PSCoerceToCanonicalValue function (propsys.h)

Article
10/13/2021

Converts the value of a property to the canonical value, according to the property description.

Syntax

PSSTDAPI PSCoerceToCanonicalValue(
  [in]      REFPROPERTYKEY key,
  [in, out] PROPVARIANT    *ppropvar
);

Parameters

[in] key

Type: REFPROPERTYKEY

Reference to a PROPERTYKEY structure that identifies the property whose value is to be coerced.

[in, out] ppropvar

Type: PROPVARIANT*

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

Return value

Type: HRESULT

Possible return values include the following:

Return code	Description
S_OK	The function succeeded. The property value specified by ppropvar is now in a canonical form.
INPLACE_S_TRUNCATED	The property value specified by ppropvar is now in a truncated, canonical form.
E_INVALIDARG	The ppropvar parameter is invalid. The PROPVARIANT structure has been cleared.
TYPE_E_TYPEMISMATCH	Coercion from the value's type to the property description's type was not possible. The PROPVARIANT structure has been cleared.
Any other failure code	Coercion from the value's type to the property description's type was not possible. The PROPVARIANT structure has been cleared.

Remarks

This function is a wrapper around the system's implementation of IPropertyDescription::CoerceToCanonicalValue.

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 be of type VT_LPWSTR. This function coerces values to this type, and then coerces the result into a canonical form.

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

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

The coercion is performed in four steps, as follows:

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.
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 by calling IPropertyDescription::GetPropertyType. For information on how the property schema influences the type of a property description, see typeInfo. 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

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 and 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 results 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 results in VT_EMPTY.

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

Examples

The following example, to be included as part of a larger program, demonstrates how to use PSCoerceToCanonicalValue to coerce a value to the type required for PKEY_Keywords.

// PROPVARIANT propvar;
// Assume variable propvar is initialized and valid.

HRESULT hr = PSCoerceToCanonicalValue(PKEY_Keywords, &propvar);

if (SUCCEEDED(hr))
{
    // The conversion succeeded and propvar now is of the correct type for 
    // PKEY_Keywords, or VT_EMPTY.
    PropVariantClear(&propvar);
}
else
{
    // The conversion failed and propvar is now VT_EMPTY.
}

Requirements

Requirement	Value
Minimum supported client	Windows XP with SP2, Windows Vista [desktop apps only]
Minimum supported server	Windows Server 2003 with SP1 [desktop apps only]
Target Platform	Windows
Header	propsys.h
Library	Propsys.lib
DLL	Propsys.dll (version 6.0 or later)
Redistributable	Windows Desktop Search (WDS) 3.0

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