macOS下从C动态库向Excel VBA返回Variant的实现问题

问题场景 #

尝试参考C++项目实现从动态库向Excel VBA返回Variant类型，但用C编写的代码返回的Variant在VBA中始终显示为Empty。通过内存检查发现，返回的Variant中vt字段值为0，无法正确标识数据类型。

原C代码 #

typedef uint16_t VARTYPE;  
typedef uint16_t WORD; 
typedef double DOUBLE;

typedef struct {
    VARTYPE vt;
    WORD    wReserved1;
    WORD    wReserved2;
    WORD    wReserved3;
    union {
        DOUBLE       dblVal;
    } data;
} VARIANT2;

enum VARENUM {
    VT_DOUBLE       = 0x0005,
};

VARIANT2 * new_VT_DOUBLE_PTR(double x) {
    VARIANT2 *answer = (VARIANT2 *) malloc(sizeof(VARIANT2));
    answer->vt = VT_DOUBLE;
    answer->data.dblVal = x;
    return answer;
}

VARIANT2 new_VT_DOUBLE(double x) {
    VARIANT2 answer;
    answer.vt = VT_DOUBLE;
    answer.data.dblVal = x;
    return answer;
}

原VBA声明与调用代码 #

Private Declare PtrSafe Function jinni_new_VT_DOUBLE _
Lib "/Library/Application Support/Microsoft/jinni.dylib" Alias "new_VT_DOUBLE" _
(ByVal x As Double) As Variant
Private Declare PtrSafe Function jinni_new_VT_DOUBLE_PTR _
Lib "/Library/Application Support/Microsoft/jinni.dylib" Alias "new_VT_DOUBLE_PTR" _
(ByVal x As Double) As Variant

Function newDoublePtr(x As Double) As Variant
    newDoublePtr = jinni_new_VT_DOUBLE_PTR(x)
End Function

Function newDouble(x As Double) As Variant
    newDouble = jinni_new_VT_DOUBLE(x)
End Function

问题根因 #

自定义的VARIANT2结构体内存布局与Excel VBA中Variant的实际布局不匹配：VBA的Variant实际占用24字节，而原结构体仅定义了16字节，导致内存错位，vt字段的值无法被VBA正确解析。

解决方法 #

在VARIANT2结构体中添加填充字段，使其总大小达到24字节。修改后的结构体定义如下：

typedef struct {
    VARTYPE vt;
    WORD    wReserved1;
    WORD    wReserved2;
    WORD    wReserved3;
    union {
        DOUBLE       dblVal;
    } data;
    WORD    padding[4]; // 添加填充字段补足至24字节
} VARIANT2;

修改后，无论是通过栈直接返回VARIANT2实例，还是返回VARIANT2指针，对应的Variant都能被VBA正确识别，不再显示为Empty。

内容的提问来源于stack exchange，提问作者DangerMouse