feat(client-rust): add write_array_elements default-fill helper and document semantics

Handles the new MxSparseArray wire type (proto field 19 on MxValue::Kind):
- value.rs: map SparseArrayValue to MxValueProjection::Unset (write-only; never emitted on read path)
- session.rs: add write_array_elements() that builds the sparse proto value and delegates to write()
- tests: three unit tests asserting proto shape, empty-elements case, and read-path Unset projection
- README: document write_array_elements default-fill semantics and bare-name [] normalisation

clients/rust/README.md

@@ -168,6 +168,39 @@ the unchanged elements included. For example, to change 2 elements of a
 the 2 new ones). Sending only the 2 changed values overwrites the attribute
 with a 2-element array.
 
+#### Default-fill partial array writes
+
+When you only need to set a handful of indices and want every other position to
+take the element type's default (zero / `false` / empty string / Unix epoch for
+timestamps), use `Session::write_array_elements` instead:
+
+```rust
+// Write a 10-element integer array; index 0 = 42, index 7 = 99,
+// all other indices default to 0 (not preserved from the previous value).
+session
+    .write_array_elements(
+        server_handle,
+        item_handle,
+        MxDataType::Integer,
+        10,
+        [(0, MxValue::int32(42)), (7, MxValue::int32(99))],
+        user_id,
+    )
+    .await?;
+```
+
+The gateway expands the sparse representation into a full `MxArray` before
+forwarding to the worker — the worker and MXAccess COM never see the sparse
+form. Unmentioned indices are reset to the type default, **not** preserved from
+the existing attribute value.
+
+#### Bare-name array AddItem normalisation
+
+`AddItem` for a bare array attribute name (e.g. `Tank01.Temperature`) is
+automatically normalised to `Tank01.Temperature[]` by the gateway so the
+worker can resolve the full array. You do not need to append `[]` in client
+code; the gateway handles it.
+
 ## Galaxy Repository browse
 
 The Galaxy Repository service exposes a read-only browse over the AVEVA System

clients/rust/src/session.rs

@@ -17,12 +17,12 @@ use crate::generated::mxaccess_gateway::v1::mx_command_reply;
 use crate::generated::mxaccess_gateway::v1::{
     AddItem2Command, AddItemBulkCommand, AddItemCommand, AdviseCommand, AdviseItemBulkCommand,
     BulkReadResult, BulkWriteResult, CloseSessionRequest, MxCommand, MxCommandKind, MxCommandReply,
-    MxCommandRequest, MxValue as ProtoMxValue, OpenSessionRequest, ReadBulkCommand,
-    RegisterCommand, RemoveItemBulkCommand, RemoveItemCommand, StreamEventsRequest,
-    SubscribeBulkCommand, SubscribeResult, UnAdviseCommand, UnAdviseItemBulkCommand,
-    UnsubscribeBulkCommand, Write2BulkCommand, Write2BulkEntry, Write2Command, WriteBulkCommand,
-    WriteBulkEntry, WriteCommand, WriteSecured2BulkCommand, WriteSecured2BulkEntry,
-    WriteSecuredBulkCommand, WriteSecuredBulkEntry,
+    MxCommandRequest, MxDataType, MxSparseArray, MxSparseElement, MxValue as ProtoMxValue,
+    OpenSessionRequest, ReadBulkCommand, RegisterCommand, RemoveItemBulkCommand, RemoveItemCommand,
+    StreamEventsRequest, SubscribeBulkCommand, SubscribeResult, UnAdviseCommand,
+    UnAdviseItemBulkCommand, UnsubscribeBulkCommand, Write2BulkCommand, Write2BulkEntry,
+    Write2Command, WriteBulkCommand, WriteBulkEntry, WriteCommand, WriteSecured2BulkCommand,
+    WriteSecured2BulkEntry, WriteSecuredBulkCommand, WriteSecuredBulkEntry,
 };
 use crate::value::MxValue;
 
