DISP_E_<\/A>BADVARTYPE<\/CODE>\nexception is raised.\n<\/P>\n\nOn the other hand, if the offending line is changed to\n<\/P>\n
\nCComQIPtr<IAwesome> pAwesome = sa.GetAt(i).punkVal;\n<\/PRE>\n\nthen the problem goes away.\n<\/P>\n
\nYour initial reaction to this code would be that\nthere is an off-by-one error in the loop control,\nbut it turns out that there isn’t because\nSAFEARRAY<\/CODE> uses inclusive upper bounds\nrather than exclusive.\n<\/P>\n\nThe first step in debugging this is seeing what is\nin the bad variant that makes the copy constructor\nthink it’s not a valid variant type.\n<\/P>\n
\nInspecting in the debugger shows that the variant\nreturned by GetAt<\/CODE> has a valid\npunk<\/CODE>, but the vt<\/CODE> is 0x1234<\/CODE>.\nWell, that’s not a valid variant type, so that’s the proximate\ncause of the problem.\n<\/P>\n\nHow did an invalid variant type get into your SAFEARRAY<\/CODE>?\n<\/P>\n\nAt this point the customer realized that maybe their code to create\nthe array was faulty,\nso they offered to share it.\n<\/P>\n
\nvoid GetAwesomeArray(SAFEARRAY **ppsa)\n{\n SAFEARRAY *psa = SafeArrayCreateVector(VT_UNKNOWN, 0, m_count);\n for (LONG i = 0; i < m_count; i++) {\n CComPtr<IAwesome> spAwesome;\n CreateAwesomeThing(i, &spAwesome);\n SafeArrayPutElement(psa, &i, spAwesome);\n }\n *ppsa = psa;\n}\n<\/PRE>\n\nOkay, now all the pieces fell into place.\n<\/P>\n
\nThe GetAwesomeArray<\/CODE>\nfunction is creating an array of VT_UNKNOWN<\/CODE>,\nbut the code fragment that calls\nGetAwesomeArray<\/CODE>\ntreats it as an array of VT_VARIANT<\/CODE>.\n<\/P>\n\nYour array of IUnknown*<\/CODE> is being misinterpreted\nas an array of VARIANT<\/CODE>.\nThat explains all the symptoms:\nThe vt<\/CODE> is wrong, because it’s really just the\nlow-order word of the first IUnknown*<\/CODE>.\nIgnoring the vt<\/CODE> and going straight for the\npunk<\/CODE> seems to work because that’s where\nthe second<\/I> IUnknown*<\/CODE> happens to be.\n(Or third, if you are compiling as 32-bit.)\n<\/P>\n\nIn other words, it’s as if you did a\nreinterpret_cast<VARIANT&>(punkArray[0])<\/CODE>.\n<\/P>\n\nIf you had used regular C-style arrays\nor a C++ collection,\nthen the compile-time type checking would have told you that\nyou mismatched the producer and consumer.\nBut since you went through a SAFEARRAY<\/CODE>,\nthat compile-time type information is lost,\nsince a\nSAFEARRAY<\/CODE> is a polymorphic array.\nIt now becomes your job to keep track of what you have an array of<\/I>,\nand its dimensions and bounds.\n<\/P>\n\nYou can keep track of this information via documentation,\n“This function returns a 1-dimensional\nSAFEARRAY<\/CODE> of VT_IUNKNOWN<\/CODE>,\nwith lower bound 0 and variable upper bound.”\nOr you can check at runtime, by calling\nSafeArrayGetVartype<\/CODE>\nto see what the base type is,\nand\nSafeGetDim<\/CODE> to see how many\ndimensions the array has,\nand\nSafeArrayGetLBound<\/CODE>\nand\nSafeArrayGetUBound<\/CODE>\nto obtain the upper and lower bounds for those dimensions.\n<\/P>\n\nThe code above seemed not to be sure which model it wanted\nto use.\nIt trusted the base type and the dimension,\nbut checked the upper and lower bounds.\n<\/P>\n
\nAnyway,\nassuming we are going with the “keep track via documentation”\napproach,\nthe solution for the original problem is to have\nthe producer and consumer agree on exactly what kind of\nSAFEARRAY<\/CODE> is being handed around.\nEither produce an array of VT_UNKNOWN<\/CODE> and\nconsume it as a CComSafeArray<IUnknown*><\/CODE>\nor produce an array of VT_VARIANT<\/CODE> and consume\nit as a\nCComSafeArray<VARIANT><\/CODE>.\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"A customer had a problem with SAFEARRAY, or more specifically, with CComSafeArray. CComSafeArray<VARIANT> sa; GetAwesomeArray(&sa); LONG lb = sa.GetLowerBound(); LONG ub = sa.GetUpperBound(); for (LONG i = lb; i <= ub; i++) { CComVariant item = sa.GetAt(i); … use the item … } The GetAt method returns a VARIANT&, and when it is copy-constructed into […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-5443","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A customer had a problem with SAFEARRAY, or more specifically, with CComSafeArray. CComSafeArray<VARIANT> sa; GetAwesomeArray(&sa); LONG lb = sa.GetLowerBound(); LONG ub = sa.GetUpperBound(); for (LONG i = lb; i <= ub; i++) { CComVariant item = sa.GetAt(i); … use the item … } The GetAt method returns a VARIANT&, and when it is copy-constructed into […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/5443","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=5443"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/5443\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=5443"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=5443"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=5443"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}