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 c82e4582e9e..11ece09eaca 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
@@ -77,7 +77,7 @@ class {{classname}} {
 
     _bodyData = {{#isMultipart}}FormData.fromMap({{/isMultipart}}<String, dynamic>{
       {{#formParams}}
-      {{^required}}{{^nullable}}if ({{{paramName}}} != null) {{/nullable}}{{/required}}r'{{{baseName}}}': {{#isFile}}MultipartFile.fromBytes({{{paramName}}}, filename: r'{{{baseName}}}'){{/isFile}}{{^isFile}}parameterToString(_serializers, {{{paramName}}}){{/isFile}},
+      {{^required}}{{^nullable}}if ({{{paramName}}} != null) {{/nullable}}{{/required}}r'{{{baseName}}}': {{#isFile}}MultipartFile.fromBytes({{{paramName}}}, filename: r'{{{baseName}}}'){{/isFile}}{{^isFile}}encodeFormParameter(_serializers, {{{paramName}}}, const FullType({{^isContainer}}{{{baseType}}}{{/isContainer}}{{#isContainer}}Built{{#isMap}}Map{{/isMap}}{{#isArray}}{{#uniqueItems}}Set{{/uniqueItems}}{{^uniqueItems}}List{{/uniqueItems}}{{/isArray}}, [{{#isMap}}FullType(String), {{/isMap}}FullType({{{baseType}}})]{{/isContainer}})){{/isFile}},
       {{/formParams}}
     }{{#isMultipart}}){{/isMultipart}};
     {{/hasFormParams}}
diff --git a/modules/openapi-generator/src/main/resources/dart-dio/api_util.mustache b/modules/openapi-generator/src/main/resources/dart-dio/api_util.mustache
index f3a39d4e80a..12cd528d2d7 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/api_util.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/api_util.mustache
@@ -1,15 +1,25 @@
 {{>header}}
 import 'dart:convert';
 
+import 'package:built_collection/built_collection.dart';
 import 'package:built_value/serializer.dart';
 
-/// Format the given parameter object into string.
-String parameterToString(Serializers serializers, dynamic value) {
-    if (value == null) {
-        return '';
-    } else if (value is String || value is num) {
-        return value.toString();
-    } else {
-        return json.encode(serializers.serialize(value));
-    }
-}
\ No newline at end of file
+/// Format the given form parameter object into something that Dio can handle.
+/// Returns primitive or String.
+/// Returns List/Map if the value is BuildList/BuiltMap.
+dynamic encodeFormParameter(Serializers serializers, dynamic value, FullType type) {
+  if (value == null) {
+    return '';
+  }
+  if (value is String || value is num || value is bool) {
+    return value;
+  }
+  final serialized = serializers.serialize(value, specifiedType: type);
+  if (serialized is String) {
+    return serialized;
+  }
+  if (value is BuiltList || value is BuiltMap) {
+    return serialized;
+  }
+  return json.encode(serialized);
+}
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 31ad6c101bb..3baaf489520 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
@@ -419,8 +419,8 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      if (name != null) r'name': parameterToString(_serializers, name),
-      if (status != null) r'status': parameterToString(_serializers, status),
+      if (name != null) r'name': encodeFormParameter(_serializers, name, const FullType(String)),
+      if (status != null) r'status': encodeFormParameter(_serializers, status, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
@@ -475,7 +475,7 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = FormData.fromMap(<String, dynamic>{
-      if (additionalMetadata != null) r'additionalMetadata': parameterToString(_serializers, additionalMetadata),
+      if (additionalMetadata != null) r'additionalMetadata': encodeFormParameter(_serializers, additionalMetadata, const FullType(String)),
       if (file != null) r'file': MultipartFile.fromBytes(file, filename: r'file'),
     });
 
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
index 150ffb3f9bb..a6ce78558cf 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
@@ -7,15 +7,25 @@
 
 import 'dart:convert';
 
+import 'package:built_collection/built_collection.dart';
 import 'package:built_value/serializer.dart';
 
