how to pass a byte array from a CSharp client application to an ATL COM object

Question

I want to write an application with csharp which reads the content of a file and passes it to an ATL COM object.

I have a problem with IDL. I defined a method named "Decode" on an ATL-COM object like below:

[id(101), helpstring("Decode")] HRESULT Decode([in] BYTE* pBuff, [in] INT nLen, [out, retval] INT* pnRetVal);

But, when I compiled my own COM dll and wanted to use it in the CSharp application I found the method with this signature:

int MyClass.Decode(ref byte pBuff, int nLen);

But, I excepted to have a method like this:

    int MyClass.Decode(byte[] pBuff, int nLen);

Do you have any suggestions? I also tried different types such as VARIANT* or BYTE**, but I look for best solution.

Pointers to C arrays are not Automation compatible types. They are indistinguishable from, in your case, a single byte passed by reference. While IDL has a way to attribute the argument to indicate it is an array, type libraries don't have that ability. Technically you can decompile the interop library and edit the method definition. — Hans Passant
– Hans Passant, Commented Aug 5, 2011 at 17:50

Alexandre C. · Accepted Answer · 2011-08-05 12:26:38Z

2

According to this link, you have to declare your function as taking a SAFEARRAY(byte) inside the IDL.

Also, passing a VARIANT (which shows as object in the C# side) will work, provided you extract the SAFEARRAY inside the C++ code.

answered Aug 5, 2011 at 12:26

Alexandre C.

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2 Comments

Olórin Over a year ago

I declared a function as taking a VARIANT* pdata in the idl and doign this : pdata->bstrVal = SysAllocString(L"TOTO"); When I pass (from c#) a string "TUTU", "TUTU" is return, not "TOTO", which is not the attended behaviour according to the link you posted. Any idea ?

Olórin Over a year ago

(Forgot to say that VARIANT* pdata is [in,out] in the idl.)

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