I've written the following methods:

[StructLayout(LayoutKind.Sequential)]
public struct TRACE_GUID_REGISTRATION
{
public IntPtr Guid;
public IntPtr RegHandle;
}

[DllImport("advapi32.dll")]
public static extern uint RegisterTraceGuids(WMIDPREQUEST request, IntPtr
context,
ref Guid controlGuid, uint guidCount, IntPtr traceGuidReg, string
mofImagePath,
string mofResourceName, out ulong session);

public static uint RegisterTraceGuids(WMIDPREQUEST request, IntPtr context,
ref Guid controlGuid, ref TRACE_GUID_REGISTRATION[] traceGuidReg, string
mofImagePath,
string mofResourceName, out ulong session)
{
if (traceGuidReg == null)
throw(new ArgumentNullException("traceGuidReg"));
if (traceGuidReg.Length == 0)
throw(new ArgumentException("traceGuidReg.Length cannot be zero."));

uint size = (uint) Marshal.SizeOf(typeof(TRACE_GUID_REGISTRATION));
IntPtr buffer = Marshal.AllocHGlobal((int) (size * traceGuidReg.Length));
try
{
for (int i = 0; i < traceGuidReg.Length; ++i)
{
Marshal.StructureToPtr(traceGuidReg[i], (IntPtr) ((uint) buffer + (i *
size)), false);
}

uint result = RegisterTraceGuids(request, context, ref controlGuid, (uint)
traceGuidReg.Length,
buffer, mofImagePath, mofResourceName, out session);
if (result != 0)
return result;

for (int i = 0; i < traceGuidReg.Length; ++i)
{
Marshal.PtrToStructure((IntPtr) ((uint) buffer + (i * size)),
traceGuidReg[i]);
}

return 0;
}
finally
{
Marshal.FreeHGlobal(buffer);
}
}


It appears calling my helper RegisterTraceGuids() works up until the point I
try to marshal the array back, from IntPtr to the managed array. Is there a
reason why I can't copy back into the array?

--
Thanks,
Nick

nicknospamdu (AT) community (DOT) nospam
remove "nospam" change community. to msn.com

What error did you see?

Is the return of RegisterTraceGuids 0? If it's not 0, what does
GetLastError() return?

What's the symptom when it unmarshals the array?

for (int i = 0; i < traceGuidReg.Length; ++i)
{
Marshal.PtrToStructure((IntPtr) ((uint) buffer + (i * size)),
traceGuidReg[i]);
}



"nickdu" <nicknospamdu (AT) community (DOT) nospam> wrote

Hello Nick

I can quickly reproduce the problem with this piece of test code:

[StructLayout(LayoutKind.Sequential)]
public struct TRACE_GUID_REGISTRATION
{
public IntPtr Guid;
public IntPtr RegHandle;
}
TRACE_GUID_REGISTRATION[] arr = new TRACE_GUID_REGISTRATION[2];
arr[0].Guid = (IntPtr)0x123;
arr[1].Guid = (IntPtr)0x456;
uint size = (uint)Marshal.SizeOf(typeof(TRACE_GUID_REGISTRATIO N));
IntPtr buffer = Marshal.AllocHGlobal((int)(size * arr.Length));
for (int i = 0; i < arr.Length; ++i)
{
Marshal.StructureToPtr(arr[i], (IntPtr)((uint)buffer + (i * size)),
false);
}
for (int i = 0; i < arr.Length; ++i)
{
Marshal.PtrToStructure((IntPtr)((uint)buffer + (i * size)), arr[i]);
}

The problem does nothing with the RegisterTraceGuids API.

According to the doc of Marshal.PtrToStructure(IntPtr, Object)
http://msdn.microsoft.com/en-us/library/30ex8z62.aspx
, it throws the ArgumentException that you saw when structure layout is not
sequential or explicit or structure is a boxed value type.

In this case, the structure is declared as sequential, however, the
elements in the array (traceGuidReg[i]) are boxed on the managed heap
because of the array object, thus you got the error "the structure must not
be a value class."

You would need to use the overload Marshal.PtrToStructure Method (IntPtr,
Type)
http://msdn.microsoft.com/en-us/library/4ca6d5z7.aspx
and assign the result of PtrToStructure to the array elements.


Regards,
Jialiang Ge (jialge (AT) online (DOT) microsoft.com, remove 'online.')
Microsoft Online Community Support

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""Jialiang Ge [MSFT]"" <jialge (AT) online (DOT) microsoft.com> wrote

Elements of arrays are not boxed unless the array is of type object[]. The
problem is passing the argument in a parameter of type object, then it is
boxed.

Of course it could not work, because changes made to a value class passed by
value are lost.

Probably the next version of .NET should add an overload

PtrToStructure<T>(IntPtr, ref T) where T : struct