Develop Reference

C# Pinvoke invalid file handle

I'm having a problem with kernal32 Pinvoke functions as they keeps throwing an INVALID_FILE_HANDLE. The program reads the first sector of the current hard disk. I can't see what is wrong with the following code.
class Program
{
const uint GENERIC_READ = 0x80000000;
const uint FILE_SHARE_READ = 0x00000001;
const uint OPEN_EXISTING = 0x00000003;
const uint FILE_FLAG_DELETE_ON_CLOSE = 0x04000000;
[DllImport("kernel32.dll", SetLastError = true)]
public static extern SafeFileHandle CreateFile(string Disk, uint Access, uint ShareMode, IntPtr SecurityAttributes, uint CreationDisposition, uint Flags, IntPtr TemplateFile);
[DllImport("kernel32.dll", SetLastError = true)]
public static extern uint SetFilePointer([In] SafeFileHandle Handle, [In] int DistanceToMove, [Out] out int DistanceToMoveHigh, [In] int MoveMethod);
[DllImport("kernel32.dll", SetLastError = true)]
unsafe public static extern int ReadFile(SafeFileHandle Handle, [Out] byte[] Buffer, int NumberOfBytesToRead, out int NumberOfBytesRead, IntPtr Overlapped);
unsafe public static void Main(string[] args)
{
string Drive = #"\\.\C";
int SectorSize = 512;
int Sector = 0;
int BytesRead, DistanceToMoveHigh;
byte[] Buffer = new byte[SectorSize];
SafeFileHandle Handle = CreateFile(Drive, GENERIC_READ, FILE_SHARE_READ, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_DELETE_ON_CLOSE, IntPtr.Zero);
SetFilePointer(Handle, Sector * SectorSize, out DistanceToMoveHigh, 0);
ReadFile(Handle, Buffer, SectorSize, out BytesRead, IntPtr.Zero);
Console.WriteLine(Marshal.GetLastWin32Error()); // It gives 6 which translates to an INVALID_FILE_HANDLE error
Console.ReadKey();
}
}

Your call to CreateFile fails. Of course you cannot know that because you omitted any error checking. Read the documentation. Errors for all three functions that you call are signaled by the return value. Which you ignore.
Your call to CreateFile returns INVALID_HANDLE_VALUE. You need to test for that. When you encounter that, and only then, call GetLastWin32Error. Likely ERROR_ACCESS_DENIED will be returned then.
Passing FILE_FLAG_DELETE_ON_CLOSE is a mistake. Remove that flag.
I believe that the share flags must be FILE_SHARE_READ | FILE_SHARE_WRITE.
The file name must be #"\\.\C:" with a trailing colon.
And you will need the process to be executed elevated.

You use GetLastWin32Error in a wrong way.
The method that fails here is CreateFile and it returns an INVALID_HANDLE_VALUE (indicating that it failed). To determine what went wrong you have to call GetLastWin32Error directly after CreateFile.
When you call it after trying to read, the error is of course ERROR_INVALID_HANDLE (6) as you passed an invalid handle to ReadFile.
If you call GetLastWin32Error directly after the failing CreateFile you get error 2:
The system cannot find the file specified.
That is because the the drive name misses a :
string Drive = #"\\.\C:"; // <- add colon :
I tried with that drive name, but then got the error 32:
The process cannot access the file because it is being used by another process.
I keep trying to figure out how that can be handled...

