fix: preserve array encoding serialization format for customized properties

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

@@ -1074,7 +1074,7 @@ private List<MethodBodyStatement> BuildDeserializePropertiesStatements(ScopedApi
                     var propertyExpression = parameter.Property?.AsVariableExpression ?? parameter.Field?.AsVariableExpression;
                     var checkIfJsonPropEqualsName = new IfStatement(jsonProperty.NameEquals(propertySerializationName))
                     {
-                        DeserializeProperty(propertyName!, propertyType!, wireInfo, propertyExpression!, jsonProperty, serializationAttributes, parameter.Property?.SerializationFormat)
+                        DeserializeProperty(propertyName!, propertyType!, wireInfo, propertyExpression!, jsonProperty, serializationAttributes, wireInfo.SerializationFormat)
                     };
                     propertyDeserializationStatements.Add(checkIfJsonPropEqualsName);
                 }
@@ -1754,7 +1754,7 @@ private MethodBodyStatement[] CreateWritePropertiesStatements(bool isDynamicMode
                     continue;
                 }
 
-                propertyStatements.Add(CreateWritePropertyStatement(property.WireInfo, property.Type, property.Name, property, property.SerializationFormat));
+                propertyStatements.Add(CreateWritePropertyStatement(property.WireInfo, property.Type, property.Name, property, property.WireInfo.SerializationFormat));
             }
 
             foreach (var field in _model.CanonicalView.Fields)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/SerializationCustomizationTests.cs

@@ -477,5 +477,29 @@ public async Task CanChangePropertyNameAndRedefineOriginal()
             var file = writer.Write();
             Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
         }
+
+        [Test]
+        public async Task CanPreserveArrayEncodingForCustomizedProperty()
+        {
+            var props = new[]
+            {
+                InputFactory.Property("Prop1", InputFactory.Array(InputPrimitiveType.String), encode: ArrayKnownEncoding.CommaDelimited)
+            };
+
+            var inputModel = InputFactory.Model("Model", properties: props, usage: InputModelTypeUsage.Json);
+            var mockGenerator = await MockHelpers.LoadMockGeneratorAsync(
+                inputModels: () => [inputModel],
+                compilation: async () => await Helpers.GetCompilationFromDirectoryAsync());
+
+            var modelProvider = mockGenerator.Object.OutputLibrary.TypeProviders.Single(t => t is ModelProvider);
+            var serializationProvider = modelProvider.SerializationProviders.Single(t => t is MrwSerializationTypeDefinition);
+            Assert.IsNotNull(serializationProvider);
+
+            var writer = new TypeProviderWriter(new FilteredMethodsTypeProvider(
+                serializationProvider!,
+                name => name == "DeserializeModel" || name == "JsonModelWriteCore"));
+            var file = writer.Write();
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
+        }
     }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/SerializationCustomizationTests/CanPreserveArrayEncodingForCustomizedProperty.cs

@@ -0,0 +1,72 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.ClientModel.Primitives;
+using System.Collections.Generic;
+using System.Text.Json;
+using Sample.Models;
+
+namespace Sample
+{
+    public partial class Model
+    {
+        protected virtual void JsonModelWriteCore(global::System.Text.Json.Utf8JsonWriter writer, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.Model>)this).GetFormatFromOptions(options) : options.Format;
+            if ((format != "J"))
+            {
+                throw new global::System.FormatException($"The model {nameof(global::Sample.Models.Model)} does not support writing '{format}' format.");
+            }
+            if (global::Sample.Optional.IsCollectionDefined(Prop2))
+            {
+                writer.WritePropertyName("prop1"u8);
+                writer.WriteStringValue(string.Join(",", Prop2));
+            }
+            if (((options.Format != "W") && (_additionalBinaryDataProperties != null)))
+            {
+                foreach (var item in _additionalBinaryDataProperties)
+                {
+                    writer.WritePropertyName(item.Key);
+#if NET6_0_OR_GREATER
+                    writer.WriteRawValue(item.Value);
+#else
+                    using (global::System.Text.Json.JsonDocument document = global::System.Text.Json.JsonDocument.Parse(item.Value))
+                    {
+                        global::System.Text.Json.JsonSerializer.Serialize(writer, document.RootElement);
+                    }
+#endif
+                }
+            }
+        }
+
+        internal static global::Sample.Models.Model DeserializeModel(global::System.Text.Json.JsonElement element, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            if ((element.ValueKind == global::System.Text.Json.JsonValueKind.Null))
+            {
+                return null;
+            }
+            global::System.Collections.Generic.IList<string> prop2 = default;
+            global::System.Collections.Generic.IDictionary<string, global::System.BinaryData> additionalBinaryDataProperties = new global::Sample.ChangeTrackingDictionary<string, global::System.BinaryData>();
+            foreach (var prop in element.EnumerateObject())
+            {
+                if (prop.NameEquals("prop1"u8))
+                {
+                    if ((prop.Value.ValueKind == global::System.Text.Json.JsonValueKind.Null))
+                    {
+                        continue;
+                    }
+                    string stringValue = prop.Value.GetString();
+                    prop2 = string.IsNullOrEmpty(stringValue) ? new global::System.Collections.Generic.List<string>() : new global::System.Collections.Generic.List<string>(stringValue.Split(','));
+                    continue;
+                }
+                if ((options.Format != "W"))
+                {
+                    additionalBinaryDataProperties.Add(prop.Name, global::System.BinaryData.FromString(prop.Value.GetRawText()));
+                }
+            }
+            return new global::Sample.Models.Model((prop2 ?? new global::Sample.ChangeTrackingList<string>()), additionalBinaryDataProperties);
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/SerializationCustomizationTests/CanPreserveArrayEncodingForCustomizedProperty/Model.cs

