oledb32: Support VT_NULL when converting to DBTYPE_BSTR.

dlls/oledb32/convert.c

@@ -534,9 +534,18 @@ static HRESULT WINAPI convert_DataConvert(IDataConvert* iface,
         }
         break;
         case DBTYPE_VARIANT:
-            VariantInit(&tmp);
-            if ((hr = VariantChangeType(&tmp, (VARIANT*)src, 0, VT_BSTR)) == S_OK)
-                *d = V_BSTR(&tmp);
+            if(V_VT((VARIANT*)src) == VT_NULL)
+            {
+                *dst_status = DBSTATUS_S_ISNULL;
+                *dst_len = get_length(DBTYPE_BSTR);
+                return S_OK;
+            }
+            else
+            {
+                VariantInit(&tmp);
+                if ((hr = VariantChangeType(&tmp, (VARIANT*)src, 0, VT_BSTR)) == S_OK)
+                    *d = V_BSTR(&tmp);
+            }
             break;
         default: FIXME("Unimplemented conversion %04x -> BSTR\n", src_type); return E_NOTIMPL;
         }

dlls/oledb32/tests/convert.c

@@ -977,6 +977,14 @@ static void test_converttobstr(void)
     ok(!lstrcmpW(b, dst), "got %s\n", wine_dbgstr_w(dst));
     SysFreeString(dst);
     SysFreeString(b);
+
+    V_VT(&v) = VT_NULL;
+    dst = (void*)0x1234;
+    hr = IDataConvert_DataConvert(convert, DBTYPE_VARIANT, DBTYPE_BSTR, 0, &dst_len, &v, &dst, sizeof(dst), 0, &dst_status, 0, 0, 0);
+    ok(hr == S_OK, "got %08x\n", hr);
+    ok(dst_status == DBSTATUS_S_ISNULL, "got %08x\n", dst_status);
+    ok(dst_len == sizeof(BSTR), "got %ld\n", dst_len);
+    ok(dst == (void*)0x1234, "got %p\n", dst);
 }
 
 static void test_converttowstr(void)