MFC中使用COleVariant获取CMFCPropertyGridProperty属性窗口某个属性值

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  获取MFC属性窗口CMFCPropertyGridProperty中某个item的值时，如果不小心写错了类型，就会导致获取的结果不正确，原因就是COleVariant其实继承自一个特殊的结构体tagVARIANT。

• COleVariant类声明

class COleVariant : public tagVARIANT
{
// Constructors
public:

    COleVariant();
 ...

    COleVariant(BYTE nSrc);
    COleVariant(short nSrc, VARTYPE vtSrc = VT_I2);
    COleVariant(long lSrc, VARTYPE vtSrc = VT_I4);
    COleVariant(const COleCurrency& curSrc);

    COleVariant(LONGLONG nSrc);
    COleVariant(ULONGLONG nSrc);

    COleVariant(float fltSrc);
    COleVariant(double dblSrc);
    COleVariant(const COleDateTime& timeSrc);

    COleVariant(const CByteArray& arrSrc);
    COleVariant(const CLongBinary& lbSrc);
    COleVariant(LPCITEMIDLIST pidl);
public:

    const COleVariant& operator=(BYTE nSrc);
    const COleVariant& operator=(short nSrc);
    const COleVariant& operator=(long lSrc);
    const COleVariant& operator=(const COleCurrency& curSrc);

    const COleVariant& operator=(LONGLONG nSrc);
    const COleVariant& operator=(ULONGLONG nSrc);

    const COleVariant& operator=(float fltSrc);
    const COleVariant& operator=(double dblSrc);
    const COleVariant& operator=(const COleDateTime& dateSrc);

// Implementation
public:
    ~COleVariant();
};

• tagVARIANT定义

struct tagVARIANT
    {
    union 
        {
        struct __tagVARIANT
            {
            VARTYPE vt;
            WORD wReserved1;
            WORD wReserved2;
            WORD wReserved3;
            union 
                {
                LONGLONG llVal;
                LONG lVal;
                BYTE bVal;
                SHORT iVal;
                FLOAT fltVal;
                DOUBLE dblVal;
                VARIANT_BOOL boolVal;
                _VARIANT_BOOL bool;
                SCODE scode;
                CY cyVal;
                DATE date;
                BSTR bstrVal;
                IUnknown *punkVal;
                IDispatch *pdispVal;
                SAFEARRAY *parray;
                BYTE *pbVal;
                SHORT *piVal;
                LONG *plVal;
                LONGLONG *pllVal;
                FLOAT *pfltVal;
                DOUBLE *pdblVal;
                VARIANT_BOOL *pboolVal;
                _VARIANT_BOOL *pbool;
                SCODE *pscode;
                CY *pcyVal;
                DATE *pdate;
                BSTR *pbstrVal;
                IUnknown **ppunkVal;
                IDispatch **ppdispVal;
                SAFEARRAY **pparray;
                VARIANT *pvarVal;
                PVOID byref;
                CHAR cVal;
                USHORT uiVal;
                ULONG ulVal;
                ULONGLONG ullVal;
                INT intVal;
                UINT uintVal;
                DECIMAL *pdecVal;
                CHAR *pcVal;
                USHORT *puiVal;
                ULONG *pulVal;
                ULONGLONG *pullVal;
                INT *pintVal;
                UINT *puintVal;
                struct __tagBRECORD
                    {
                    PVOID pvRecord;
                    IRecordInfo *pRecInfo;
                    }   __VARIANT_NAME_4;
                }   __VARIANT_NAME_3;
            }   __VARIANT_NAME_2;
        DECIMAL decVal;
        }   __VARIANT_NAME_1;
    } ;

可以看到tagVARIANT内部是一个union，union的大小是里面成员最大的一个的大小，union是共享内存的，在某个时刻只能有一个值是有效的。
因此在用COleVariant获取属性值时，最初的属性值设置为什么类型就要用tagVARIANT对应的成员去获取，如果short用double去获取，获取出来的结果是错误的。

    // float类型
    CMFCPropertyGridProperty* pPropItem1； 
    pPropItem1 = new CMFCPropertyGridProperty(_T("float类型"), (_variant_t)(float)12.3, _T(""));
    COleVariant val1 = pPropItem1->GetValue();
    float fValue = val1.fltVal;
    // double类型
    CMFCPropertyGridProperty* pPropItem2； 
    pPropItem2 = new CMFCPropertyGridProperty(_T("double类型"), (_variant_t)(double)212.3, _T(""));
    COleVariant val2 = pPropItem2->GetValue();
    double dValue = val2.dblVal;
    // long类型
    CMFCPropertyGridProperty* pPropItem3； 
    pPropItem3 = new CMFCPropertyGridProperty(_T("long类型"), (_variant_t)(long)100, _T(""));
    COleVariant val3 = pPropItem3->GetValue();
    long lValue = val3.lVal;

• union

union MyUnion
{
    int n;
    float f;
    double d;
};

int main(int argc, _TCHAR* argv[])
{

    MyUnion un;
    un.n = 10;    // 此时联合体中n为10
    un.d = 3.14;  // 此时联合体中d为3.14，而n为0
    int n;
    n = un.n;
    return 0;
}