diff --git a/modules/openapi-generator/src/main/resources/dart-dio/api.mustache b/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
index 884dfb2fe56..9a4cc29beb3 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
@@ -40,25 +40,28 @@ class {{classname}} {
         List<String> contentTypes = [{{#consumes}}"{{{mediaType}}}"{{^-last}},{{/-last}}{{/consumes}}];
 
         {{#hasFormParams}}
-        Map<String, dynamic> formData = {};
-        {{#formParams}}
+        final Map<String, dynamic> formData = {};
         {{#isMultipart}}
-            {{^isFile}}
-                if ({{paramName}} != null) {
-                    formData[r'{{baseName}}'] = parameterToString(_serializers, {{paramName}});
-                }
-            {{/isFile}}
-            {{#isFile}}
-                if ({{paramName}} != null) {
-                    formData[r'{{baseName}}'] = MultipartFile.fromBytes({{paramName}}, filename: r'{{baseName}}');
-                }
-            {{/isFile}}
-        {{/isMultipart}}
-        {{^isMultipart}}
+        {{#formParams}}
+        {{^isFile}}
+        if ({{paramName}} != null) {
             formData[r'{{baseName}}'] = parameterToString(_serializers, {{paramName}});
-        {{/isMultipart}}
+        }
+        {{/isFile}}
+        {{#isFile}}
+        if ({{paramName}} != null) {
+            formData[r'{{baseName}}'] = MultipartFile.fromBytes({{paramName}}, filename: r'{{baseName}}');
+        }
+        {{/isFile}}
         {{/formParams}}
         bodyData = FormData.fromMap(formData);
+        {{/isMultipart}}
+        {{^isMultipart}}
+        {{#formParams}}
+        formData['{{baseName}}'] = parameterToString(_serializers, {{paramName}});
+        {{/formParams}}
+        bodyData = formData;
+        {{/isMultipart}}
         {{/hasFormParams}}
 
         {{#bodyParam}}
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
index 5d3e4e37ac6..bf19ec00cdf 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
@@ -295,10 +295,10 @@ class PetApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'name'] = parameterToString(_serializers, name);
-            formData[r'status'] = parameterToString(_serializers, status);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['name'] = parameterToString(_serializers, name);
+        formData['status'] = parameterToString(_serializers, status);
+        bodyData = formData;
 
 
             return _dio.request(
@@ -334,13 +334,13 @@ class PetApi {
 
         List<String> contentTypes = ["multipart/form-data"];
 
-        Map<String, dynamic> formData = {};
-                if (additionalMetadata != null) {
-                    formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
-                }
-                if (file != null) {
-                    formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
-                }
+        final Map<String, dynamic> formData = {};
+        if (additionalMetadata != null) {
+            formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
+        }
+        if (file != null) {
+            formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
+        }
         bodyData = FormData.fromMap(formData);
 
 
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
index 28c906b775a..62982eb8239 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
@@ -323,10 +323,10 @@ class PetApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'name'] = parameterToString(_serializers, name);
-            formData[r'status'] = parameterToString(_serializers, status);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['name'] = parameterToString(_serializers, name);
+        formData['status'] = parameterToString(_serializers, status);
+        bodyData = formData;
 
 
             return _dio.request(
@@ -362,13 +362,13 @@ class PetApi {
 
         List<String> contentTypes = ["multipart/form-data"];
 
-        Map<String, dynamic> formData = {};
-                if (additionalMetadata != null) {
-                    formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
-                }
-                if (file != null) {
-                    formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
-                }
+        final Map<String, dynamic> formData = {};
+        if (additionalMetadata != null) {
+            formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
+        }
+        if (file != null) {
+            formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
+        }
         bodyData = FormData.fromMap(formData);
 
