How do you pass an array to a COM object using interop?

I need to extend an existing COM object to add a function that takes as input an array of bytes, which is readonly. I haven't done COM in a while and I don't remember the gory details so hopefully someone here can help me fill in the blanks.

The interface definition looks something like this:

[object, dual, uuid("12345678-1234-1234-1234-123456789ABC") ]
__interface IMyFoo {
  [id(1), helpstring("method foo1")] HRESULT foo1([in] BSTR str, [out,retval] BSTR* result);
  [id(2), helpstring("method foo2")] HRESULT foo2([in] BYTE buf[], [in] LONG numBytes, [out,retval] BSTR* result);  // <-- #1 below
};

[ coclass, uuid("87654321-4321-4321-4321-CBA987654321"), default(IMyFoo) ]
class CMyFoo : public IMyFoo
{
public:
  CMyFoo(void);
  ~CMyFoo void);
  STDMETHOD(foo1)(BSTR str, LONG* result);
  STDMETHOD(foo2)(BYTE []buf, LONG numBytes, BSTR* result);  // <--#1 below
};

The implementation of foo2 is conceptually something like:

STDMETHODIMP CMyFoo::foo2(BYTE []buf, LONG numBytes, BSTR* result)
{

  int sum = 0;
  for (int i=0; i<numBytes; i++) {
     // process buf here
     sum +=  buf[i]; // <--- #2 below
  }
 
  char outstr[50];
  sprintf(outstr, "answer is %ld", sum);

  *result = ::SysAllocString((OLECHAR *) outstr);

  return S_OK;
}

From the C# side, I want to do something like:

byte[] buf = new byte[1234];
// buf gets data in it, not important how

mycom.CMyFooClass myfoo = new mycomCMyFooClass();
string sum= myfoo.foo2(buf, buf.length);  // <--- #3 below

I've already got everything working to call the foo1() function in C#, but I need help with

#1) setting up the interface to be able to pass an array of bytes in foo2()
#2) accessing that array within the COM object function foo2() - don't need to modify it, just access it
#3) how do I call foo2() from the C# side to pass the array

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