docs(csharp): document the per-operation serializer override (Step/Callback/Invoke/WaitForCondition/ChildContext + Map/Parallel ItemSerializer)

docs/sdk-reference/state/serialization.md

@@ -99,8 +99,13 @@ Each SDK uses a default SerDes when you do not provide one.
 
 === "C#"
 
-    There is no per-operation default SerDes. Register a single `ILambdaSerializer` at the
-    host boundary and the SDK uses it for every durable operation result.
+    The default is the single `ILambdaSerializer` registered at the host boundary — the SDK
+    uses it for every durable operation result and for the handler's return value. An
+    optional per-operation override is available (see
+    [Custom SerDes on durable operations](#custom-serdes-on-durable-operations)): set
+    `Serializer` on `StepConfig`, `CallbackConfig`, `InvokeConfig`,
+    `WaitForConditionConfig<TState>`, or `ChildContextConfig` to use a different
+    `ILambdaSerializer` for that one operation.
 
     Use `DefaultLambdaJsonSerializer` (from `Amazon.Lambda.Serialization.SystemTextJson`)
     for reflection-based serialization. For AOT or trim-friendly functions, use
@@ -163,8 +168,10 @@ Each SDK uses a default SerDes when you do not provide one.
 
 === "C#"
 
-    .NET has no per-operation SerDes interface. Serialization is controlled by the single
-    `ILambdaSerializer` registered on `ILambdaContext.Serializer`.
+    .NET has no separate per-operation SerDes interface — it reuses the standard
+    `ILambdaSerializer` contract. Serialization defaults to the single `ILambdaSerializer`
+    registered on `ILambdaContext.Serializer`, and an individual operation can override it
+    by setting `Serializer` (an `ILambdaSerializer`) on that operation's config.
 
     ```csharp
     --8<-- "examples/csharp/sdk-reference/serialization/SerdesInterface.cs"
@@ -240,9 +247,10 @@ same handler continue to use the default.
 
 === "C#"
 
-    `StepConfig` does not expose a serializer. The step result is serialized with the
-    `ILambdaSerializer` registered at the host boundary. Register a custom
-    `ILambdaSerializer` there to change how step results are serialized.
+    Set `StepConfig.Serializer` to serialize this step's result with a specific
+    `ILambdaSerializer`. When it is `null` (default), the step result is serialized with the
+    `ILambdaSerializer` registered at the host boundary. Only this step is affected; other
+    operations and the handler's return value continue to use the registered serializer.
 
     ```csharp
     --8<-- "examples/csharp/sdk-reference/serialization/StepConfigExample.cs"
@@ -276,9 +284,10 @@ system sends when it completes the callback.
 
 === "C#"
 
-    `CallbackConfig` does not expose a serializer. The payload the external system delivers
-    is deserialized with the `ILambdaSerializer` registered at the host boundary. Register a
-    custom `ILambdaSerializer` there to change how the callback payload is deserialized.
+    Set `CallbackConfig.Serializer` to deserialize the callback payload with a specific
+    `ILambdaSerializer`. When it is `null` (default), the payload the external system
+    delivers is deserialized with the `ILambdaSerializer` registered at the host boundary.
+    Only the deserialize path is used for callbacks.
 
     ```csharp
     --8<-- "examples/csharp/sdk-reference/serialization/CallbackConfigExample.cs"
@@ -320,9 +329,12 @@ Map and parallel perations support two SerDes fields that apply at different lev
 
 === "C#"
 
-    `MapConfig` does not expose a serializer, and there is no separate item-level
-    serializer. Each item result is serialized with the `ILambdaSerializer` registered at
-    the host boundary. `ParallelConfig` likewise has no serializer field.
+    Set `MapConfig<TItem>.ItemSerializer` (and `ParallelConfig.ItemSerializer`) to serialize
+    each item / branch **result** with a specific `ILambdaSerializer`. When `null` (default),
+    item results use the `ILambdaSerializer` registered at the host boundary. There is no
+    separate whole-result serializer: the aggregated batch envelope (per-item statuses and
+    completion reason) is an SDK-internal, source-generated structure and is not
+    user-serialized — only the per-item results are.
 