 
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
index 916e4fbda3f..26de26914aa 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
@@ -459,22 +459,22 @@ class FakeApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'integer'] = parameterToString(_serializers, integer);
-            formData[r'int32'] = parameterToString(_serializers, int32);
-            formData[r'int64'] = parameterToString(_serializers, int64);
-            formData[r'number'] = parameterToString(_serializers, number);
-            formData[r'float'] = parameterToString(_serializers, float);
-            formData[r'double'] = parameterToString(_serializers, double_);
-            formData[r'string'] = parameterToString(_serializers, string);
-            formData[r'pattern_without_delimiter'] = parameterToString(_serializers, patternWithoutDelimiter);
-            formData[r'byte'] = parameterToString(_serializers, byte);
-            formData[r'binary'] = parameterToString(_serializers, binary);
-            formData[r'date'] = parameterToString(_serializers, date);
-            formData[r'dateTime'] = parameterToString(_serializers, dateTime);
-            formData[r'password'] = parameterToString(_serializers, password);
-            formData[r'callback'] = parameterToString(_serializers, callback);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['integer'] = parameterToString(_serializers, integer);
+        formData['int32'] = parameterToString(_serializers, int32);
+        formData['int64'] = parameterToString(_serializers, int64);
+        formData['number'] = parameterToString(_serializers, number);
+        formData['float'] = parameterToString(_serializers, float);
+        formData['double'] = parameterToString(_serializers, double_);
+        formData['string'] = parameterToString(_serializers, string);
+        formData['pattern_without_delimiter'] = parameterToString(_serializers, patternWithoutDelimiter);
+        formData['byte'] = parameterToString(_serializers, byte);
+        formData['binary'] = parameterToString(_serializers, binary);
+        formData['date'] = parameterToString(_serializers, date);
+        formData['dateTime'] = parameterToString(_serializers, dateTime);
+        formData['password'] = parameterToString(_serializers, password);
+        formData['callback'] = parameterToString(_serializers, callback);
+        bodyData = formData;
 
 
             return _dio.request(
@@ -516,10 +516,10 @@ class FakeApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'enum_form_string_array'] = parameterToString(_serializers, enumFormStringArray);
-            formData[r'enum_form_string'] = parameterToString(_serializers, enumFormString);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['enum_form_string_array'] = parameterToString(_serializers, enumFormStringArray);
+        formData['enum_form_string'] = parameterToString(_serializers, enumFormString);
+        bodyData = formData;
 
 
             return _dio.request(
@@ -634,10 +634,10 @@ class FakeApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'param'] = parameterToString(_serializers, param);
-            formData[r'param2'] = parameterToString(_serializers, param2);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['param'] = parameterToString(_serializers, param);
+        formData['param2'] = parameterToString(_serializers, param2);
+        bodyData = formData;
 
 
             return _dio.request(
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
index 45a34674ae6..20ef02e734f 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
@@ -295,10 +295,10 @@ class PetApi {
 
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
-        Map<String, dynamic> formData = {};
-            formData[r'name'] = parameterToString(_serializers, name);
-            formData[r'status'] = parameterToString(_serializers, status);
-        bodyData = FormData.fromMap(formData);
+        final Map<String, dynamic> formData = {};
+        formData['name'] = parameterToString(_serializers, name);
+        formData['status'] = parameterToString(_serializers, status);
+        bodyData = formData;
 
 
             return _dio.request(
@@ -334,13 +334,13 @@ class PetApi {
 
         List<String> contentTypes = ["multipart/form-data"];
 
-        Map<String, dynamic> formData = {};
-                if (additionalMetadata != null) {
-                    formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
-                }
-                if (file != null) {
-                    formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
-                }
+        final Map<String, dynamic> formData = {};
+        if (additionalMetadata != null) {
+            formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
+        }
+        if (file != null) {
+            formData[r'file'] = MultipartFile.fromBytes(file, filename: r'file');
+        }
         bodyData = FormData.fromMap(formData);
 
 
@@ -391,13 +391,13 @@ class PetApi {
 
         List<String> contentTypes = ["multipart/form-data"];
 
-        Map<String, dynamic> formData = {};
-                if (additionalMetadata != null) {
-                    formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
-                }
-                if (requiredFile != null) {
-                    formData[r'requiredFile'] = MultipartFile.fromBytes(requiredFile, filename: r'requiredFile');
-                }
+        final Map<String, dynamic> formData = {};
+        if (additionalMetadata != null) {
+            formData[r'additionalMetadata'] = parameterToString(_serializers, additionalMetadata);
+        }
+        if (requiredFile != null) {
+            formData[r'requiredFile'] = MultipartFile.fromBytes(requiredFile, filename: r'requiredFile');
+        }
         bodyData = FormData.fromMap(formData);