@@ -0,0 +1,13 @@
+
+using SampleTypeSpec;
+using System.Collections.Generic;
+using Microsoft.TypeSpec.Generator.Customizations;
+
+namespace Sample.Models
+{
+    public partial class Model
+    {
+        [CodeGenMember("Prop1")]
+        public IList<string> Prop2 { get; internal set; }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Primitives/PropertyWireInformation.cs

@@ -34,7 +34,7 @@ public PropertyWireInformation(SerializationFormat serializationFormat, bool isR
         /// </summary>
         /// <param name="inputProperty">The input model property.</param>
         internal PropertyWireInformation(InputProperty inputProperty)
-            : base(CodeModelGenerator.Instance.TypeFactory.GetSerializationFormat(inputProperty.Type), inputProperty.SerializedName)
+            : base(CodeModelGenerator.Instance.TypeFactory.GetSerializationFormat(inputProperty), inputProperty.SerializedName)
         // TODO -- this is only temporary because we do not support other type of serialization, improvement tracking https://github.com/microsoft/typespec/issues/5861
         {
             InputModelProperty? modelProperty = inputProperty as InputModelProperty;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/PropertyProvider.cs

@@ -92,7 +92,7 @@ private PropertyProvider(InputProperty inputProperty, CSharpType propertyType, T
             }
 
             EnclosingType = enclosingType;
-            _serializationFormat = GetSerializationFormat(inputProperty);
+            _serializationFormat = CodeModelGenerator.Instance.TypeFactory.GetSerializationFormat(inputProperty);
             _isRequiredNonNullableConstant = inputProperty.IsRequired && propertyType is { IsLiteral: true, IsNullable: false };
             var propHasSetter = PropertyHasSetter(propertyType, inputProperty);
             MethodSignatureModifiers setterModifier = propHasSetter ? MethodSignatureModifiers.Public : MethodSignatureModifiers.None;
@@ -335,21 +335,5 @@ public void Update(
                 BuildDocs();
             }
         }
-
-        private SerializationFormat GetSerializationFormat(InputProperty inputProperty)
-        {
-            // Handle array encoding from InputModelProperty
-            if (inputProperty is InputModelProperty modelProperty &&
-                inputProperty.Type is InputArrayType)
-            {
-                var arrayEncoding = modelProperty.Encode;
-                if (arrayEncoding.HasValue)
-                {
-                    return arrayEncoding.Value.ToSerializationFormat();
-                }
-            }
-
-            return CodeModelGenerator.Instance.TypeFactory.GetSerializationFormat(inputProperty.Type);
-        }
     }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/TypeFactory.cs

@@ -385,6 +385,25 @@ or InputPrimitiveTypeKind.Int64 or InputPrimitiveTypeKind.UInt8 or InputPrimitiv
             _ => SerializationFormat.Default
         };
 
+        /// <summary>
+        /// Retrieves the serialization format for a given input property. For array-typed properties
+        /// this checks the property-level <see cref="InputModelProperty.Encode"/> before falling
+        /// back to <see cref="GetSerializationFormat(InputType)"/>.
+        /// </summary>
+        /// <param name="inputProperty">The <see cref="InputProperty"/> to retrieve the serialization format for.</param>
+        /// <returns>The <see cref="SerializationFormat"/> for the input property.</returns>
+        internal SerializationFormat GetSerializationFormat(InputProperty inputProperty)
+        {
+            if (inputProperty is InputModelProperty modelProperty &&
+                inputProperty.Type is InputArrayType &&
+                modelProperty.Encode.HasValue)
+            {
+                return modelProperty.Encode.Value.ToSerializationFormat();
+            }
+
+            return GetSerializationFormat(inputProperty.Type);
+        }
+
         /// <summary>
         /// The initialization type of list properties. This type should implement both <see cref="IList{T}"/> and <see cref="IReadOnlyList{T}"/>.
         /// </summary>

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/common/InputFactory.cs

@@ -278,7 +278,8 @@ public static InputModelProperty Property(
             string? summary = null,
             string? serializedName = null,
             string? doc = null,
-            InputSerializationOptions? serializationOptions = null)
+            InputSerializationOptions? serializationOptions = null,
+            ArrayKnownEncoding? encode = null)
         {
             serializationOptions ??= new InputSerializationOptions();
             return new InputModelProperty(
@@ -294,7 +295,8 @@ public static InputModelProperty Property(
                 access: null,
                 isDiscriminator: isDiscriminator,
                 serializedName: serializedName ?? wireName ?? name.ToVariableName(),
-                serializationOptions: serializationOptions);
+                serializationOptions: serializationOptions,
+                encode: encode);
         }
 
         public static InputHeaderParameter HeaderParameter(