microsoft · Copilot · Mar 6, 2026 · Mar 6, 2026 · Mar 6, 2026
diff --git a/.chronus/changes/copilot-update-propagateget-pattern-2026-03-06-20-49-06.md b/.chronus/changes/copilot-update-propagateget-pattern-2026-03-06-20-49-06.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/http-client-csharp"
+---
+
+Generator now emits V2 `PropagateGet` pattern for list/array properties backed by CLR types: adds an `IsEmpty` check at the outermost array level and generates `TryResolve{PropertyName}Array` / `Active{PropertyName}` helpers to support `JsonPatch.EnumerateArray` on CLR-backed collections.
@@ -382,7 +382,7 @@ private MethodBodyStatement[] BuildDynamicPropertyIfStatements(
             return [.. statements];
         }
 
-        private static MethodBodyStatement[] BuildCollectionIfStatements(
+        private MethodBodyStatement[] BuildCollectionIfStatements(
             PropertyProvider property,
             bool propagateGet,
             ParameterProvider valueParameter,
@@ -392,6 +392,7 @@ private static MethodBodyStatement[] BuildCollectionIfStatements(
             var currentType = property.Type;
             var accessorChain = new List<ValueExpression> { new IndexableExpression(property) };
             ValueExpression? remainderSlice = null;
+            bool isFirstLevel = true;
 
             statements.Add(Declare("propertyLength", typeof(int),
                 LiteralU8(GetJsonSerializedName(property.WireInfo!)).Property("Length"),
@@ -407,6 +408,20 @@ private static MethodBodyStatement[] BuildCollectionIfStatements(
 
                 if (currentType.IsList || currentType.IsArray)
                 {
+                    // V2: when PropagateGet and the outermost element is a direct dynamic model,
+                    // add an IsEmpty check so that array-level paths (e.g. "$.properties.items")
+                    // can be resolved via serialization of the CLR collection.
+                    if (propagateGet && isFirstLevel && IsDirectDynamicListProperty(property))
+                    {
+                        string tryResolveMethodName = $"TryResolve{property.Name}Array";
+                        VariableExpression valueVar = valueParameter;
+                        statements.Add(new IfStatement(currentSlice.Property("IsEmpty"))
+                        {
+                            Return(new InvokeMethodExpression(null, tryResolveMethodName,
+                                [new VariableExpression(valueVar.Type, valueVar.Declaration, IsOut: true)]))
+                        });
+                    }
+
                     statements.Add(new IfStatement(Not(currentSlice.Invoke(
                         "TryGetIndex",
                         [
@@ -457,6 +472,7 @@ private static MethodBodyStatement[] BuildCollectionIfStatements(
                 }
 
                 currentType = currentType.ElementType;
+                isFirstLevel = false;
             }
 
             var finalAccessor = accessorChain.Last();
@@ -481,6 +497,127 @@ private static MethodBodyStatement[] BuildCollectionIfStatements(
 