     ```csharp
     --8<-- "examples/csharp/sdk-reference/serialization/MapConfigExample.cs"

examples/csharp/sdk-reference/serialization/CallbackConfigExample.cs

@@ -1,5 +1,6 @@
 using Amazon.Lambda.Core;
 using Amazon.Lambda.DurableExecution;
+using Amazon.Lambda.Serialization.SystemTextJson;
 
 public class CallbackConfigExample
 {
@@ -9,13 +10,14 @@ public Task<DurableExecutionInvocationOutput> Handler(
 
     private async Task<ApprovalResult> Workflow(object input, IDurableContext ctx)
     {
-        // CallbackConfig has no serializer slot. The callback payload delivered by
-        // the external system is deserialized with the ILambdaSerializer registered
-        // on ILambdaContext.Serializer. To customize deserialization, register a
-        // custom ILambdaSerializer at the host boundary.
+        // Set CallbackConfig.Serializer to deserialize the callback payload with a specific
+        // ILambdaSerializer. When null (default), the payload the external system delivers is
+        // deserialized with the ILambdaSerializer registered on ILambdaContext.Serializer.
+        // Only the deserialize path is used for callbacks.
         var config = new CallbackConfig
         {
             Timeout = TimeSpan.FromHours(1),
+            Serializer = new DefaultLambdaJsonSerializer(),
         };
 
         ICallback<ApprovalResult> callback =

examples/csharp/sdk-reference/serialization/MapConfigExample.cs

@@ -1,5 +1,6 @@
 using Amazon.Lambda.Core;
 using Amazon.Lambda.DurableExecution;
+using Amazon.Lambda.Serialization.SystemTextJson;
 
 public class MapConfigExample
 {
@@ -9,12 +10,15 @@ public Task<DurableExecutionInvocationOutput> Handler(
 
     private async Task<IReadOnlyList<ProcessedItem>> Workflow(object input, IDurableContext ctx)
     {
-        // MapConfig has no serializer slot. Each item result is serialized with the
-        // ILambdaSerializer registered on ILambdaContext.Serializer. To customize
-        // serialization, register a custom ILambdaSerializer at the host boundary.
+        // Set MapConfig.ItemSerializer to serialize each item's RESULT with a specific
+        // ILambdaSerializer. When null (default), item results are serialized with the
+        // ILambdaSerializer registered on ILambdaContext.Serializer. This controls only the
+        // per-item result — not the aggregated batch envelope (statuses / completion
+        // reason), which is SDK-internal. ParallelConfig has the same ItemSerializer field.
         var config = new MapConfig<string>
         {
             MaxConcurrency = 3,
+            ItemSerializer = new CamelCaseLambdaJsonSerializer(),
         };
 
         var items = new[] { "a", "b", "c" };

examples/csharp/sdk-reference/serialization/StepConfigExample.cs

@@ -1,5 +1,6 @@
 using Amazon.Lambda.Core;
 using Amazon.Lambda.DurableExecution;
+using Amazon.Lambda.Serialization.SystemTextJson;
 
 public class StepConfigExample
 {
@@ -9,13 +10,14 @@ public Task<DurableExecutionInvocationOutput> Handler(
 
     private async Task<Order> Workflow(object input, IDurableContext ctx)
     {
-        // StepConfig has no serializer slot. The step result is serialized with
-        // the ILambdaSerializer registered on ILambdaContext.Serializer. To
-        // customize serialization, register a custom ILambdaSerializer at the
-        // host boundary instead of setting a per-step SerDes.
+        // Set StepConfig.Serializer to serialize THIS step's result with a specific
+        // ILambdaSerializer. When null (default), the step result is serialized with the
+        // ILambdaSerializer registered on ILambdaContext.Serializer. Only this step is
+        // affected — other operations and the handler's return value are unchanged.
         var config = new StepConfig
         {
             RetryStrategy = RetryStrategy.Exponential(maxAttempts: 3),
+            Serializer = new CamelCaseLambdaJsonSerializer(),
         };
 
         Order order = await ctx.StepAsync(