-/// Format the given parameter object into string.
-String parameterToString(Serializers serializers, dynamic value) {
-    if (value == null) {
-        return '';
-    } else if (value is String || value is num) {
-        return value.toString();
-    } else {
-        return json.encode(serializers.serialize(value));
-    }
-}
\ No newline at end of file
+/// Format the given form parameter object into something that Dio can handle.
+/// Returns primitive or String.
+/// Returns List/Map if the value is BuildList/BuiltMap.
+dynamic encodeFormParameter(Serializers serializers, dynamic value, FullType type) {
+  if (value == null) {
+    return '';
+  }
+  if (value is String || value is num || value is bool) {
+    return value;
+  }
+  final serialized = serializers.serialize(value, specifiedType: type);
+  if (serialized is String) {
+    return serialized;
+  }
+  if (value is BuiltList || value is BuiltMap) {
+    return serialized;
+  }
+  return json.encode(serialized);
+}
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 13e809798f7..ff33d7ca5d8 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
@@ -449,8 +449,8 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      if (name != null) r'name': parameterToString(_serializers, name),
-      if (status != null) r'status': parameterToString(_serializers, status),
+      if (name != null) r'name': encodeFormParameter(_serializers, name, const FullType(String)),
+      if (status != null) r'status': encodeFormParameter(_serializers, status, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
@@ -505,7 +505,7 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = FormData.fromMap(<String, dynamic>{
-      if (additionalMetadata != null) r'additionalMetadata': parameterToString(_serializers, additionalMetadata),
+      if (additionalMetadata != null) r'additionalMetadata': encodeFormParameter(_serializers, additionalMetadata, const FullType(String)),
       if (file != null) r'file': MultipartFile.fromBytes(file, filename: r'file'),
     });
 
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
index 150ffb3f9bb..a6ce78558cf 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api_util.dart
@@ -7,15 +7,25 @@
 
 import 'dart:convert';
 
+import 'package:built_collection/built_collection.dart';
 import 'package:built_value/serializer.dart';
 
-/// Format the given parameter object into string.
-String parameterToString(Serializers serializers, dynamic value) {
-    if (value == null) {
-        return '';
-    } else if (value is String || value is num) {
-        return value.toString();
-    } else {
-        return json.encode(serializers.serialize(value));
-    }
-}
\ No newline at end of file
+/// Format the given form parameter object into something that Dio can handle.
+/// Returns primitive or String.
+/// Returns List/Map if the value is BuildList/BuiltMap.
+dynamic encodeFormParameter(Serializers serializers, dynamic value, FullType type) {
+  if (value == null) {
+    return '';
+  }
+  if (value is String || value is num || value is bool) {
+    return value;
+  }
+  final serialized = serializers.serialize(value, specifiedType: type);
+  if (serialized is String) {
+    return serialized;
+  }
+  if (value is BuiltList || value is BuiltMap) {
+    return serialized;
+  }
+  return json.encode(serialized);
+}
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 59106fd810e..61849d6e4e0 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
@@ -587,20 +587,20 @@ class FakeApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      if (integer != null) r'integer': parameterToString(_serializers, integer),
-      if (int32 != null) r'int32': parameterToString(_serializers, int32),
-      if (int64 != null) r'int64': parameterToString(_serializers, int64),
-      r'number': parameterToString(_serializers, number),
-      if (float != null) r'float': parameterToString(_serializers, float),
-      r'double': parameterToString(_serializers, double_),
-      if (string != null) r'string': parameterToString(_serializers, string),
-      r'pattern_without_delimiter': parameterToString(_serializers, patternWithoutDelimiter),
-      r'byte': parameterToString(_serializers, byte),
+      if (integer != null) r'integer': encodeFormParameter(_serializers, integer, const FullType(int)),
+      if (int32 != null) r'int32': encodeFormParameter(_serializers, int32, const FullType(int)),
+      if (int64 != null) r'int64': encodeFormParameter(_serializers, int64, const FullType(int)),
+      r'number': encodeFormParameter(_serializers, number, const FullType(num)),
+      if (float != null) r'float': encodeFormParameter(_serializers, float, const FullType(double)),
+      r'double': encodeFormParameter(_serializers, double_, const FullType(double)),
+      if (string != null) r'string': encodeFormParameter(_serializers, string, const FullType(String)),
+      r'pattern_without_delimiter': encodeFormParameter(_serializers, patternWithoutDelimiter, const FullType(String)),
+      r'byte': encodeFormParameter(_serializers, byte, const FullType(String)),
       if (binary != null) r'binary': MultipartFile.fromBytes(binary, filename: r'binary'),
-      if (date != null) r'date': parameterToString(_serializers, date),
-      if (dateTime != null) r'dateTime': parameterToString(_serializers, dateTime),
-      if (password != null) r'password': parameterToString(_serializers, password),
-      if (callback != null) r'callback': parameterToString(_serializers, callback),
+      if (date != null) r'date': encodeFormParameter(_serializers, date, const FullType(DateTime)),
+      if (dateTime != null) r'dateTime': encodeFormParameter(_serializers, dateTime, const FullType(DateTime)),
+      if (password != null) r'password': encodeFormParameter(_serializers, password, const FullType(String)),
+      if (callback != null) r'callback': encodeFormParameter(_serializers, callback, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
@@ -661,8 +661,8 @@ class FakeApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      if (enumFormStringArray != null) r'enum_form_string_array': parameterToString(_serializers, enumFormStringArray),
-      if (enumFormString != null) r'enum_form_string': parameterToString(_serializers, enumFormString),
+      if (enumFormStringArray != null) r'enum_form_string_array': encodeFormParameter(_serializers, enumFormStringArray, const FullType(BuiltList, [FullType(String)])),
+      if (enumFormString != null) r'enum_form_string': encodeFormParameter(_serializers, enumFormString, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
@@ -818,8 +818,8 @@ class FakeApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      r'param': parameterToString(_serializers, param),
-      r'param2': parameterToString(_serializers, param2),
+      r'param': encodeFormParameter(_serializers, param, const FullType(String)),
+      r'param2': encodeFormParameter(_serializers, param2, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
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 fd5bb342a49..857a07bf209 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
@@ -419,8 +419,8 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = <String, dynamic>{
-      if (name != null) r'name': parameterToString(_serializers, name),
-      if (status != null) r'status': parameterToString(_serializers, status),
+      if (name != null) r'name': encodeFormParameter(_serializers, name, const FullType(String)),
+      if (status != null) r'status': encodeFormParameter(_serializers, status, const FullType(String)),
     };
 