             return [.. statements];
         }
+
+        /// <summary>
+        /// Returns true if the property is a list or array whose direct element type is a dynamic model with a Patch property.
+        /// Only such properties support the V2 IsEmpty / TryResolveArray pattern.
+        /// </summary>
+        private static bool IsDirectDynamicListProperty(PropertyProvider property)
+        {
+            if (!property.Type.IsList && !property.Type.IsArray)
+                return false;
+            return ScmCodeModelGenerator.Instance.TypeFactory.CSharpTypeMap.TryGetValue(
+                property.Type.ElementType,
+                out var provider) &&
+                provider is ScmModelProvider { JsonPatchProperty: not null };
+        }
+
+        /// <summary>
+        /// Builds the <c>TryResolve{PropertyName}Array</c> helper method that serializes the active
+        /// (non-removed) CLR items as a JSON array and returns an <see cref="JsonPatch.EncodedValue"/>.
+        /// </summary>
+        private MethodProvider BuildTryResolveArrayMethod(PropertyProvider property)
+        {
+            var valueParameter = new ParameterProvider("value", $"", typeof(JsonPatch.EncodedValue), isOut: true);
+
+            var signature = new MethodSignature(
+                $"TryResolve{property.Name}Array",
+                $"",
+                MethodSignatureModifiers.Private,
+                typeof(bool),
+                $"",
+                [valueParameter]);
+
+            var dataDeclStatement = Declare("data", typeof(BinaryData),
+                Static(typeof(ModelReaderWriter)).Invoke(nameof(ModelReaderWriter.Write), [
+                    new InvokeMethodExpression(null, $"Active{property.Name}", []),
+                    New.Instance(typeof(ModelReaderWriterOptions), Literal("J"))
+                ]),
+                out var dataVar);
+
+            var tempPatchDeclStatement = Declare("tempPatch", typeof(JsonPatch), New.Instance(typeof(JsonPatch)), out var tempPatchVar);
+
+            var bodyStatements = new MethodBodyStatement[]
+            {
+                valueParameter.Assign(Default).Terminate(),
+                dataDeclStatement,
+                tempPatchDeclStatement,
+                tempPatchVar.As<JsonPatch>().Set(LiteralU8("$"), dataVar.Invoke("ToMemory").Property("Span")),
+                Return(tempPatchVar.As<JsonPatch>().TryGetEncodedValue(LiteralU8("$"), valueParameter))
+            };
+
+            return new MethodProvider(
+                signature,
+                bodyStatements,
+                _model,
+                suppressions: [ScmModelProvider.JsonPatchSuppression]);
+        }
+
+        /// <summary>
+        /// Builds the <c>Active{PropertyName}</c> iterator that yields non-removed items from the CLR collection.
+        /// </summary>
+        private MethodProvider BuildActiveItemsMethod(PropertyProvider property)
+        {
+            var elementType = property.Type.ElementType;
+            var returnType = new CSharpType(typeof(IEnumerable<>), elementType);
+
+            var signature = new MethodSignature(
+                $"Active{property.Name}",
+                $"",
+                MethodSignatureModifiers.Private,
+                returnType,
+                $"",
+                []);
+
+            var propertyExpr = new IndexableExpression(property);
+            string lengthPropertyName = property.Type.IsArray ? "Length" : "Count";
+
+            var indexDeclaration = Declare<int>("i", out var indexVar);
+            var forStatement = new ForStatement(
+                indexDeclaration.Assign(Literal(0)),
+                indexVar.LessThan(((ValueExpression)property).Property(lengthPropertyName)),
+                indexVar.Increment())
+            {
+                new IfStatement(Not(propertyExpr[indexVar].Property("Patch").As<JsonPatch>().IsRemoved(LiteralU8("$"))))
+                {
+                    YieldReturn(propertyExpr[indexVar])
+                }
+            };
+
+            var bodyStatements = new MethodBodyStatement[]
+            {
+                new IfStatement(Not(OptionalSnippets.IsCollectionDefined((ValueExpression)property)))
+                {
+                    YieldBreak()
+                },
+                forStatement
+            };
+
+            return new MethodProvider(
+                signature,
+                bodyStatements,
+                _model,
+                suppressions: [ScmModelProvider.JsonPatchSuppression]);
+        }
+
+        /// <summary>
+        /// Returns all collection properties from this model and its base models that qualify for the
+        /// V2 PropagateGet pattern (outermost element is a direct dynamic model with a Patch property).
+        /// </summary>
+        private List<PropertyProvider> GetQualifyingDynamicListProperties()
+        {
+            var result = new List<PropertyProvider>();
+            var currentModel = _model;
+            while (currentModel != null)
+            {
+                var qualifyingProps = currentModel.CanonicalView.Properties
+                    .Where(p => p.WireInfo != null && IsDirectDynamicListProperty(p));
+                result.AddRange(qualifyingProps);
+                currentModel = currentModel.BaseModelProvider as ScmModelProvider;
+            }
+            return result;
+        }
+
 #pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
 
         private static string BuildJsonPathForElement(string propertySerializedName, List<ValueExpression> indices)

@@ -244,6 +244,12 @@ protected override MethodProvider[] BuildMethods()
             if (_model is ScmModelProvider { IsDynamicModel: true, HasDynamicProperties: true })
             {
                 methods.AddRange(BuildPropagateGetMethod(), BuildPropagateSetMethod());
+
+                // Add V2 helper methods for every qualifying list/array property
+                foreach (var prop in GetQualifyingDynamicListProperties())
+                {
+                    methods.AddRange(BuildTryResolveArrayMethod(prop), BuildActiveItemsMethod(prop));
+                }
             }
 
             return [.. methods];

@@ -357,6 +357,41 @@ public void PropagateModelListProperty()
             Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
         }
 
+        [Test]
+        public void PropagateModelListPropertyHelperMethods()
+        {
+            var inputModel = InputFactory.Model(
+                "dynamicModel",
+                isDynamicModel: true,
+                properties:
+                [
+                    InputFactory.Property("p1",
+                        InputFactory.Array(InputFactory.Model(
+                            "anotherDynamic",
+                            isDynamicModel: true,
+                            properties:
+                            [
+                                InputFactory.Property("a1", InputPrimitiveType.String, isRequired: true)
+                            ])))
+                ]);
+
+            MockHelpers.LoadMockGenerator(inputModels: () => [inputModel]);
+            var model = ScmCodeModelGenerator.Instance.TypeFactory.CreateModel(inputModel) as ClientModel.Providers.ScmModelProvider;
+
+            Assert.IsNotNull(model);
+            Assert.IsTrue(model!.IsDynamicModel);
+            var serialization = model.SerializationProviders.SingleOrDefault();
+            Assert.IsNotNull(serialization);
+
+            var writer = new TypeProviderWriter(new FilteredMethodsTypeProvider(
+                serialization!,
+                name => name is "TryResolveP1Array" or "ActiveP1"));
+
+            var file = writer.Write();
+
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
+        }
+
         [Test]
         public void PropagateModelDictionaryProperty()
         {

