diff --git a/modules/openapi-generator/src/main/resources/dart/class.mustache b/modules/openapi-generator/src/main/resources/dart/class.mustache
index 64726de30675..b69af46ffaa9 100644
--- a/modules/openapi-generator/src/main/resources/dart/class.mustache
+++ b/modules/openapi-generator/src/main/resources/dart/class.mustache
@@ -58,14 +58,24 @@ class {{classname}} {
     return {
     {{#vars}}
       {{#isDateTime}}
-      '{{baseName}}': {{name}} == null ? '' : {{name}}.toUtc().toIso8601String(){{^-last}},{{/-last}}
+        '{{baseName}}': {{name}} == null ? '' : {{name}}.toUtc().toIso8601String(){{^-last}},{{/-last}}
       {{/isDateTime}}
       {{#isDate}}
-      '{{baseName}}': {{name}} == null ? '' : {{name}}.toUtc().toIso8601String(){{^-last}},{{/-last}}
+        '{{baseName}}': {{name}} == null ? '' : {{name}}.toUtc().toIso8601String(){{^-last}},{{/-last}}
       {{/isDate}}
       {{^isDateTime}}
       {{^isDate}}
-      '{{baseName}}': {{name}}{{^-last}},{{/-last}}
+        {{#complexType}}
+          {{#isListContainer}}
+            '{{baseName}}': {{name}} == null ? null : {{name}}.map((item) => item.toJson()).toList(){{^-last}},{{/-last}}
+          {{/isListContainer}}
+          {{^isListContainer}}
+            '{{baseName}}': {{name}} == null ? null : {{name}}.toJson(){{^-last}},{{/-last}}
+          {{/isListContainer}}
+        {{/complexType}}
+        {{^complexType}}
+          '{{baseName}}': {{name}}{{^-last}},{{/-last}}
+        {{/complexType}}
       {{/isDate}}
       {{/isDateTime}}
     {{/vars}}
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/api_response.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/api_response.dart
index f2fddde347ae..07ab760f4f8c 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/api_response.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/api_response.dart
@@ -35,9 +35,9 @@ class ApiResponse {
 
   Map<String, dynamic> toJson() {
     return {
-      'code': code,
-      'type': type,
-      'message': message
+          'code': code,
+          'type': type,
+          'message': message
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/category.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/category.dart
index 1750c6a0acb1..817df42fa46c 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/category.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/category.dart
@@ -28,8 +28,8 @@ class Category {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object.dart
index 01da636b114a..70b56f81369f 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object.dart
@@ -28,8 +28,8 @@ class InlineObject {
 
   Map<String, dynamic> toJson() {
     return {
-      'name': name,
-      'status': status
+          'name': name,
+          'status': status
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object1.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object1.dart
index 9a50a7b88be3..f0a458ab2802 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object1.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/inline_object1.dart
@@ -28,8 +28,8 @@ class InlineObject1 {
 
   Map<String, dynamic> toJson() {
     return {
-      'additionalMetadata': additionalMetadata,
-      'file': file
+          'additionalMetadata': additionalMetadata,
+            'file': file == null ? null : file.toJson()
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/order.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/order.dart
index 51d15f730415..6bd66bb554a3 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/order.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/order.dart
@@ -57,12 +57,12 @@ class Order {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'petId': petId,
-      'quantity': quantity,
-      'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
-      'status': status,
-      'complete': complete
+          'id': id,
+          'petId': petId,
+          'quantity': quantity,
+        'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
+          'status': status,
+          'complete': complete
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/pet.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/pet.dart
index c64406368d87..30db0ebc8417 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/pet.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/pet.dart
@@ -57,12 +57,12 @@ class Pet {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'category': category,
-      'name': name,
-      'photoUrls': photoUrls,
-      'tags': tags,
-      'status': status
+          'id': id,
+            'category': category == null ? null : category.toJson(),
+          'name': name,
+          'photoUrls': photoUrls,
+            'tags': tags == null ? null : tags.map((item) => item.toJson()).toList(),
+          'status': status
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/tag.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/tag.dart
index 980c6e016302..4657fde0d393 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/tag.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/tag.dart
@@ -28,8 +28,8 @@ class Tag {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/user.dart b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/user.dart
index 1555eb0a3ef5..41ed8d742f1d 100644
--- a/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/user.dart
+++ b/samples/client/petstore/dart/flutter_petstore/openapi/lib/model/user.dart
@@ -70,14 +70,14 @@ class User {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'username': username,
-      'firstName': firstName,
-      'lastName': lastName,
-      'email': email,
-      'password': password,
-      'phone': phone,
-      'userStatus': userStatus
+          'id': id,
+          'username': username,
+          'firstName': firstName,
+          'lastName': lastName,
+          'email': email,
+          'password': password,
+          'phone': phone,
+          'userStatus': userStatus
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/api_response.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/api_response.dart
index f2fddde347ae..07ab760f4f8c 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/api_response.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/api_response.dart
@@ -35,9 +35,9 @@ class ApiResponse {
 