     final _response = await _dio.request<dynamic>(
@@ -475,7 +475,7 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = FormData.fromMap(<String, dynamic>{
-      if (additionalMetadata != null) r'additionalMetadata': parameterToString(_serializers, additionalMetadata),
+      if (additionalMetadata != null) r'additionalMetadata': encodeFormParameter(_serializers, additionalMetadata, const FullType(String)),
       if (file != null) r'file': MultipartFile.fromBytes(file, filename: r'file'),
     });
 
@@ -546,7 +546,7 @@ class PetApi {
     dynamic _bodyData;
 
     _bodyData = FormData.fromMap(<String, dynamic>{
-      if (additionalMetadata != null) r'additionalMetadata': parameterToString(_serializers, additionalMetadata),
+      if (additionalMetadata != null) r'additionalMetadata': encodeFormParameter(_serializers, additionalMetadata, const FullType(String)),
       r'requiredFile': MultipartFile.fromBytes(requiredFile, filename: r'requiredFile'),
     });
 
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api_util.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api_util.dart
index 150ffb3f9bb..a6ce78558cf 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api_util.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api_util.dart
@@ -7,15 +7,25 @@
 
 import 'dart:convert';
 
+import 'package:built_collection/built_collection.dart';
 import 'package:built_value/serializer.dart';
 
