client/dotnet: make LazyBrowseNode.ExpandAsync thread-safe

clients/dotnet/ZB.MOM.WW.MxGateway.Client.Tests/LazyBrowseNodeTests.cs

@@ -142,6 +142,37 @@ public sealed class LazyBrowseNodeTests
         Assert.Equal("7:abc:2", transport.BrowseChildrenCalls[2].Request.PageToken);
     }
 
+    /// <summary>
+    /// Verifies that ten concurrent ExpandAsync calls issue exactly one RPC, not ten.
+    /// </summary>
+    [Fact]
+    public async Task Expand_CalledConcurrently_OnlyFiresOneRpc()
+    {
+        FakeGalaxyRepositoryTransport transport = CreateTransport();
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(1, "Plant", isArea: true)],
+            childHasChildren: [true],
+            cacheSequence: 7));
+        transport.BrowseChildrenReplies.Enqueue(BuildReply(
+            children: [BuildObject(2, "Mixer_001")],
+            childHasChildren: [false],
+            cacheSequence: 7));
+
+        await using GalaxyRepositoryClient client = CreateClient(transport);
+        IReadOnlyList<LazyBrowseNode> roots = await client.BrowseAsync();
+
+        // Fire ten concurrent expands of the same node.
+        Task[] tasks = Enumerable.Range(0, 10)
+            .Select(_ => roots[0].ExpandAsync())
+            .ToArray();
+        await Task.WhenAll(tasks);
+
+        Assert.True(roots[0].IsExpanded);
+        Assert.Single(roots[0].Children);
+        // 1 roots fetch + exactly 1 expand fetch = 2 total
+        Assert.Equal(2, transport.BrowseChildrenCalls.Count);
+    }
+
     /// <summary>
     /// Verifies that BrowseChildrenOptions filter fields are forwarded to the BrowseChildren request.
     /// </summary>

clients/dotnet/ZB.MOM.WW.MxGateway.Client/LazyBrowseNode.cs

@@ -13,6 +13,7 @@ public sealed class LazyBrowseNode
     private readonly GalaxyRepositoryClient _client;
     private readonly BrowseChildrenOptions _options;
     private readonly List<LazyBrowseNode> _children = [];
+    private readonly SemaphoreSlim _expandLock = new(1, 1);
     private bool _isExpanded;
 
     internal LazyBrowseNode(
@@ -43,6 +44,10 @@ public sealed class LazyBrowseNode
     ///     Fetches direct children from the gateway and populates <see cref="Children"/>.
     ///     Idempotent: subsequent calls are no-ops.
     /// </summary>
+    /// <remarks>
+    ///     Thread-safe: concurrent callers see exactly one fetch; subsequent callers
+    ///     (after the first completes) return immediately.
+    /// </remarks>
     /// <param name="cancellationToken">Token to observe for cancellation.</param>
     public async Task ExpandAsync(CancellationToken cancellationToken = default)
     {
@@ -51,6 +56,14 @@ public sealed class LazyBrowseNode
             return;
         }
 
+        await _expandLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        try
+        {
+            if (_isExpanded)
+            {
+                return;
+            }
+
             string pageToken = string.Empty;
             HashSet<string> seenPageTokens = new(StringComparer.Ordinal);
             do
@@ -80,4 +93,9 @@ public sealed class LazyBrowseNode
 
             _isExpanded = true;
         }
+        finally
+        {
+            _expandLock.Release();
+        }
+    }
 }