   Map<String, dynamic> toJson() {
     return {
-      'code': code,
-      'type': type,
-      'message': message
+          'code': code,
+          'type': type,
+          'message': message
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/category.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/category.dart
index 1750c6a0acb1..817df42fa46c 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/category.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/category.dart
@@ -28,8 +28,8 @@ class Category {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/order.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/order.dart
index 51d15f730415..6bd66bb554a3 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/order.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/order.dart
@@ -57,12 +57,12 @@ class Order {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'petId': petId,
-      'quantity': quantity,
-      'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
-      'status': status,
-      'complete': complete
+          'id': id,
+          'petId': petId,
+          'quantity': quantity,
+        'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
+          'status': status,
+          'complete': complete
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/pet.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/pet.dart
index c64406368d87..30db0ebc8417 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/pet.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/pet.dart
@@ -57,12 +57,12 @@ class Pet {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'category': category,
-      'name': name,
-      'photoUrls': photoUrls,
-      'tags': tags,
-      'status': status
+          'id': id,
+            'category': category == null ? null : category.toJson(),
+          'name': name,
+          'photoUrls': photoUrls,
+            'tags': tags == null ? null : tags.map((item) => item.toJson()).toList(),
+          'status': status
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/tag.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/tag.dart
index 980c6e016302..4657fde0d393 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/tag.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/tag.dart
@@ -28,8 +28,8 @@ class Tag {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi-browser-client/lib/model/user.dart b/samples/client/petstore/dart/openapi-browser-client/lib/model/user.dart
index 1555eb0a3ef5..41ed8d742f1d 100644
--- a/samples/client/petstore/dart/openapi-browser-client/lib/model/user.dart
+++ b/samples/client/petstore/dart/openapi-browser-client/lib/model/user.dart
@@ -70,14 +70,14 @@ class User {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'username': username,
-      'firstName': firstName,
-      'lastName': lastName,
-      'email': email,
-      'password': password,
-      'phone': phone,
-      'userStatus': userStatus
+          'id': id,
+          'username': username,
+          'firstName': firstName,
+          'lastName': lastName,
+          'email': email,
+          'password': password,
+          'phone': phone,
+          'userStatus': userStatus
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/api_response.dart b/samples/client/petstore/dart/openapi/lib/model/api_response.dart
index f2fddde347ae..07ab760f4f8c 100644
--- a/samples/client/petstore/dart/openapi/lib/model/api_response.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/api_response.dart
@@ -35,9 +35,9 @@ class ApiResponse {
 
   Map<String, dynamic> toJson() {
     return {
-      'code': code,
-      'type': type,
-      'message': message
+          'code': code,
+          'type': type,
+          'message': message
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/category.dart b/samples/client/petstore/dart/openapi/lib/model/category.dart
index 1750c6a0acb1..817df42fa46c 100644
--- a/samples/client/petstore/dart/openapi/lib/model/category.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/category.dart
@@ -28,8 +28,8 @@ class Category {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/order.dart b/samples/client/petstore/dart/openapi/lib/model/order.dart
index 51d15f730415..6bd66bb554a3 100644
--- a/samples/client/petstore/dart/openapi/lib/model/order.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/order.dart
@@ -57,12 +57,12 @@ class Order {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'petId': petId,
-      'quantity': quantity,
-      'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
-      'status': status,
-      'complete': complete
+          'id': id,
+          'petId': petId,
+          'quantity': quantity,
+        'shipDate': shipDate == null ? '' : shipDate.toUtc().toIso8601String(),
+          'status': status,
+          'complete': complete
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/pet.dart b/samples/client/petstore/dart/openapi/lib/model/pet.dart
index c64406368d87..30db0ebc8417 100644
--- a/samples/client/petstore/dart/openapi/lib/model/pet.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/pet.dart
@@ -57,12 +57,12 @@ class Pet {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'category': category,
-      'name': name,
-      'photoUrls': photoUrls,
-      'tags': tags,
-      'status': status
+          'id': id,
+            'category': category == null ? null : category.toJson(),
+          'name': name,
+          'photoUrls': photoUrls,
+            'tags': tags == null ? null : tags.map((item) => item.toJson()).toList(),
+          'status': status
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/tag.dart b/samples/client/petstore/dart/openapi/lib/model/tag.dart
index 980c6e016302..4657fde0d393 100644
--- a/samples/client/petstore/dart/openapi/lib/model/tag.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/tag.dart
@@ -28,8 +28,8 @@ class Tag {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'name': name
+          'id': id,
+          'name': name
     };
   }
 
diff --git a/samples/client/petstore/dart/openapi/lib/model/user.dart b/samples/client/petstore/dart/openapi/lib/model/user.dart
index 1555eb0a3ef5..41ed8d742f1d 100644
--- a/samples/client/petstore/dart/openapi/lib/model/user.dart
+++ b/samples/client/petstore/dart/openapi/lib/model/user.dart
@@ -70,14 +70,14 @@ class User {
 
   Map<String, dynamic> toJson() {
     return {
-      'id': id,
-      'username': username,
-      'firstName': firstName,
-      'lastName': lastName,
-      'email': email,
-      'password': password,
-      'phone': phone,
-      'userStatus': userStatus
+          'id': id,
+          'username': username,
+          'firstName': firstName,
+          'lastName': lastName,
+          'email': email,
+          'password': password,
+          'phone': phone,
+          'userStatus': userStatus
     };
   }