.NET COM Interop Method Signature

Question

What interop signature would you use for the following COM method? I am interested particularly in the final two parameters, and whether to try to use MarshalAs with a SizeParamIndex or not.

HRESULT GetOutputSetting(
  DWORD    dwOutputNum,
  LPCWSTR  pszName,
  WMT_ATTR_DATATYPE*  pType,
  BYTE*    pValue,
  WORD*    pcbLength
);

Documentation states:

pValue [out] Pointer to a byte buffer containing the value. Pass NULL to retrieve the length of the buffer required.

pcbLength [in, out] On input, pointer to a variable containing the length of pValue. On output, the variable contains the number of bytes in pValue used.

Answer 1

You could try the PInvoke Signature Toolkit. It's rather useful for getting marshaling right when performing platform interops. It quite possibly won't cover your particular problem, but you may find a comparable one that gives you the information you seek.

Answer 2

I would use the SizeParamIndex, because your scenario is exactly the one this feature is for: To specify the length of a variable sized array.

So the last to parameters would be in C# signature:

byte[] pValue,
ref ushort pcbLength

The byte-Array is passed without ref, because the array corresponds to a pointer in native code. If you pass NULL (or null in C#) for pValue in order to retrieve the size of the buffer needed. That means also that the caller has to allocate the byte-Array. The parameter pcbLength is passed by ref, because it is used as an in/out-parameter.