-/// Format the given parameter object into string.
-String parameterToString(Serializers serializers, dynamic value) {
-    if (value == null) {
-        return '';
-    } else if (value is String || value is num) {
-        return value.toString();
-    } else {
-        return json.encode(serializers.serialize(value));
-    }
-}
\ No newline at end of file
+/// Format the given form parameter object into something that Dio can handle.
+/// Returns primitive or String.
+/// Returns List/Map if the value is BuildList/BuiltMap.
+dynamic encodeFormParameter(Serializers serializers, dynamic value, FullType type) {
+  if (value == null) {
+    return '';
+  }
+  if (value is String || value is num || value is bool) {
+    return value;
+  }
+  final serialized = serializers.serialize(value, specifiedType: type);
+  if (serialized is String) {
+    return serialized;
+  }
+  if (value is BuiltList || value is BuiltMap) {
+    return serialized;
+  }
+  return json.encode(serialized);
+}
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api/fake_api_test.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api/fake_api_test.dart
new file mode 100644
index 00000000000..c634c386e92
--- /dev/null
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api/fake_api_test.dart
@@ -0,0 +1,119 @@
+import 'dart:typed_data';
+
+import 'package:built_collection/built_collection.dart';
+import 'package:dio/dio.dart';
+import 'package:http_mock_adapter/http_mock_adapter.dart';
+import 'package:openapi/api.dart';
+import 'package:openapi/api/fake_api.dart';
+import 'package:test/test.dart';
+
+void main() {
+  Openapi client;
+  DioAdapter server;
+
+  setUp(() {
+    server = DioAdapter();
+    client = Openapi(dio: Dio()..httpClientAdapter = server);
+  });
+
+  tearDown(() {
+    server.close();
+  });
+
+  group(FakeApi, () {
+    group('testEndpointParameters', () {
+      test('complete', () async {
+        server.onPost(
+          '/fake',
+          data: {
+            'number': '3',
+            'double': '-13.57',
+            'pattern_without_delimiter': 'patternWithoutDelimiter',
+            'byte': '0',
+            'float': '1.23',
+            'integer': '45',
+            'int32': '2147483647',
+            'int64': '9223372036854775807',
+            'date': '2020-08-11T00:00:00.000Z',
+            'dateTime': '2020-08-11T12:30:55.123Z',
+            'binary': "Instance of 'MultipartFile'",
+          },
+          headers: <String, dynamic>{
+            'content-type': 'application/x-www-form-urlencoded',
+            'content-length': 255,
+          },
+          handler: (response) => response.reply(200, null),
+        );
+
+        final response = await client.getFakeApi().testEndpointParameters(
+              3,
+              -13.57,
+              'patternWithoutDelimiter',
+              '0',
+              float: 1.23,
+              integer: 45,
+              int32: 2147483647,
+              int64: 9223372036854775807,
+              date: DateTime.utc(2020, 8, 11),
+              dateTime: DateTime.utc(2020, 8, 11, 12, 30, 55, 123),
+              binary: Uint8List.fromList([0, 1, 2, 3, 4, 5]),
+            );
+
+        expect(response.statusCode, 200);
+      });
+
+      test('minimal', () async {
+        server.onPost(
+          '/fake',
+          data: {
+            'byte': '0',
+            'double': '-13.57',
+            'number': '3',
+            'pattern_without_delimiter': 'patternWithoutDelimiter',
+          },
+          headers: <String, dynamic>{
+            'content-type': 'application/x-www-form-urlencoded',
+            'content-length': 79,
+          },
+          handler: (response) => response.reply(200, null),
+        );
+
+        final response = await client.getFakeApi().testEndpointParameters(
+              3,
+              -13.57,
+              'patternWithoutDelimiter',
+              '0',
+            );
+
+        expect(response.statusCode, 200);
+      });
+    });
+
+    group('testEnumParameters', () {
+      test('in body data', () async {
+        // Not sure if this is correct, we are not sending
+        // form data in the body but some weird map
+        server.onGet(
+          '/fake',
+          data: {
+            'enum_form_string': 'formString',
+            'enum_form_string_array': '[foo, bar]',
+          },
+          headers: <String, dynamic>{
+            'content-type': 'application/x-www-form-urlencoded',
+          },
+          handler: (response) => response.reply(200, null),
+        );
+
+        final response = await client.getFakeApi().testEnumParameters(
+              enumFormString: 'formString',
+              enumFormStringArray: ListBuilder<String>(
+                <String>['foo', 'bar'],
+              ).build(),
+            );
+
+        expect(response.statusCode, 200);
+      });
+    });
+  });
+}
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api_util_test.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api_util_test.dart
new file mode 100644
index 00000000000..f48210be33e
--- /dev/null
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake_tests/test/api_util_test.dart
@@ -0,0 +1,230 @@
+import 'package:built_collection/built_collection.dart';
+import 'package:built_value/serializer.dart';
+import 'package:dio/dio.dart';
+import 'package:openapi/api_util.dart';
+import 'package:openapi/model/cat.dart';
+import 'package:openapi/serializers.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('api_utils', () {
+    group('encodeFormParameter should return', () {
+      test('empty String for null', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            null,
+            const FullType(Cat),
+          ),
+          '',
+        );
+      });
+
+      test('String for String', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            'foo',