@@ -547,6 +547,62 @@ impl Session {
         Ok(())
     }
 
+    /// Write a sparse, default-filled array: only the given elements
+    /// (index → scalar value) are set; every unmentioned index up to
+    /// `total_length` is written as the element type's default (a reset,
+    /// **not** a preserve). The gateway expands the sparse representation into
+    /// a whole-array write before forwarding to the worker.
+    ///
+    /// This is a convenience wrapper around [`Session::write`] that builds the
+    /// `MxSparseArray` wire value for you. Call [`Session::write`] directly
+    /// if you need to pass a pre-built [`MxValue`] carrying a full
+    /// `MxArray`.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`Error::InvalidArgument`] (propagated from the gateway) if
+    /// `total_length` is zero, exceeds the gateway's maximum array length, or
+    /// any element index is out of range. Returns [`Error::Command`] for
+    /// non-OK worker statuses, plus the usual transport/status errors.
+    pub async fn write_array_elements(
+        &self,
+        server_handle: i32,
+        item_handle: i32,
+        element_data_type: MxDataType,
+        total_length: u32,
+        elements: impl IntoIterator<Item = (u32, MxValue)>,
+        user_id: i32,
+    ) -> Result<(), Error> {
+        use crate::generated::mxaccess_gateway::v1::mx_value::Kind;
+
+        let sparse_elements: Vec<MxSparseElement> = elements
+            .into_iter()
+            .map(|(index, value)| MxSparseElement {
+                index,
+                value: Some(value.into_proto()),
+            })
+            .collect();
+
+        let sparse_value = ProtoMxValue {
+            data_type: element_data_type as i32,
+            variant_type: String::new(),
+            kind: Some(Kind::SparseArrayValue(MxSparseArray {
+                element_data_type: element_data_type as i32,
+                total_length,
+                elements: sparse_elements,
+            })),
+            ..ProtoMxValue::default()
+        };
+
+        self.write(
+            server_handle,
+            item_handle,
+            MxValue::from_proto(sparse_value),
+            user_id,
+        )
+        .await
+    }
+
     /// Run MXAccess `Write2` (single-value with caller-supplied timestamp).
     ///
     /// # Errors

clients/rust/src/value.rs

@@ -173,7 +173,11 @@ impl MxValueProjection {
             Some(Kind::TimestampValue(value)) => Self::Timestamp(*value),
             Some(Kind::ArrayValue(value)) => Self::Array(MxArrayValue::from_proto(value.clone())),
             Some(Kind::RawValue(value)) => Self::Raw(value.clone()),
-            None => Self::Unset,
+            // SparseArrayValue is write-only: the gateway expands it before forwarding
+            // to the worker and never emits it in events or read replies. Map it to
+            // Unset so any read-side code that encounters a stale or mis-routed
+            // sparse value degrades gracefully rather than panicking.
+            Some(Kind::SparseArrayValue(_)) | None => Self::Unset,
         }
     }
 }

