\nCOM methods cannot be variadic.\nWhy not?\n<\/p>\n
\nAnswer:\nBecause the marshaler doesn’t know when to stop.\n<\/p>\n
\nSuppose variadic COM methods were possible.\nAnd then you wrote this code:\n<\/p>\n
\ninterface IVariadic\n{\n HRESULT Mystery([in] int code, ...);\n};\nIVariadic *variadic = something;\nuint32_t ipaddr;\nHRESULT hr = variadic->Mystery(9, 192, 168, 1, 1, &ipaddr);\n<\/pre>\n\nHow would COM know how to marshal this function call?\nIn other words, suppose that variadic<\/code>\nis a pointer to a proxy that refers to an object in\nanother process.\nThe COM marshaler needs to take all the parameters\nto IVariadic::Mystery<\/code>,\npackage them up,\nsend them to the other process,\nthen unpack the parameters,\nand pass them to the implementation.\nAnd then when the implementation returns,\nit needs to take the return value\nand any output parameters,\npackage them up,\nsend them back to the originating process,\nwhere they are unpacked and applied to the\noriginal parameters.\n<\/p>\n\nConsider, for example,\n<\/p>\n
\ninterface IDyadic\n{\n HRESULT Enigma([in] int a, [out] int *b);\n};\nIDyadic *dyadic = something;\nint b;\nHRESULT hr = dyadic->Enigma(1, &b);\n<\/pre>\n\nIf dyadic<\/code> refers to an object\nin another process,\nthe marshaler does this:\n<\/p>\n\n- Allocate a block of memory containing the\n following information:<\/p>\n
\n- Information to identify the
dyadic<\/code>\n object in the other process,<\/p>\n- the integer 1.\n <\/ul>\n
- Transmit that block of memory to the other process.\n<\/ul>\n
\nThe other process receives the block of memory and\ndoes the following:\n<\/p>\n
\n- Use the information in the memory block\n to identify the
dyadic<\/code> object.<\/p>\n- Extract the parameter
1<\/code> from the\n memory block.<\/p>\n- Allocate a local integer variable, call it
x<\/code>.\n- Call
dyadic->Enigma(1, &x)<\/code>.\n Let’s say that the function stores 42 into x<\/code>,\n and it returns E_PENDING<\/code>.<\/p>\n- Allocate a block of memory containing the following\n information:<\/p>\n
\n- The value
E_PENDING<\/code>\n (the HRESULT<\/code> returned by\n dyadic->Enigma<\/code>),<\/p>\n- The integer 42 (the value that
dyadic->Enigma<\/code>\n stored in the local variable x<\/code>).\n <\/ul>\n- Transmit that block of memory to the originating process.\n<\/ul>\n
\nThe originating process receives the block of memory\nand does the following:\n<\/p>\n
\n- Extracts the
HRESULT<\/code> E_PENDING<\/code>.\n- Extracts the value 42.\n
- Stores the value 42 into
b<\/code>.\n- Returns the value
E_PENDING<\/code> to the caller.\n<\/ul>\n\nNote that in order for the marshaler to do its job,\nit needs to know every parameter to the method,\nwhether that parameter is an input parameter\n(which is sent from the originating process to the remote process),\nan output parameter\n(which is sent from the remote process to the originating process),\nand how to send that parameter.\nIn our case, the parameter is just an integer, so sending it is just\ncopying the bits,\nbut in the more general case, the parameter could be a more complicated\ndata structure.\n<\/p>\n
\nNow let’s look at that variadic method again.\nHow is the marshaler supposed to know what to do with the\n...<\/code>?\nIt doesn’t know how many parameters it needs to transfer.\nIt doesn’t know what types those parameters are.\nIt doesn’t know which ones are input parameters\nand which ones are output parameters.\n<\/p>\n\nIn order to know that, it would have to reverse-engineer the\nimplementation of the IVariadic::Mystery<\/code>\nfunction and figure out that the first parameter, the\n\nnumber 9<\/a>,\nis a code that means that the method takes four 8-bit integers as\ninput and outputs a 32-bit integer.\n<\/p>\n\nThis is a rather tall order for the client side of the marshaler,\nsince it has to do its work without access\nto the other process.\nIt would have to use its psychic powers to figure out how to package\nup the parameters, as well as how to unpack them afterward.\n<\/p>\n
\nTherefore, COM says,\n“Sorry, you can’t do that.”\n<\/p>\n
\nBut what you can do is encode the parameters in a form that\nthe marshaler understands.\nFor example, you might use a counted array of VARIANT<\/code>s\nor a SAFEARRAY<\/code>.\nThe COM folks already did the work to teach the marshaler how\nto, for example,\ndecode the vt<\/code> member of the VARIANT<\/code>\nand understand that,\n“Oh, if the value is VT_I4<\/code>, then the VARIANT<\/code>\ncontains a 32-bit signed integer.”\n<\/p>\n