how to resolve access violation error:writing into protected memory

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Iam calling a method in C++ DLL form C# application:

Method in C++ DLL:
int ImageSize_(PUC i_puc_Header,UL i_ul_HeaderSize,PUL o_pul_UnHeaderSize)
{
//code here

*o_pul_UnHeaderSize=somevalue;/*i am assigning some calculated value to this unsigned long pointer in this line only am getting error. am confirming this by putting debug statements i tried using out modifier and UintPtr ref type.....*/
}
Here am declaring that method in C# as follows

public static extern int ImageSize(byte[] i_puc_Header, long i_ul_HeaderSize, ref UInt32 o_pul_UnHeaderSize);

Then am calling the method as follows.

ImageSize( Buffer1, Sizeofbuffer,ref image_size);

I searched in net to resolve it but i could not figure it out.Any help highly appreciated.
Thanks in advance

Posted 17-Jul-14 3:12am

D Mahesh

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[no name] 17-Jul-14 12:48pm

Please include the DllImport attribute in your declaration of ImageSize.

D Mahesh 18-Jul-14 0:15am

//sorry bling, still same error for unsigned long pointer as Access violation exception
//I changed declaration like this
[DllImport("Image.dll", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true, SetLastError = false)]
public static extern int ImageSize([In] byte[] i_puc_Header, long i_ul_HeaderSize,[In] [Out] ref UInt32 o_pul_UnHeaderSize);
//and am calling like this
int retval1 = ImageSize( RawBuffer1, Sizeofbuffer,ref image_size);
//earler also my issue is with unsigned long pointer

Bernhard Hiller 18-Jul-14 2:56am

Does the C++ function do anything with o_pul_UnHeaderSize before it sets it to somevalue? If not, "out" instead of "ref" could do the trick.

D Mahesh 18-Jul-14 3:31am

ya tried following way as u said not worked same error
Declaration:
[DllImport("Image.dll", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true, SetLastError = false)]
public static extern int ImageSize([In] byte[] i_puc_Header, long i_ul_HeaderSize, [Out] out UInt32 o_pul_UnHeaderSize);
Calling:
int retval1 = ImageSize( Buffer1, Sizeofbuffer,out image_size);

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bling · Answer 1 · 2014-07-17T06:51:00

I'll assume your DLL is named "image.dll". Any arrays need to be marked as [In] or [Out]. Try this ...

VB

[DllImport("image.dll", CallingConvention=CallingConvention.Cdecl,
             ExactSpelling=true, SetLastError=false)]
public static extern int ImageSize([In] byte[] i_puc_Header, long i_ul_HeaderSize, [Out] ref UInt32 o_pul_UnHeaderSize);

D Mahesh · Answer 2 · 2014-07-22T00:43:00

Solution 2

actual error was on 2 nd argument.
c++ long has 4 bytes so we should replace in our c# application with int(4 bytes)

Posted 22-Jul-14 0:43am