在下面的代码示例中，在.NET 4.5.1或.NET 4.5.2上执行（相同的结果），当代码查询＆＃34;不存在＆＃34;变量。另一个完全存在的变量＆＃34; myvar＆＃34;其值为空字符串，将通过调用GetEnvironmentVariable而停止显示，但仍可通过整个环境的迭代看到它。
这种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;但是原生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，这感觉很奇怪。
目标框架设置为.NET 2.0的行为略有不同。直接获取myvar和迭代之间的不匹配在本地调用SetEnvironmentVariable之后立即发生 - 没有必要查询另一个变量来查看它。
编辑：添加Charset=CharSet.Auto，正如Hans Passant所建议的那样（谢谢！）将.NET 4.5.x的疯狂程度降低到.NET 2.0的程度，完全如上一段，除了修复Unicode处理。 DllImport也缺少SetLastError参数，但这只是一个人为的例子，我们知道这个本机调用在Win32级别上是成功的。所以，到目前为止还没有解释。我知道解决这个问题的几种方法，但我想更好地了解我所看到的内容。
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");
}
}
}
答案 0 :(得分：2)
原因是：
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 ""
步骤2调用GetEnvironmentStringsW()，它返回指向环境字符串块的指针。
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http://www.rohitab.com/apimonitor获得的。当为.NET EXE使用API监视器时，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 - ＆gt;流程和主题 - ＆gt;过程 - ＆gt; Kernel32.dll和未命中的GetCurrentProcess（）
调用的.NET源代码在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();
}
在Win32 C ++中，使用以下代码按照您的预期工作，与.NET不同：
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
从.NET 4调用时在这种情况下，我直接调用了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
显示流程环境块（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价值位置......
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点击断点...
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])
从这里跟踪执行单词，我们看到错误设置在这里：
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。