Use the OpenAPI Generator for Dart

Oct 13, 2025 by wbrum.org 84 views

The method 'listFromJson' isn't defined for the type 'MultipartFile'

In this article, we will discuss a common issue encountered when using the OpenAPI Generator for Dart to generate code from an OpenAPI specification. The issue arises when the generated code includes a call to MultipartFile.listFromJson(json[r'photos']), but the MultipartFile class does not have a listFromJson method. This results in a compilation error.

The OpenAPI Generator for Dart is generating code that includes a call to MultipartFile.listFromJson(json[r'photos']), but the MultipartFile class (from the http package) does not have a listFromJson method. This results in a compilation error:

The method 'listFromJson' isn't defined for the type 'MultipartFile'.

Use the OpenAPI Generator for Dart: Use the OpenAPI Generator for Dart to generate models and API code from an OpenAPI specification that includes a field expecting a list of MultipartFile objects (e.g., photos).
Generated code: The generated code will include a line like this: photos: MultipartFile.listFromJson(json[r'photos']),
Compilation error: Attempt to compile the generated code. The compilation will fail with the error mentioned above.

Here’s a minimal OpenAPI specification that reproduces the issue:

openapi: 3.0.0
info:
  title: Sample API
  version: 1.0.0
paths:
  /upload:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                photos:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        '200':
          description: Success
components:
  schemas:
    AddIncomeExpenseRequestDTO:
      type: object
      properties:
        photos:
          type: array
          items:
            type: string
            format: binary

Here’s the @Openapi annotation I used:

@Openapi(
  inputSpec: InputSpec(
    path: 'bin/restrox-swagger.json',
  ),
  generatorName: Generator.dart,
  outputDirectory: 'lib/data/generated',
  cleanSubOutputDirectory: ['lib/data/generated'],
)
class ApiClient {}

The generated code should handle the photos field correctly, either by:

Providing a valid method to deserialize a list of MultipartFile objects, or
Generating code that uses a supported method for handling file uploads (e.g., MultipartFile.fromFileSync or similar).

No specific logs are available as this is a compilation error. The error message is:

The method 'listFromJson' isn't defined for the type 'MultipartFile'.

No response

Linux

6.1.0

3.27.3

stable

To solve this issue, you can use the MultipartFile.fromFileSync method to handle file uploads. Here's an example of how you can modify the generated code to use this method:

@Openapi(
  inputSpec: InputSpec(
    path: 'bin/restrox-swagger.json',
  ),
  generatorName: Generator.dart,
  outputDirectory: 'lib/data/generated',
  cleanSubOutputDirectory: ['lib/data/generated'],
)
class ApiClient {
  Future<void> uploadFile() async {
    final file = File('path/to/file');
    final multipartFile = MultipartFile.fromFileSync(file.path, filename: file.path.split('/').last);
    final response = await http.post(
      Uri.parse('https://example.com/upload'),
      headers: {
        'Content-Type': 'multipart/form-data',
      },
      body: {
        'photos': [multipartFile],
      },
    );
    if (response.statusCode == 200) {
      print('File uploaded successfully');
    } else {
      print('Error uploading file');
    }
  }
}

In this example, we're using the MultipartFile.fromFileSync method to create a MultipartFile object from a local file. We're then sending this file in a multipart request to the server.

In conclusion, the issue of the listFromJson method not being defined for the MultipartFile type is a common problem encountered when using the OpenAPI Generator for Dart. To solve this issue, you can use the MultipartFile.fromFileSync method to handle file uploads. By modifying the generated code to use this method, you can ensure that your code compiles and runs correctly.
The method 'listFromJson' isn't defined for the type 'MultipartFile' - Q&A

In our previous article, we discussed a common issue encountered when using the OpenAPI Generator for Dart to generate code from an OpenAPI specification. The issue arises when the generated code includes a call to MultipartFile.listFromJson(json[r'photos']), but the MultipartFile class does not have a listFromJson method. This results in a compilation error.

In this article, we will provide a Q&A section to help you better understand the issue and its solution.

A: The OpenAPI Generator for Dart is a tool that generates Dart code from an OpenAPI specification. It allows you to define your API endpoints, models, and other components in a single file, and then generates the corresponding Dart code for you.

A: The issue is that the generated code includes a call to MultipartFile.listFromJson(json[r'photos']), but the MultipartFile class does not have a listFromJson method. This results in a compilation error.

A: To reproduce the issue, follow these steps:

Use the OpenAPI Generator for Dart to generate models and API code from an OpenAPI specification that includes a field expecting a list of MultipartFile objects (e.g., photos).
The generated code will include a line like this: photos: MultipartFile.listFromJson(json[r'photos']),
Attempt to compile the generated code. The compilation will fail with the error mentioned above.

A: The expected behavior is that the generated code should handle the photos field correctly, either by:

Providing a valid method to deserialize a list of MultipartFile objects, or
Generating code that uses a supported method for handling file uploads (e.g., MultipartFile.fromFileSync or similar).

A: To solve the issue, you can use the MultipartFile.fromFileSync method to handle file uploads. Here's an example of how you can modify the generated code to use this method:

@Openapi(
  inputSpec: InputSpec(
    path: 'bin/restrox-swagger.json',
  ),
  generatorName: Generator.dart,
  outputDirectory: 'lib/data/generated',
  cleanSubOutputDirectory: ['lib/data/generated'],
)
class ApiClient {
  Future<void> uploadFile() async {
    final file = File('path/to/file');
    final multipartFile = MultipartFile.fromFileSync(file.path, filename: file.path.split('/').last);
    final response = await http.post(
      Uri.parse('https://example.com/upload'),
      headers: {
        'Content-Type': 'multipart/form-data',
      },
      body: {
        'photos': [multipartFile],
      },
    );
    if (response.statusCode == 200) {
      print('File uploaded successfully');
    } else {
      print('Error uploading file');
    }
  }
}

In this example, we're using the MultipartFile.fromFileSync method to create a MultipartFile object from a local file. We're then sending this file in a multipart request to the server.

A: The benefits of using the OpenAPI Generator for Dart include:

Faster development: The OpenAPI Generator for Dart allows you to generate code quickly and easily, saving you time and effort.
Improved code quality: The generated code is high-quality and well-structured, making it easier to maintain and extend.
Better API documentation: The OpenAPI Generator for Dart generates API documentation automatically, making it easier to understand and use your API.

In conclusion, the issue of the listFromJson method not being defined for the MultipartFile type is a common problem encountered when using the OpenAPI Generator for Dart. By understanding the issue and its solution, you can ensure that your code compiles and runs correctly.