RegSetValueEx returns 998 (ERROR_NOACCESS) if called without ref

I'm trying to write a DWORD to registry using c#.
Using p/invoke because of registry reddirection.
I've searched for this issue and finally could get it working but i don't understand.
[DllImport("advapi32.dll", SetLastError = true)]
static extern uint RegSetValueEx(
IntPtr hKey,
[MarshalAs(UnmanagedType.LPStr)]
string lpValueName,
int Reserved,
RegistryValueKind dwType,
ref IntPtr lpData,
int cbData);
int checkreturn = RegOpenKeyEx(HKeyLocalMachine, #"SOFTWARE\Test", 0, (int) RegistrySecurity.KEY_WOW64_64KEY | (int) RegistrySecurity.KEY_SET_VALUE, ref keyHandle);
const int dataStored = 0;
IntPtr p = new IntPtr(dataStored);
int size = Marshal.SizeOf(dataStored);
uint checkreturn2 = RegSetValueEx(keyHandle, "valueName", 0, RegistryValueKind.DWord, ref p, size);
This works if i put out or ref on lpData parameter, if i don't it returns error 998 (ERROR_NOACCESS), why is that? The same thing happens if i change the IntPtr to int, and pass the actual value, but this time i get an first exception AccessViolation on my code.
the winapi declaration for that it's *lpData, which i assume is what passing a IntPtr is.
_In_ const BYTE *lpData,

The api requires a pointer to the data plus the size of the data. You can't pass an int, or a char, or a bool. You need to pass a pointer to the data. If you pass something else, the API will interpret it as a pointer to the data, and random results will happen.
With P/Invoke, a ref to something is translated to a pointer to that something.
Now, you can
[DllImport("advapi32.dll", SetLastError = true)]
static extern uint RegSetValueEx(
IntPtr hKey,
[MarshalAs(UnmanagedType.LPStr)]
string lpValueName,
int Reserved,
RegistryValueKind dwType,
ref uint lpData,
int cbData);
and then in cbData pass sizeof(uint) and this will work, because a ref for P/Invoke is a ref.
Only thing, I would suggest removing the
[MarshalAs(UnmanagedType.LPStr)]
because without it the P/Invoke will use the Unicode version of the method, that is more correct.

How does one pass nullptr through P/Invoke declared with a StringBuilder?

Consider a typical P/Invoke declaration like this:
[DllImport("advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool LookupAccountName(
string SystemName,
string accountName,
IntPtr pSid,
ref uint cbSid,
[MarshalAs(UnmanagedType.LPWStr)] StringBuilder ReferencedDomainName,
ref uint ReferencedDomainNameCount,
out SID_NAME_USE SIDUse);
MSDN documentation for LookupAccountName says that pSid and ReferencedDomainName may be nullptr if the customer wishes. Passing nullptr for pSid is easy; just pass IntPtr.Zero. But what should one pass for a StringBuilder?
I don't want to pass an empty StringBuilder, because I don't want this call to fail with ERROR_INSUFFICIENT_BUFFER.

You can just pass null for this parameter; it will be marshalled as a null pointer.

Access violation calling Win API from C# using DllImport

The task is to determine the last write time for the registry key. As standard RegistryKey class doesn't provide this feature, I have to use WinAPI function "RegQueryInfoKey". To get the key handle I open it by "RegOpenKeyEx".
This is the WinAPI prototype of the function (taken from MSDN):
LONG WINAPI RegOpenKeyEx(
__in HKEY hKey,
__in LPCTSTR lpSubKey,
DWORD ulOptions,
__in REGSAM samDesired,
__out PHKEY phkResult
);
I use the following declaration:
[DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern int RegOpenKeyEx(UIntPtr hkey, string lpSubKey, uint samDesired, ref UIntPtr phkResult);
Then I call it in the following way:
UIntPtr hKey = UIntPtr.Zero;
string myKeyName = "blablabla";
UIntPtr HKEY_USERS = (UIntPtr)0x80000003;
uint KEY_READ = 0x20019;
RegOpenKeyEx(HKEY_USERS, myKeyName, KEY_READ, ref hKey);
At this point I get "Access violation" exception. What am I doing wrong?
I think something is wrong with parameters passing, but how to do it right?
Thank you.

