ERROR #" << dwResult << std::endl;
}

std::wcout << "Buffer : '" << buffer << "'";

发生以下 API 调用:

ConsoleTest.exe!SetEnvironmentVariableW ( "myvar", "" ) TRUE        
    KERNELBASE.dll!RtlSetEnvironmentVar ( NULL, "myvar", 5, "", 0 ) STATUS_SUCCESS      
ConsoleTest.exe!GetEnvironmentVariableW ( "myvar", <pointer 1>, 128 )   0   0 = The operation completed successfully.   
    KERNELBASE.dll!RtlQueryEnvironmentVariable ( NULL, "myvar", 5, <pointer 1>, 128, <pointer 2> )  STATUS_SUCCESS  

当从 .NET 4 调用时，在这种情况下，我直接依次调用 SetEnvironmentVariable 和 GetEnvironmentVariable:

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern bool SetEnvironmentVariable(string lpName, string lpValue);

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern uint GetEnvironmentVariable(string lpName, [Out] StringBuilder lpBuffer, uint nSize);

StringBuilder buffer = new StringBuilder(128);

SetEnvironmentVariable("myvar", String.Empty);
uint result=GetEnvironmentVariable("myvar", buffer, 128);
if (result==0)
{
    Console.WriteLine(string.Format("Error: {0}", Marshal.GetLastWin32Error()));
}

这会导致这些 API 调用/结果

 clr.dll->SetEnvironmentVariableW ( "myvar", "" )   TRUE    
       KERNELBASE.dll!RtlSetEnvironmentVar ( NULL, "myvar", 5, "", 0 )  STATUS_SUCCESS

    clr.dll->SetLastError ( ERROR_ENVVAR_NOT_FOUND )        
    clr.dll->SetLastError ( ERROR_ENVVAR_NOT_FOUND )            
    clr.dll->SetLastError ( ERROR_ENVVAR_NOT_FOUND )            
    clr.dll->SetLastError ( ERROR_ENVVAR_NOT_FOUND )            

    clr.dll!GetEnvironmentVariableW ( "myvar", <pointer 1>, 128 )   0   203 = The system could not find the environment option that was entered. 
       KERNELBASE.dll!RtlQueryEnvironmentVariable ( NULL, "myvar", 5, <pointer 1>, 128, <pointer 2> )   STATUS_SUCCESS  

在 .NET 版本中调用 KERNELBASE.dll!RtlQueryEnvironmentVariable 成功

据我所知，clr.dll 正在影响 GetEnvironmentVariableW 的返回值。

如果在 WinDbg 中启动 .NET exe，并在设置“somevalue”后中断

加载 .NET 调试扩展

0:003> .loadby sos clr 

显示进程环境 block (PEB) 以查找环境的地址

0:003> !peb
PEB at 7f40d000
    InheritedAddressSpace:    No
    ReadImageFileExecOptions: No
    BeingDebugged:            Yes
    ImageBaseAddress:         00250000
    Ldr                       77d891e0
      etc...
    Environment:  **006b0c68**
      etc..

在内存中搜索我们的值的位置...

0:003> s -u **006b0c68** **006b0c68**+1000 "somevalue"
**006b1214**  0073 006f 006d 0065 0076 0061 006c 0075  s.o.m.e.v.a.l.u.

在该内存位置设置读取断点...