@@ -19,6 +19,10 @@ private bool PropagateGet(global::System.ReadOnlySpan<byte> jsonPath, out global
             {
                 int propertyLength = "prop1"u8.Length;
                 global::System.ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveProp2Array(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;

@@ -19,6 +19,10 @@ private bool PropagateGet(global::System.ReadOnlySpan<byte> jsonPath, out global
             {
                 int propertyLength = "prop1"u8.Length;
                 global::System.ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveProp2Array(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;

@@ -19,6 +19,10 @@ private bool PropagateGet(global::System.ReadOnlySpan<byte> jsonPath, out global
             {
                 int propertyLength = "p1"u8.Length;
                 global::System.ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveP1Array(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;

@@ -0,0 +1,42 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.ClientModel.Primitives;
+using System.Collections.Generic;
+using Sample.Models;
+
+namespace Sample
+{
+    public partial class DynamicModel
+    {
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+        private bool TryResolveP1Array(out global::System.ClientModel.Primitives.JsonPatch.EncodedValue value)
+        {
+            value = default;
+            global::System.BinaryData data = global::System.ClientModel.Primitives.ModelReaderWriter.Write(ActiveP1(), new global::System.ClientModel.Primitives.ModelReaderWriterOptions("J"));
+            global::System.ClientModel.Primitives.JsonPatch tempPatch = new global::System.ClientModel.Primitives.JsonPatch();
+            tempPatch.Set("$"u8, data.ToMemory().Span);
+            return tempPatch.TryGetEncodedValue("$"u8, out value);
+        }
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+        private global::System.Collections.Generic.IEnumerable<global::Sample.Models.AnotherDynamic> ActiveP1()
+        {
+            if (!global::Sample.Optional.IsCollectionDefined(P1))
+            {
+                yield break;
+            }
+            for (int i = 0; (i < P1.Count); i++)
+            {
+                if (!P1[i].Patch.IsRemoved("$"u8))
+                {
+                    yield return P1[i];
+                }
+            }
+        }
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+    }
+}
@@ -32,6 +32,10 @@ private bool PropagateGet(global::System.ReadOnlySpan<byte> jsonPath, out global
             {
                 int propertyLength = "p2"u8.Length;
                 global::System.ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveP2Array(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;
@@ -42,6 +46,10 @@ private bool PropagateGet(global::System.ReadOnlySpan<byte> jsonPath, out global
             {
                 int propertyLength = "p3"u8.Length;
                 global::System.ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveP3Array(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;

@@ -773,6 +773,10 @@ private bool PropagateGet(ReadOnlySpan<byte> jsonPath, out JsonPatch.EncodedValu
             {
                 int propertyLength = "listFoo"u8.Length;
                 ReadOnlySpan<byte> currentSlice = local.Slice(propertyLength);
+                if (currentSlice.IsEmpty)
+                {
+                    return TryResolveListFooArray(out value);
+                }
                 if (!currentSlice.TryGetIndex(out int index, out int bytesConsumed))
                 {
                     return false;
@@ -966,5 +970,38 @@ private bool PropagateSet(ReadOnlySpan<byte> jsonPath, JsonPatch.EncodedValue va
             return false;
         }
 #pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+
+        /// <summary></summary>
+        /// <param name="value"></param>
+        /// <returns></returns>
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+        private bool TryResolveListFooArray(out JsonPatch.EncodedValue value)
+        {
+            value = default;
+            BinaryData data = ModelReaderWriter.Write(ActiveListFoo(), new ModelReaderWriterOptions("J"));
+            JsonPatch tempPatch = new JsonPatch();
+            tempPatch.Set("$"u8, data.ToMemory().Span);
+            return tempPatch.TryGetEncodedValue("$"u8, out value);
+        }
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+
+        /// <summary></summary>
+        /// <returns></returns>
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+        private IEnumerable<AnotherDynamicModel> ActiveListFoo()
+        {
+            if (!Optional.IsCollectionDefined(ListFoo))
+            {
+                yield break;
+            }
+            for (int i = 0; (i < ListFoo.Count); i++)
+            {
+                if (!ListFoo[i].Patch.IsRemoved("$"u8))
+                {
+                    yield return ListFoo[i];
+                }
+            }
+        }
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
     }
 }