ADO对象——获取数据库字段的数据类型 .

如果想根据字段的数据类型不同，进行不同的操作，就必须知道字段的数据类型。

例如：

如果字段的数据类型是日期：则Format类似于***年***月***日的字符串。

如果字段的数据类型是时间：则Format类似于***:***:***的字符串

本以为使用VARIANT类型的变量获得字段的值，VARIANT中的数据类型就可以区分。

VARIANT的数据类型如下：（在wtypes.h中声明）

* VARENUM usage key,
*
* * [V] - may appear in a VARIANT
* * [T] - may appear in a TYPEDESC
* * [P] - may appear in an OLE property set
* * [S] - may appear in a Safe Array
*
*
* VT_EMPTY [V] [P] nothing
* VT_NULL [V] [P] SQL style Null
* VT_I2 [V][T][P][S] 2 byte signed int
* VT_I4 [V][T][P][S] 4 byte signed int
* VT_R4 [V][T][P][S] 4 byte real
* VT_R8 [V][T][P][S] 8 byte real
* VT_CY [V][T][P][S] currency
* VT_DATE [V][T][P][S] date
* VT_BSTR [V][T][P][S] OLE Automation string
* VT_DISPATCH [V][T] [S] IDispatch *
* VT_ERROR [V][T][P][S] SCODE
* VT_BOOL [V][T][P][S] True=-1, False=0
* VT_VARIANT [V][T][P][S] VARIANT *
* VT_UNKNOWN [V][T] [S] IUnknown *
* VT_DECIMAL [V][T] [S] 16 byte fixed point
* VT_RECORD [V] [P][S] user defined type
* VT_I1 [V][T][P][s] signed char
* VT_UI1 [V][T][P][S] unsigned char
* VT_UI2 [V][T][P][S] unsigned short
* VT_UI4 [V][T][P][S] unsigned long
* VT_I8 [T][P] signed 64-bit int
* VT_UI8 [T][P] unsigned 64-bit int
* VT_INT [V][T][P][S] signed machine int
* VT_UINT [V][T] [S] unsigned machine int
* VT_INT_PTR [T] signed machine register size width
* VT_UINT_PTR [T] unsigned machine register size width
* VT_VOID [T] C style void
* VT_HRESULT [T] Standard return type
* VT_PTR [T] pointer type
* VT_SAFEARRAY [T] (use VT_ARRAY in VARIANT)
* VT_CARRAY [T] C style array
* VT_USERDEFINED [T] user defined type
* VT_LPSTR [T][P] null terminated string
* VT_LPWSTR [T][P] wide null terminated string
* VT_FILETIME [P] FILETIME
* VT_BLOB [P] Length prefixed bytes
* VT_STREAM [P] Name of the stream follows
* VT_STORAGE [P] Name of the storage follows
* VT_STREAMED_OBJECT [P] Stream contains an object
* VT_STORED_OBJECT [P] Storage contains an object
* VT_VERSIONED_STREAM [P] Stream with a GUID version
* VT_BLOB_OBJECT [P] Blob contains an object
* VT_CF [P] Clipboard format
* VT_CLSID [P] A Class ID
* VT_VECTOR [P] simple counted array
* VT_ARRAY [V] SAFEARRAY*
* VT_BYREF [V] void* for local use
* VT_BSTR_BLOB Reserved for system use
*/

但有些情况不满足，比如VT_DATE代表日期和时间，却没有把日期和时间分开定义。

使用ADO对象中的数据类型获得字段的数据类型。

DataTypeEnum adoType;
adoType = pRecordset->GetFields()->GetItem( _variant_t( strFieldName ) )->Type;
DataTypeEnum枚举类型在msado15.tlh中定义。

DataTypeEnum
{
adEmpty = 0,
adTinyInt = 16,
adSmallInt = 2,
adInteger = 3,
adBigInt = 20,
adUnsignedTinyInt = 17,
adUnsignedSmallInt = 18,
adUnsignedInt = 19,
adUnsignedBigInt = 21,
adSingle = 4,
adDouble = 5,
adCurrency = 6,
adDecimal = 14,
adNumeric = 131,
adBoolean = 11,
adError = 10,
adUserDefined = 132,
adVariant = 12,
adIDispatch = 9,
adIUnknown = 13,
adGUID = 72,
adDate = 7,
adDBDate = 133,
adDBTime = 134,
adDBTimeStamp = 135,
adBSTR = 8,
adChar = 129,
adVarChar = 200,
adLongVarChar = 201,
adWChar = 130,
adVarWChar = 202,
adLongVarWChar = 203,
adBinary = 128,
adVarBinary = 204,
adLongVarBinary = 205,
adChapter = 136,
adFileTime = 64,
adPropVariant = 138,
adVarNumeric = 139,
adArray = 8192
};

可以看到，这里面日期和时间就分开定义了，分别为adDBDate和adDBTime。