0:003> ba r 1 **006b1214**
0:000> du 6b1214
006b1214  "somevalue"
0:000> g
Breakpoint 1 hit
eax=00000000 ebx=006b1228 ecx=006b1214 edx=00000000 esi=00000000 edi=006b293a
eip=77ce37ad esp=003df094 ebp=003df108 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246
ntdll!RtlSetEnvironmentVar+0x516:
77ce37ad 83c102          add     ecx,2
0:000> du 6b1214
006b1214  ""
0:000> kv
ChildEBP RetAddr  Args to Child              
003df108 7774029e 00000000 01fe2b00 00000005 ntdll!RtlSetEnvironmentVar+0x516 (FPO: [SEH])
003df12c 005d04a3 01fe2b00 01fe1230 ed4737ca KERNELBASE!SetEnvironmentVariableW+0x47 (FPO: [Non-Fpo])
WARNING: Frame IP not in any known module. Following frames may be wrong.
003df1f8 73cc2552 0068e698 003df258 73ccf237 0x5d04a3
003df204 73ccf237 003df29c 003df248 73e18ad2 clr!CallDescrWorkerInternal+0x34
003df258 73ccff60 00000000 00000001 003df2b8 clr!CallDescrWorkerWithHandler+0x6b (FPO: [Non-Fpo])
003df2d0 73de671c 003df3cc efea5369 004e37fc clr!MethodDescCallSite::CallTargetWorker+0x152 (FPO: [Non-Fpo])
003df3f4 73de6840 01fe2a68 00000000 efea5495 clr!RunMain+0x1aa (FPO: [Non-Fpo])
003df668 73e23dc5 00000000 efea56e5 00250000 clr!Assembly::ExecuteMainMethod+0x124 (FPO: [1,149,0])
003dfb68 73e23e68 efea5b5d 00000000 00000000 clr!SystemDomain::ExecuteMainMethod+0x63c (FPO: [0,313,0])
003dfbc0 73e23f7a efea5c9d 00000000 00000000 clr!ExecuteEXE+0x4c (FPO: [Non-Fpo])
003dfc00 73e26b86 efea5ca1 00000000 00000000 clr!_CorExeMainInternal+0xdc (FPO: [Non-Fpo])
003dfc3c 7436ffcc eff9e4d3 7573980c 74360000 clr!_CorExeMain+0x4d (FPO: [Non-Fpo])
003dfc74 743ebbb7 003dfc8c 743ebbcc 00000000 mscoreei!_CorExeMain+0x10a (FPO: [0,10,4])
003dfc7c 743ebbcc 00000000 00000000 003dfc98 MSCOREE!_CorExeMain_Exported+0x77 (FPO: [Non-Fpo])
003dfc8c 7573919f 7f40d000 003dfcdc 77cda8cb MSCOREE!_CorExeMain_Exported+0x8c (FPO: [Non-Fpo])
003dfc98 77cda8cb 7f40d000 eea2cea9 00000000 KERNEL32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
003dfcdc 77cda8a1 ffffffff 77ccf663 00000000 ntdll!__RtlUserThreadStart+0x20 (FPO: [SEH])
003dfcec 00000000 743ebb40 7f40d000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])
0:000> !clrstack
OS Thread Id: 0x3464 (0)
Child SP       IP Call Site
003df140 77ce37ad [InlinedCallFrame: 003df140] 
003df13c 005d04a3 *** WARNING: Unable to verify checksum for ConsoleApplicationTest.exe
DomainBoundILStubClass.IL_STUB_PInvoke(System.String, System.String)
003df140 005d0106 [InlinedCallFrame: 003df140] ConsoleApplication1.Program.SetEnvironmentVariable(System.String, System.String)
003df1a4 005d0106 ConsoleApplication1.Program.Main(System.String[]) [c:\Users\mccafferym\Documents\Visual Studio 2013\Projects\ConsoleApplicationTest\ConsoleApplicationTest\Program.cs @ 32]
003df378 73cc2552 [GCFrame: 003df378] 
0:000> g

我们现在在读回值时遇到断点...

Breakpoint 1 hit
eax=0000003d ebx=006b1212 ecx=006b1214 edx=01fe2b0a esi=006b1208 edi=00000001
eip=77cda2f7 esp=003def5c ebp=003def74 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246
ntdll!RtlpScanEnvironment+0xa7:
77cda2f7 75f7            jne     ntdll!RtlpScanEnvironment+0xa0 (77cda2f0) [br=0]
0:000> kv
ChildEBP RetAddr  Args to Child              
003def74 77cda138 01fe2b0a 003df000 00000080 ntdll!RtlpScanEnvironment+0xa7 (FPO: [Non-Fpo])
003defc8 77732b88 00000000 01fe2b00 00000005 ntdll!RtlQueryEnvironmentVariable+0xa7 (FPO: [SEH])
003defec 005d0682 01fe2b00 003df000 00000080 KERNELBASE!GetEnvironmentVariableW+0x39 (FPO: [Non-Fpo])

从这里跟踪执行 onwords，我们看到这里设置了错误:

KERNELBASE!GetEnvironmentVariableW+0x52:
77732ba9 c20c00          ret     0Ch
006f0682 8b4d98          mov     ecx,dword ptr [ebp-68h] ss:002b:0033f040=004ee698
006f0685 c6410801        mov     byte ptr [ecx+8],1         ds:002b:004ee6a0=00
006f0689 833d64a32f7400  cmp     dword ptr [clr!g_TrapReturningThreads (742fa364)],0 ds:002b:742fa364=00000000
006f0690 7407            je      006f0699                                [br=1]
006f0699 c7458000000000  mov     dword ptr [ebp-80h],0 ss:002b:0033f028=006f0682
006f06a0 8945ac          mov     dword ptr [ebp-54h],eax ss:002b:0033f054=0033f288
006f06a3 e801545e73      call    clr!StubHelpers::SetLastError (73cd5aa9)

如果此行为需要更改，我建议向 Microsoft 提出支持案例。