+            const FullType(String),
+          ),
+          'foo',
+        );
+      });
+
+      test('List<String> for BuiltList<String>', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            ListBuilder<String>(['foo', 'bar', 'baz']).build(),
+            const FullType(BuiltList, [FullType(String)]),
+          ),
+          ['foo', 'bar', 'baz'],
+        );
+      });
+
+      test('Map<String, String> for BuiltList<String, String>', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            MapBuilder<String, String>({
+              'foo': 'foo-value',
+              'bar': 'bar-value',
+              'baz': 'baz-value',
+            }).build(),
+            const FullType(BuiltMap, [FullType(String), FullType(String)]),
+          ),
+          {
+            'foo': 'foo-value',
+            'bar': 'bar-value',
+            'baz': 'baz-value',
+          },
+        );
+      });
+
+      test('num for num', () {
+        expect(
+          encodeFormParameter(standardSerializers, 0, const FullType(int)),
+          0,
+        );
+        expect(
+          encodeFormParameter(standardSerializers, 1, const FullType(int)),
+          1,
+        );
+        expect(
+          encodeFormParameter(standardSerializers, 1.0, const FullType(num)),
+          1.0,
+        );
+        expect(
+          encodeFormParameter(
+              standardSerializers, 1.234, const FullType(double)),
+          1.234,
+        );
+      });
+
+      test('List<num> for BuiltList<num>', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            ListBuilder<num>([0, 1, 2, 3, 4.5, -123.456]).build(),
+            const FullType(BuiltList, [FullType(num)]),
+          ),
+          [0, 1, 2, 3, 4.5, -123.456],
+        );
+      });
+
+      test('bool for bool', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            true,
+            const FullType(bool),
+          ),
+          true,
+        );
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            false,
+            const FullType(bool),
+          ),
+          false,
+        );
+      });
+
+      test('String for Date', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            DateTime.utc(2020, 8, 11),
+            const FullType(DateTime),
+          ),
+          '2020-08-11T00:00:00.000Z',
+        );
+      });
+
+      test('String for DateTime', () {
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            DateTime.utc(2020, 8, 11, 12, 30, 55, 123),
+            const FullType(DateTime),
+          ),
+          '2020-08-11T12:30:55.123Z',
+        );
+      });
+
+      test('JSON String for Cat', () {
+        // Not sure that is even a valid case,
+        // sending complex objects via FormData may not work as expected
+        expect(
+          encodeFormParameter(
+            standardSerializers,
+            (CatBuilder()
+                  ..color = 'black'
+                  ..className = 'cat'
+                  ..declawed = false)
+                .build(),
+            const FullType(Cat),
+          ),
+          '{"className":"cat","color":"black","declawed":false}',
+        );
+      });
+    });
+
+    test('encodes FormData correctly', () {
+      final data = FormData.fromMap({
+        'null': encodeFormParameter(
+          standardSerializers,
+          null,
+          const FullType(num),
+        ),
+        'empty': encodeFormParameter(
+          standardSerializers,
+          '',
+          const FullType(String),
+        ),
+        'string_list': encodeFormParameter(
+          standardSerializers,
+          ListBuilder<String>(['foo', 'bar', 'baz']).build(),
+          const FullType(BuiltList, [FullType(String)]),
+        ),
+        'num_list': encodeFormParameter(
+          standardSerializers,
+          ListBuilder<num>([0, 1, 2, 3, 4.5, -123.456]).build(),
+          const FullType(BuiltList, [FullType(num)]),
+        ),
+        'string_map': encodeFormParameter(
+          standardSerializers,
+          MapBuilder<String, String>({
+            'foo': 'foo-value',
+            'bar': 'bar-value',
+            'baz': 'baz-value',
+          }).build(),
+          const FullType(BuiltMap, [FullType(String), FullType(String)]),
+        ),
+        'bool': encodeFormParameter(
+          standardSerializers,
+          true,
+          const FullType(bool),
+        ),
+        'double': encodeFormParameter(
+          standardSerializers,
+          -123.456,
+          const FullType(double),
+        ),
+        'date_time': encodeFormParameter(
+          standardSerializers,
+          DateTime.utc(2020, 8, 11, 12, 30, 55, 123),
+          const FullType(DateTime),
+        ),
+      });
+
+      expect(
+        data.fields,
+        pairwiseCompare<MapEntry<String, String>, MapEntry<String, String>>(
+          <MapEntry<String, String>>[
+            MapEntry('null', ''),
+            MapEntry('empty', ''),
+            MapEntry('string_list[]', 'foo'),
+            MapEntry('string_list[]', 'bar'),
+            MapEntry('string_list[]', 'baz'),
+            MapEntry('num_list[]', '0'),
+            MapEntry('num_list[]', '1'),
+            MapEntry('num_list[]', '2'),
+            MapEntry('num_list[]', '3'),
+            MapEntry('num_list[]', '4.5'),
+            MapEntry('num_list[]', '-123.456'),
+            MapEntry('string_map[foo]', 'foo-value'),
+            MapEntry('string_map[bar]', 'bar-value'),
+            MapEntry('string_map[baz]', 'baz-value'),
+            MapEntry('bool', 'true'),
+            MapEntry('double', '-123.456'),
+            MapEntry('date_time', '2020-08-11T12:30:55.123Z'),
+          ],
+          (e, a) => e.key == a.key && e.value == a.value,
+          'Compares map entries by key and value',
+        ),
+      );
+    });
+  });
+}