There are 5 parameters in the native function's prototype, and only 4 in your P/Invoke signature.
In particular, you're missing DWORD ulOptions. This parameter is "reserved and must be zero" according to the MSDN documentation, but it must still be passed in the function call.
Also, you don't need to set the SetLastError field because the RegOpenKeyEx function returns its error code; you don't have to retrieve it by calling GetLastError. Consequently, you don't need the marshaler to save that value for you automatically. Just check the return value for the error code.
Change your P/Invoke signature to look like this:
[DllImport("advapi32.dll", CharSet = CharSet.Auto)]
public static extern int RegOpenKeyEx(UIntPtr hkey, string lpSubKey,
uint ulOptions, uint samDesired,
out UIntPtr phkResult);
The wrong P/Invoke signature is almost always the cause of "access violation" errors. When you see one of those, make sure you double-check it twice!

You've missed ulOptions from your P/Invoke signature.

A Problem in Pinvoke

I have the following function in C++ native dll, and I want to use it in a C# app.
DWORD __cdecl Foo(
LPCTSTR Input,
TCHAR** Output,
DWORD Options,
ErroneousWord** List = NULL,
LPDWORD Count = 0
);
Using Pinvoke
[DllImport("dllName", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern UInt32 Foo(string InputWord, out string Output, UInt32 Options, out object List,out UInt32 Count);
Calling code:
string output;
object dummyError = null;
uint dummyCount = 0;
uint x = 0;
Foo(Text, out output, x | y,out dummyError,out dummyCount);
I got the following exception
Attempted to read or write protected
memory. This is often an indication
that other memory is corrupt
P.S:
ErroneousWord is struct and I do not need its output, so I marshal it as object

That error more than likely means that you have a marshaling problem.
You don't show us what the ErroneousWord type is, but I assume it's some kind of class defined in your C++ code. My guess is that it's not being marshaled correctly to a .NET object.
Considering that it's a pointer (or a pointer to a pointer), try changing that parameter to an IntPtr type to represent a pointer, instead. It shouldn't matter, since you're simply passing NULL for the argument anyway, easily represented using the static IntPtr.Zero field.
You probably also want to marshal Output the exact same way. If you change the parameter to an IntPtr type, you'll receive a pointer to a TCHAR*, which you can then pass to the other unmanaged functions however you see fit (e.g., to free it).
Try the following code:
[
DllImport("dllName",
CharSet = CharSet.Unicode,
CallingConvention = CallingConvention.Cdecl)
]
public static extern UInt32 Foo(
string InputWord,
out IntPtr Output, // change to IntPtr
UInt32 Options,
out IntPtr List, // change to IntPtr
out UInt32 Count);
IntPtr output;
IntPtr dummyError = IntPtr.Zero;
uint dummyCount = 0;
uint x = 0;
Foo(Text, out output, x | y, out dummyError, out dummyCount);
You might also need to use the Marshal.AllocHGlobal method to allocate unmanaged memory from your process that is accessible to the C++ code. Make sure that if you do so, you also call the corresponding Marshal.FreeHGlobal method to release the memory.

Given Cody's answer and the comments, you will have to do it this way:
[DllImport("dllName", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
extern static UInt32 Foo(string InputWord, out IntPtr Output, UInt32 Options, out IntPtr List, out UInt32 Count);
Now to get the string value in Output marshalled over to managed memory you will do:
string outputValue = Marshal.PtrToStringAnsi(Output);
You must know if TCHAR is Ansi or Unicode and use the appropriate marshal.
Remember to hang onto the Output IntPtr so you can pass that to the native Free method.

Thanks Cody for your answer but I want to make a seperate one, first Output is created by Foo from the native side, and I call FreeFoo to free the allocated memory by Foo.
The following is the code
[DllImport("dllname", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern UInt32 Correct(string InputWord, out IntPtr Output, UInt32 Options, out object List,out UInt32 Count);
[DllImport("dllname", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern void FreeFoo(IntPtr Output);
}
To use it:
public string FooWrapper(string Text)
{
IntPtr output;
object dummyError = null;
uint dummyCount = 0;
uint x = 0;
Foo(Text, out output, x,out dummyError,out dummyCount);
string str = Marshal.PtrToStringUni(output);
FreeFoo(output);
return str;
}

Whatever the ErroneousWord type is, you can't marshal an array as a single out object. If it is at all possible to marshal as an object...