VariantChangeType - OLE Programmer's Reference (ole.hlp)

VariantChangeType

pvargDest, pvargSrc, wFlags, vtNew)
VARIANTARG FAR* pvargDest
VARIANTARG FAR* pvargSrc
unsigned short wFlags
VARTYPE vtNew

This function converts a variant from one type to another.

Parameters
pvargDest
Pointer to the VARIANTARG to receive the coerced type. If this is the same as pvargSrc, the variant will be converted in place.

pvargSrc
Pointer to the source VARIANTARG to be coerced.

wFlags
Flags that control the coercion. The only defined flag is VARIANT_NOVALUEPROP, which prevents the function from attempting to coerce an object to a fundamental type by getting the Value property. Applications should set this flag only if necessary, because it makes their behavior inconsistent with other applications.

tNew

The type to coerce to. If the return code is S_OK, the vt field of the *pvargDest will always be the same as this value.

Return Value
The SCODE obtained from the returned HRESULT is one of the following:

SCODE	Meaning
S_OK	Success.
DISP_E_BADVARTYPE	The variant type vtNew is not a valid type of variant.
DISP_E_OVERFLOW	The data pointed to by pvargSrc does not fit in the destination type.
DISP_E_TYPEMISMATCH	The argument could not be coerced to the specified type.
E_INVALIDARG	One of the arguments is invalid.
E_OUTOFMEMORY	Memory could not be allocated for the conversion.

Comments
The VariantChangeType function handles coercions between the fundamental types (including numeric-to-string and string-to-numeric coercions). A variant that has VT_BYREF set is coerced to a value by fetching the referenced value. An object is coerced to a value by invoking the object's Value property (DISPID_VALUE).

Typically, the implementor of IDispatch::Invoke determines which member is being accessed, then calls VariantChangeType to get the value of one or more arguments. For example, if the IDispatch call specifies a SetTitle member that takes one string argument, the implementor would call VariantChangeType to attempt to coerce the argument to VT_BSTR. If VariantChangeType did not return an error, the argument could then be fetched directly from the bstrVal field of the VARIANTARG. If VariantChangeType returned DISP_E_TYPEMISMATCH, the implementor would set *puArgErr to 0 (indicating the argument in error) and return DISP_E_TYPEMISMATCH from IDispatch::Invoke.

Note that you should not attempt to change the type of a VARIANTARG in the rgvarg array in place.

Arrays of one type can't be converted to arrays of another type with this function.