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WS_UNION_DESCRIPTION structure (webservices.h)

  • Article

Information about the choices within a union type. This is used with WS_UNION_TYPE.

Syntax

typedef struct _WS_UNION_DESCRIPTION {
  ULONG                      size;
  ULONG                      alignment;
  WS_UNION_FIELD_DESCRIPTION **fields;
  ULONG                      fieldCount;
  ULONG                      enumOffset;
  int                        noneEnumValue;
  ULONG                      *valueIndices;
} WS_UNION_DESCRIPTION;

Members

size

The size in bytes of the structure.

alignment

The alignment requirement of the structure. This must be a power of two between 1 and 8.

fields

An array of pointers to the descriptions of the fields of the union.

See the Remarks section for information about ordering of the fields in this array.

fieldCount

The number of fields in the fields array. Any part of the structure that is not represented by a field will be left uninitialized. Fields descriptions may reference the same offset of the structure (for example if they are all part of a single union).

enumOffset

The offset of the enumeration field which controls which choice is selected within the union. The size of the field is assumed to be the size of an enumeration (32-bit signed integer).

noneEnumValue

This value corresponds to the enum value used when none of the choices are currently set. This field is only used when the field is optional (WS_FIELD_OPTIONAL was specified).

valueIndices

This optional array provides information which can improve the performance of looking up fields of the union either by element or by enum value. This array may NULL, in which case an O(n) lookup is used, which may be sufficient for small numbers of fields.

If non-NULL, the following must be true:

  • The fields array is required to be sorted by element, in ascending order. When comparing an element the namespace should be compared first, then the local name. Each of the names should be compared by performing a byte-wide comparison of the utf-8 string. The field that uses WS_ANY_ELEMENT_FIELD_MAPPING, if present, should always be last in the fields array.
  • The valueIndices array points to an array that has fieldCount items. The valueIndices array provides the indices of the items in the fields array as if they were sorted by value in ascending order.

Remarks

This description assumes a structure which contains both the selector value (an integer enumerated value) and a union which contains a field which corresponds to each of the possible choices, for example:

// Enumeration of choices of different values
enum Choice
{
    ChoiceA = 20,
    ChoiceB = 10,
    None = 0,
};

// Struct containing union of values, and enum "selector"
struct StructType
{
    Choice choice;
    union
    {
        int a;
        WS_STRING b;
    } value;
};

The following examples illustrates initializing a union description for the previous example. This example fills out the nameIndices field, but this field could be NULL instead.

WS_XML_STRING choiceAString = WS_XML_STRING_VALUE("choiceA");
WS_XML_STRING choiceANs = WS_XML_STRING_VALUE("http://examples.org/a");

WS_UNION_FIELD_DESCRIPTION fieldA = { };
fieldA.value = ChoiceA;
fieldA.field.localName = &choiceAString;
fieldA.field.ns = &choiceANs;
fieldA.field.type = WS_INT32_TYPE;
fieldA.field.offset = WsOffsetOf(StructType, value.a);

WS_XML_STRING choiceBString = WS_XML_STRING_VALUE("choiceB");
WS_XML_STRING choiceBNs = WS_XML_STRING_VALUE("http://examples.org/b");

WS_UNION_FIELD_DESCRIPTION fieldB = { };
fieldB.value = ChoiceB;
fieldB.field.localName = &choiceBString;
fieldB.field.ns = &choiceBNs;
fieldB.field.type = WS_STRING_TYPE;
fieldB.field.offset = WsOffsetOf(StructType, value.b);

// Sorted by ascending element name (first ns, then localName)
WS_UNION_FIELD_DESCRIPTION* fieldsArray[] =
{
    &fieldA, // "http://example.com/a", "choiceA"
    &fieldB, // "http://example.com/b", "choiceB"
};

// Sorted by ascending enum value
ULONG valueIndices[] =
{
    1, // ChoiceB (10)
    0, // ChoiceA (20)
};

WS_UNION_DESCRIPTION unionDescription;
unionDescription.size = sizeof(StructType);
unionDescription.alignment = __alignof(StructType);
unionDescription.fields = fieldsArray;
unionDescription.fieldCount = WsCountOf(fieldsArray);
unionDescription.enumOffset = WsOffsetOf(StructType, choice);
unionDescription.noneEnumValue = None;
unionDescription.valueIndices = valueIndices;

The above would allow either of the following elements to appear:

<choiceA xmlns="http://example.com/a">123</choiceA>
<choiceB xmlns="http://example.com/b">hello</choiceB>

The following is an example of setting values:

StructType structType;

// Set ChoiceA
structType.choice = ChoiceA;
structType.value.a = 123;

// Set ChoiceB
static const WS_STRING = WS_STRING_VALUE(L"hello");
structType.choice = ChoiceB;
structType.value.b = helloString;

// Set "none" choice
structType.choice = None;

The following is the grammar describing the order of the WS_FIELD_DESCRIPTION that make up a WS_UNION_DESCRIPTION. The order is defined based on the mapping field of the WS_FIELD_DESCRIPTION.


Fields := ElementContentFields AnyElementField?
ElementContentFields := (ElementField | RepeatingElementField)*
ElementField := WS_ELEMENT_FIELD_MAPPING
RepeatingElementField := WS_REPEATING_ELEMENT_FIELD_MAPPING
AnyElementField := WS_ANY_ELEMENT_FIELD_MAPPING

The WS_ELEMENT_FIELD_MAPPING and WS_REPEATING_ELEMENT_FIELD_MAPPING represent the element choices and their corresponding fields in the union.

The WS_ANY_ELEMENT_FIELD_MAPPING is the field used when none of the other elements matched.

The following restrictions apply to the field descriptions:

Requirements

Requirement Value
Minimum supported client Windows 7 [desktop apps only]
Minimum supported server Windows Server 2008 R2 [desktop apps only]
Header webservices.h