IntPtr when calling C++ functions

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Richards

The .NET documentation has the following statement about

"The IntPtr type can be used ...... as a common means of referring to data
between languages that do and do not support pointers."

Can someone give me an example of how I pass a pointer (ByRef in VB terms)
parameter from a VB program to a C++ function?

I currently am using IntPtr, and I can get the code to compile, but the ByRef
paramenter doesn't seem to be working. When I call the C++ function.

Thanks
 
Can someone give me an example of how I pass a pointer (ByRef in VB terms)
parameter from a VB program to a C++ function?
Click to expand...

Why don't you instead post the signature of the C++ function you want
to call and your current calling VB code? There are too many
variations on this to show it all in just one example.


Mattias
 
Thank you for your reply. Here is the .Net call and the C++ method:


Private MyIntPtr As System.IntPtr

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.
EventArgs) Handles Timer1.Tick

'For testing only
Dim x As Integer = 100

'Mattias - I believe the following creates a pointer to an interger
value. The integer value is initialized to 1
MyIntPtr = New IntPtr(1)

'Mattias - I believe this chages the value of the integer pointed to
by MyIntPtr to 2
MyIntPtr = op_Explicit(2)

'Mattias, this will test that I can pass a "ByValue" parameter (x) and
a "ByRef" parameter (MyIntPtr)
x= MyImprovedNS.CalcDataCrc(x, MyIntPtr)

End Sub

Here is the C++ (abridged):

int ImprovedNetsen::NetsenMethods::CalcDataCrc(long by_value, IntPtr
by_reference )
{
// Mattias, I belived this changes the integer pointed to by "by_reference"
to 1000
*&by_reference = 1000;
return true;
}

The Debugger Watch window appears to confirm that the C++ code works as I
expected. However, when returning to the VB program, MyIntPtr is not changed.

Perhaps I do not properly understand the IntPtr type. I am not a C++
programmer, so my knowledge of that language is not great.

Thank you again for your assistance.

Best Regards,

Richard
(PS: I am in the United States, so I will be on holiday until 28/November. It
is not necessary to respond before Monday.)
 
'Mattias - I believe the following creates a pointer to an interger
value. The integer value is initialized to 1
MyIntPtr = New IntPtr(1)
Click to expand...

No, it creates a pointer-sized integer (which is what an IntPtr is)
with the value 1. If you treat it as a pointer it will "point to"
address 1 in memory.

'Mattias - I believe this chages the value of the integer pointed to
by MyIntPtr to 2
MyIntPtr = op_Explicit(2)
Click to expand...

I don't think it even compiles, but if it did all it would do is
change the "address" to 2.

int ImprovedNetsen::NetsenMethods::CalcDataCrc(long by_value, IntPtr
by_reference )
Click to expand...

OK I assume you're using managed C++ since you're using the IntPtr
type here as well.

// Mattias, I belived this changes the integer pointed to by "by_reference"
to 1000
*&by_reference = 1000;
Click to expand...

That doesn't really make sense. You're just taking the address of
by_reference and then immediately dereferencing that address again. So
the code is the same as

by_reference = 1000;

You're not passing anything by reference here, the IntPtr parameter is
still passed by value. So all you're changing is the local copy of
by_reference.

It looks like you're trying to return an integer value to the caller
through a byref parameter. To do that you'd declare the function

int ImprovedNetsen::NetsenMethods::CalcDataCrc(long by_value, int __gc
*by_reference )
{
*by_reference = 1000;

No need for IntPtrs here.

return true;
Click to expand...

This also doesn't really make sense since the return type is int.

(PS: I am in the United States, so I will be on holiday until 28/November. It
is not necessary to respond before Monday.)
Click to expand...

Have a great Thanksgiving.


Mattias
 
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