Returning object from C# COM method and memory ownership

Question

I have implemented a COM component in C#:

[ComVisible(true)]
[System.Runtime.InteropServices.Guid("E052BB1C-7ADC-47F4-99E1-9407E2FA0AA2")]
public interface IColorRamps
{
    IColorRamp getColorRamp();
}

[ComVisible(true)]
[Guid("EE47F2F2-0AD9-437C-8815-D570EACF2C07")]
[ClassInterface(ClassInterfaceType.None)]
[ProgId("ColorRamps.ColorRamps")]
public class ColorRamps : IColorRamps
{
    public IColorRamp getColorRamp() { ... } 
}

I call this from C++:

IColorRampPtr colorRamp;
{
    ColorRamps::IColorRampsPtr colorRamps(ColorRamps::CLSID_ColorRamps);
    HRESULT hr = colorRamps->getColorRamp(&colorRamp);
    colorRamp.AddRef(); // Should I do this??
 }

At first I did not have the AddRef() call and things seemed to work except I got strange crashes on "R6025 (pure virtual function call) run-time error" after running this code many times.

The signature in the autogenerated .tlh file is:

virtual HRESULT __stdcall getColorRamp(/*[out,retval]*/ struct IColorRamp * * pRetVal ) = 0;

When calling functions like this in C++ I am used to the function doing an AddRef() itself and passing on memory ownership to the caller. Is this not the case in C# COM?

I did NOT call Marshal.AddRef() inside ColorRamps.getColorRamp().

acelent · Accepted Answer · 2015-06-02 10:20:19Z

3

That is most likely to happen because you Release() the pointer than because .NET forgot to AddRef() before returning (hint: .NET surely did not forget this).

IColorRampPtr is itself a smart pointer, you must be using it because you #imported the .NET generated type library in VC++. As such, you should never call Release() on the smart pointers, as it'll be released when it goes out of scope (or, if used in a class member, when the object is destroyed).

If you want a raw pointer, on which you must later call Release(), use the raw interface pointer (e.g. IColorRamp*) or Detach() the smart pointer. Generally, you need a raw interface pointer when its scope becomes indeterminate. If the scope is well defined and you can use a smart pointer, keep it as a smart pointer.

answered Jun 2, 2015 at 10:20

acelent

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1 Comment

Andy Over a year ago

I am pretty sure I do not Release() the pointer. Since it is a smart pointer I assumed that it should take care of itself. The crash I encountered may come from some other place in the code, but the fact is that when I called AddRef() I could not reproduce it. I was thinking that the pure virtual function call exception may come from the Runtime Callable Wrapper?

محمد · Accepted Answer · 2015-06-02 09:14:30Z

2

From MSDN (https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.addref%28v=vs.80%29.aspx) :

The common language runtime manages the reference count of a COM object for you, making it unnecessary to use this method directly. In rare cases, such as testing a custom marshaler, you might find it necessary to manipulate an object's lifetime manually. After calling AddRef, you must decrement the reference count by using a method such as Marshal.Release. Do not rely on the return value of AddRef, as it can sometimes be unstable.

Worth a look?

answered Jun 2, 2015 at 9:14

محمد

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