Calling C# com object from c++, who should free the memory?

Question

For ex. i have c# COM object with such function:

int GetString([In, Out, MarshalAs(UnmanagedType.LPStr)]ref string str)

then i'm calling int from c++ (in c++ COM object is used via #import keyword), in generated wrapper method is declared as:

GetString(LPSTR * str, long * retVal)

i'm calling it this way:

char myStr[40];
LPSTR buf = (LPSTR)myStr;
LPSTR pBuf = &buf;

pComObject->GetString(pBuf);

what is strange:

1) myStr is not filled, and buf value is changed (it doesnt the same with myStr after the call) so, i suppose that c# marshaller allocates new block of memory

2) if c# allocates memory so should i call free(buf); or not?

Igor Chornous · Accepted Answer · 2012-03-26 15:44:56Z

3

Yes, the client should release the memory. In this case, one should use the CoTaskMemFree function to free the received buffer. Since the buffer may not be allocated on the client's heap. CoTaskMemFree will call an appropriate deleter.

answered Mar 26, 2012 at 15:44

Igor Chornous

1,0681 gold badge6 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Dr. Wily's Apprentice · Accepted Answer · 2012-03-26 16:25:55Z

2

Yes, I think that you do need to free the buffer in your C++ code after calling the GetString function.

According to the COM "Memory Management Rules", the caller (your C++ code) should free memory that is returned as an output parameter. In the case of an input/output parameter, the callee (the CLR and your C# code) may free the memory of the parameter that you passed in and then allocate new memory for the output value, and it is again the caller's responsibility to free that memory.

answered Mar 26, 2012 at 16:25

Dr. Wily's Apprentice

10.3k1 gold badge29 silver badges27 bronze badges

1 Comment

Igor Chornous Over a year ago

Upvoted! Good explanation, and I should have mentioned the rules.

Collectives™ on Stack Overflow

Calling C# com object from c++, who should free the memory?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related