IPropertyDescription::CoerceToCanonicalValue method (propsys.h)

Coerces the value to the canonical value, according to the property description.

Syntax

HRESULT CoerceToCanonicalValue(
  [in, out] PROPVARIANT *ppropvar
);

Parameters

[in, out] ppropvar

Type: PROPVARIANT*

On entry, contains a pointer to a PROPVARIANT structure that contains the original value. When this method returns, contains the canonical value.

Return value

Type: HRESULT

If the failure code is not INPLACE_S_TRUNCATED or E_INVALIDARG, then coercion from the value's type to the property description's type was not possible, and the PROPVARIANT structure has been cleared.

Possible results include the following:

Remarks

For more information, see the type attribute of the typeInfo element in the property's .propdesc file.

Most property descriptions specify the type that their values are expected to use. For example, the property description for System.Title specifies that System.Title values should use VT_LPWSTR. This method coerces values to this type, and coerces the result into a canonical form.

It is important to note that if this method fails, it will have already called the PropVariantClear on the input PROPVARIANT structure. Only if this method succeeds is the calling application responsible for calling the PropVariantClear on ppropvar when the structure is no longer needed.

The coercion performed by this method is also performed by the property system during IPropertyStore::GetValue and IPropertyStore::SetValue calls. Applications may depend on the property system to perform the coercions, or may use this method to perform the coercion at a time of the application's choosing.

The coercion is performed in four steps, as follows:

1. The following values are converted to VT_EMPTY.
   • Values of type VT_NULL.
   • Values of type VT_LPWSTR, VT_BSTR, or VT_LPSTR whose pointer is NULL.
   • Values of type VT_LPWSTR, VT_BSTR, or VT_LPSTR that are empty or consist entirely of spaces.
   • Values of type VT_FILETIME prior to midnight 1601/01/02.
2. If the value is not of type VT_EMPTY after Step 1, it is converted to the type specified by the property description. The type of a property description can be obtained using IPropertyDescription::GetPropertyType. See typeInfo for information about how the property schema influences the type of a property description. Conversions are performed as follows:
   • Values of type VT_LPWSTR, VT_BSTR, or VT_LPSTR are converted to VT_VECTOR | VT_LPWSTR using InitPropVariantFromStringAsVector.
   • All other conversions are performed using PropVariantChangeType
3. After Steps 2 and 3, the value is coerced into a canonical form based on its type. The canonical forms are summarized in the following table.

   Value Type	Canonical Form
   VT_EMPTY	Always canonical.
   VT_LPWSTR	No leading or trailing spaces. The string is non-empty. The string is non-NULL. For example, L"Alice". If this is a tree property (that is, if the typeInfo element's isTreeProperty attribute is TRUE), then it must not have leading or trailing forward slashes (/), must not have spaces between the text and the forward slashes, and must not have two consecutive forward slashes(/). For example, L"Friend/Bob" Coercion removes unnecessary characters and will result in VT_EMPTY if there was no content.
   VT_VECTOR | VT_LPWSTR	Each string in the vector must adhere to the rules for VT_LPWSTR listed above. In addition, the vector must have no duplicates and have no null pointers. If this is a tree property, then no value can be the ancestor of another value. For example, L"Friend" is an ancestor of L"Friend/Bob". If there is no content, coercion removes duplicate and ancestor characters and will result in VT_EMPTY.

    
4. If applicable, the value is checked against the property description type enumeration. The following checks apply.

   Enumeration Type	Value Type	Canonical Form
   Discrete or Ranged	VT_EMPTY	Always canonical
   Discrete	VT_LPWSTR	The string matches one of the enumerated strings allowed for the property. Comparisons are case-insensitive. If not, convert the value to VT_EMPTY.
   Discrete	Numeric	The number matches one of the enumerated values allowed for the property. If not, convert the value to VT_EMPTY.
   Discrete	VT_VECTOR | VT_LPWSTR	Each string in the vector matches one of the enumerated strings allowed for the property. Comparisons are case-insensitive. If not, remove that string from the vector. If the resulting vector is empty, convert the value to VT_EMPTY.
   Discrete	VT_VECTOR | Numeric	Each number in the vector matches one of the enumerated values allowed for the property. If not, remove that number from the vector. If the resulting vector is empty, convert the value to VT_EMPTY.
   Ranged	VT_LPWSTR	The string exists in the range allowed for the property. Comparisons are case-sensitive. If not, convert the value to VT_EMPTY.
   Ranged	Numeric	The number exists in the range allowed for the property. If not, convert the value to VT_EMPTY.
   Ranged	VT_VECTOR | VT_LPWSTR	Each string in the vector exists in the range allowed for the property. Comparisons are case-sensitive. If not, remove that string from the vector. If the resulting vector is empty, convert the value to VT_EMPTY.
   Ranged	VT_VECTOR | Numeric	Each number in the vector exists in the range allowed for the property. If not, remove that number from the vector. If the resulting vector is empty, convert the value to VT_EMPTY.

    

Requirements

See also

IPropertyDescription

Property Description Schema