resolve issue 4296

src/TypeSpec.Extension/Emitter.Csharp/src/lib/model.ts

@@ -348,7 +348,7 @@ export function getInputType(
                 } as InputPrimitiveType;
         }
     } else if (type.kind === "Union") {
-        return getInputTypeForUnion(type);
+        return getInputTypeForUnion(type, formattedType.encode);
     } else if (type.kind === "UnionVariant") {
         return getInputType(
             context,
@@ -808,7 +808,10 @@ export function getInputType(
         }
     }
 
-    function getInputTypeForUnion(union: Union): InputUnionType | InputType {
+    function getInputTypeForUnion(
+        union: Union,
+        encodeData: EncodeData | undefined = undefined
+    ): InputUnionType | InputType {
         var clientType = getClientType(context, union);
         if (clientType.kind === "enum" && clientType.isFixed === false) {
             return fromSdkEnumType(clientType, context, enums);
@@ -819,9 +822,12 @@ export function getInputType(
 
         let hasNullType = false;
         for (const variant of variants) {
+            var formattedType = getFormattedType(program, variant.type);
+            // propagate the encode data from parent.
+            if (!formattedType.encode) formattedType.encode = encodeData;
             const inputType = getInputType(
                 context,
-                getFormattedType(program, variant.type),
+                formattedType,
                 models,
                 enums
             );

src/TypeSpec.Extension/Emitter.Csharp/test/Unit/encode.test.ts

@@ -259,4 +259,34 @@ describe("Test encode duration", () => {
             )
         );
     });
+    it("encode iso8601 on duration|null union model property", async () => {
+        const program = await typeSpecCompile(
+            `
+            @doc("This is a model.")
+            model ISO8601DurationProperty {
+                @encode(DurationKnownEncoding.ISO8601)
+                value: duration | null;
+            }
+      `,
+            runner
+        );
+        const context = createEmitterContext(program);
+        const sdkContext = createNetSdkContext(context);
+        const [services] = getAllHttpServices(program);
+        const modelMap = new Map<string, InputModelType>();
+        const enumMap = new Map<string, InputEnumType>();
+        navigateModels(sdkContext, services[0].namespace, modelMap, enumMap);
+        const durationProperty = modelMap.get("ISO8601DurationProperty");
+        assert(durationProperty !== undefined);
+        assert(
+            isEqual(
+                {
+                    Kind: InputTypeKind.Primitive,
+                    Name: InputPrimitiveTypeKind.DurationISO8601,
+                    IsNullable: true
+                } as InputPrimitiveType,
+                durationProperty.Properties[0].Type
+            )
+        );
+    });
 });

test/TestProjects/FirstTest-TypeSpec/FirstTest-TypeSpec.tsp

@@ -548,3 +548,13 @@ model AzureLocationModel
 @put
 @convenientAPI(true)
 op azureLocationOp(@query location: azureLocation, @header regenLocation: azureLocation, @body body?: AzureLocationModel): void;
+
+model EncodedUnionProperty{
+    @encode("unixTimestamp", int32)
+    started_at: utcDateTime | null;
+}
+@route("/encodedUnionProperty")
+@doc("test nullableTime.")
+@put
+@convenientAPI(true)
+op EncodedUnionPropertyOp(@body body: EncodedUnionProperty):EncodedUnionProperty;

test/TestProjects/FirstTest-TypeSpec/src/Generated/Docs/FirstTestTypeSpecClient.xml

@@ -2221,6 +2221,106 @@ using RequestContent content = RequestContent.Create(new
 Response response = client.AzureLocationOp(default, default, content);
 
 Console.WriteLine(response.Status);
+]]></code></example>
+    </member>
+    <member name="EncodedUnionPropertyOpAsync(EncodedUnionProperty,CancellationToken)">
+      <example>
+This sample shows how to call EncodedUnionPropertyOpAsync.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+EncodedUnionProperty body = new EncodedUnionProperty(DateTimeOffset.FromUnixTimeSeconds(1652209051L));
+Response<EncodedUnionProperty> response = await client.EncodedUnionPropertyOpAsync(body);
+]]></code>
+This sample shows how to call EncodedUnionPropertyOpAsync with all parameters.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+EncodedUnionProperty body = new EncodedUnionProperty(DateTimeOffset.FromUnixTimeSeconds(1652209051L));
+Response<EncodedUnionProperty> response = await client.EncodedUnionPropertyOpAsync(body);
+]]></code></example>
+    </member>
+    <member name="EncodedUnionPropertyOp(EncodedUnionProperty,CancellationToken)">
+      <example>
+This sample shows how to call EncodedUnionPropertyOp.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+EncodedUnionProperty body = new EncodedUnionProperty(DateTimeOffset.FromUnixTimeSeconds(1652209051L));
+Response<EncodedUnionProperty> response = client.EncodedUnionPropertyOp(body);
+]]></code>
+This sample shows how to call EncodedUnionPropertyOp with all parameters.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+EncodedUnionProperty body = new EncodedUnionProperty(DateTimeOffset.FromUnixTimeSeconds(1652209051L));
+Response<EncodedUnionProperty> response = client.EncodedUnionPropertyOp(body);
+]]></code></example>
+    </member>
+    <member name="EncodedUnionPropertyOpAsync(RequestContent,RequestContext)">
+      <example>
+This sample shows how to call EncodedUnionPropertyOpAsync and parse the result.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+using RequestContent content = RequestContent.Create(new
+{
+    started_at = 1652209051,
+});
+Response response = await client.EncodedUnionPropertyOpAsync(content);
+
+JsonElement result = JsonDocument.Parse(response.ContentStream).RootElement;
+Console.WriteLine(result.GetProperty("started_at").ToString());
+]]></code>
+This sample shows how to call EncodedUnionPropertyOpAsync with all request content and parse the result.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+using RequestContent content = RequestContent.Create(new
+{
+    started_at = 1652209051,
+});
+Response response = await client.EncodedUnionPropertyOpAsync(content);
+
+JsonElement result = JsonDocument.Parse(response.ContentStream).RootElement;
+Console.WriteLine(result.GetProperty("started_at").ToString());
+]]></code></example>
+    </member>
+    <member name="EncodedUnionPropertyOp(RequestContent,RequestContext)">
+      <example>
+This sample shows how to call EncodedUnionPropertyOp and parse the result.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+using RequestContent content = RequestContent.Create(new
+{
+    started_at = 1652209051,
+});
+Response response = client.EncodedUnionPropertyOp(content);
+
+JsonElement result = JsonDocument.Parse(response.ContentStream).RootElement;
+Console.WriteLine(result.GetProperty("started_at").ToString());
+]]></code>
+This sample shows how to call EncodedUnionPropertyOp with all request content and parse the result.
+<code><![CDATA[
+Uri endpoint = new Uri("<https://my-service.azure.com>");
+FirstTestTypeSpecClient client = new FirstTestTypeSpecClient(endpoint);
+
+using RequestContent content = RequestContent.Create(new
+{
+    started_at = 1652209051,
+});
+Response response = client.EncodedUnionPropertyOp(content);
+
+JsonElement result = JsonDocument.Parse(response.ContentStream).RootElement;
+Console.WriteLine(result.GetProperty("started_at").ToString());
 ]]></code></example>
     </member>
   </members>