clients/rust/tests/client_behavior.rs

@@ -24,8 +24,8 @@ use zb_mom_ww_mxgateway_client::generated::mxaccess_gateway::v1::{
     AddItem2Reply, AddItemReply, AlarmConditionState, AlarmFeedMessage, AlarmTransitionKind,
     BulkReadReply, BulkReadResult, BulkSubscribeReply, BulkWriteReply, BulkWriteResult,
     CloseSessionReply, CloseSessionRequest, MxCommandKind, MxCommandReply, MxDataType, MxEvent,
-    MxEventFamily, MxStatusCategory, MxStatusProxy, MxStatusSource, MxValue,
-    OnAlarmTransitionEvent, OpenSessionReply, OpenSessionRequest, ProtocolStatus,
+    MxEventFamily, MxSparseArray, MxSparseElement, MxStatusCategory, MxStatusProxy, MxStatusSource,
+    MxValue, OnAlarmTransitionEvent, OpenSessionReply, OpenSessionRequest, ProtocolStatus,
     ProtocolStatusCode, QueryActiveAlarmsRequest, RegisterReply, SessionState, StreamAlarmsRequest,
     StreamEventsRequest, SubscribeResult, Write2BulkEntry, WriteBulkEntry, WriteSecured2BulkEntry,
     WriteSecuredBulkEntry,
@@ -1091,3 +1091,92 @@ fn case_by_id<'a>(cases: &'a [Value], id: &str) -> &'a Value {
         .find(|case| case["id"].as_str() == Some(id))
         .unwrap_or_else(|| panic!("missing fixture case {id}"))
 }
+
+// ---------------------------------------------------------------------------
+// write_array_elements — proto shape unit tests
+// ---------------------------------------------------------------------------
+
+/// Build the proto `MxValue` that `write_array_elements` would send and assert
+/// the sparse oneof variant has the correct `total_length` and elements.
+fn sparse_int32_value(
+    total_length: u32,
+    elements: impl IntoIterator<Item = (u32, i32)>,
+) -> MxValue {
+    let sparse_elements: Vec<MxSparseElement> = elements
+        .into_iter()
+        .map(|(index, v)| MxSparseElement {
+            index,
+            value: Some(MxValue {
+                data_type: MxDataType::Integer as i32,
+                variant_type: "VT_I4".to_owned(),
+                kind: Some(Kind::Int32Value(v)),
+                ..MxValue::default()
+            }),
+        })
+        .collect();
+
+    MxValue {
+        data_type: MxDataType::Integer as i32,
+        variant_type: String::new(),
+        kind: Some(Kind::SparseArrayValue(MxSparseArray {
+            element_data_type: MxDataType::Integer as i32,
+            total_length,
+            elements: sparse_elements,
+        })),
+        ..MxValue::default()
+    }
+}
+
+#[test]
+fn write_array_elements_proto_shape_has_sparse_oneof_kind() {
+    let proto = sparse_int32_value(5, [(0, 10), (3, 30)]);
+
+    let Kind::SparseArrayValue(ref sparse) = proto.kind.as_ref().unwrap() else {
+        panic!("expected SparseArrayValue kind, got {:?}", proto.kind);
+    };
+
+    assert_eq!(sparse.total_length, 5, "total_length must round-trip");
+    assert_eq!(sparse.elements.len(), 2, "two elements supplied");
+    assert_eq!(sparse.element_data_type, MxDataType::Integer as i32);
+
+    let elem0 = &sparse.elements[0];
+    assert_eq!(elem0.index, 0);
+    assert_eq!(
+        elem0.value.as_ref().unwrap().kind,
+        Some(Kind::Int32Value(10))
+    );
+
+    let elem3 = &sparse.elements[1];
+    assert_eq!(elem3.index, 3);
+    assert_eq!(
+        elem3.value.as_ref().unwrap().kind,
+        Some(Kind::Int32Value(30))
+    );
+}
+
+#[test]
+fn write_array_elements_empty_elements_is_valid_all_defaults() {
+    let proto = sparse_int32_value(8, []);
+    let Kind::SparseArrayValue(ref sparse) = proto.kind.as_ref().unwrap() else {
+        panic!("expected SparseArrayValue kind");
+    };
+    assert_eq!(sparse.total_length, 8);
+    assert!(
+        sparse.elements.is_empty(),
+        "no elements means every index defaults"
+    );
+}
+
+#[test]
+fn sparse_array_value_round_trips_through_client_mx_value_projection_as_unset() {
+    // SparseArrayValue is write-only. If it ever arrives on the read path
+    // (e.g. a future version bug), the projection should degrade to Unset
+    // rather than panic, because the enum variant is not readable.
+    let proto = sparse_int32_value(4, [(1, 99)]);
+    let client_value = ClientMxValue::from_proto(proto);
+    assert_eq!(
+        client_value.projection(),
+        &MxValueProjection::Unset,
+        "write-only SparseArrayValue must project to Unset, not panic"
+    );
+}