Implementing API Design First in .NET for Efficient Development, Testing, and CI/CD

In today's agile software landscape, building robust and scalable applications hinges on well-defined APIs. The API design-first approach offers a structured methodology where the API contract is meticulously crafted before any code is written. This strategy is powered by the OpenAPI Specification (OAS) for Synchronous APIs and AsyncAPI Specification for Asynchronous APIs.

This article (an opinionanted tutorial) provides a practical guide to implementing API design-first in Synchronous .NET API projects, with a strong focus on its benefits for development, testing, and seamless integration into DevOps CI/CD pipelines. I will follow up with an AsyncAPI article later.

Understanding the API Design First Principle

The API design-first principle means prioritizing the design and specification of the API's interface using a contract language like the OpenAPI Specification before diving into the implementation code. This approach is more than a decade old however, many organizations are still on code first approach. There are numerous articles available on this, covering it in brief to set the stage:

• Enhanced Team Alignment: The API contract as the blueprint to make sure teams building from the other ends of the bridge meets at one point.
• Accelerated Development: Frontend and client teams can start their work based on the OAS using mock servers, even before the backend implementation is complete.
• Improved API Usability: Thoughtful design upfront leads to more intuitive, consistent, and easier-to-consume APIs.
• Automation Ready: The OAS acts as a machine-readable blueprint, enabling automation of various development lifecycle stages.

The Central Role of the OpenAPI Specification (OAS)

The OpenAPI Specification (OAS) is the cornerstone of the API design-first workflow. It provides a standardized format (YAML or JSON) to describe your RESTful APIs, detailing every aspect from endpoints and operations to request and response schemas, authentication, and more. Think of the OAS as the definitive blueprint for your API. I am personally biased towards YAML for its better fitness in the code repo (merging nested JSON files can become tricky at times).

Leveraging API Design First Approach for Design and Development in .NET

By embracing API design first in .NET projects, we can unlock significant benefits across the development lifecycle:

Design

Swagger editor or VS Code plugins for OpenAPI is an excellent tool to define OAS. The Open API initiative acts as the definitive guide. There are nuanced differences between the major release version 2 and 3. More and more tools are supporting version 3 release.

Development

Server: Generate .NET Interface and DTOs using NSwag or openapi-generator. I prefer openapi-generator-cli purely based on my comfort and wide community support. These interfaces and DTOs can be implemented in your ecosystem. Generally .Net ecosystem supports a template based platform—these DTOs and Interfaces can be imported into the solutions generated using those templates. If not, use the project and solution definition generated by the openapi-generator.

1. Prerequisite: npm and Java jdk 11 or higher
2. Install openapi-generator-cli - npm install -g @openapitools/openapi-generator-cli
3. Validate OAS though openapi-generator-cli
4. Create an openapi-generator-cli input template as its helpful in experimentation and reusable
5. Generate Solution scaffolding containing interfaces and DTOs using openapi-generator-cli
6. Update the Program.cs or Startup.cs to serve the generated Swagger. This ensures that the code generated Swagger gives an assurance of what is being implemented
7. Create a static file hosting ability in the Program.cs or Startup.cs and host the static file and make sure this is also being served in the Swagger UI. The availability of static OAS in SwaggerUI serves as the requirement and the generated OAS in the SwaggerUI serves as the proof of what has been built.
8. The code generators in the .NET ecosystem do not work as seamlessly as in the Java ecosystem with build plugins, however the DTOs and the Controller scaffolding with all the formatters and validators are extremely useful.

Client: Client generation ecosystem is relatively more matured and there are nuget packages as well as other cli utilities. I will stick to openapi-generator-cli for the sake of consistency in this article.

1. Prerequisite: OAS file, npm and Java jdk 11 or higher
2. Install openapi-generator-cli (if not already)
3. Validate OAS through openapi-generator-cli (if not already)
4. Create Generator Configuration File
5. Generate Client using openapi-generator-cli
6. Add Generated Code to Client Project:
7. (Optional) Customize Generated Client

CLI samples:

# install openapi-generator-cli refer to https://openapi-generator.tech/docs/installation
npm install -g @openapitools/openapi-generator-cli
# install validation openapi-generator-cli
openapi-generator-cli version
#validate OAS refer to https://openapi-generator.tech/docs/usage#validate
openapi-generator-cli validate -i <OASfile>

# input yaml sample for generator
# csharp as generator name for client and aspnetcore for server
# openapi-generator-cli version
# refer to https://openapi-generator.tech/docs/usage#configuration-file
generatorName: csharp
inputSpec: ./path/to/your/openapi.yaml  # Path to your OpenAPI specification file
outputDir: ./GeneratedCode  # Output directory for the generated code
additionalProperties:
 OperationIsAsunc: "true"
 packageName: "your meaningful namespace"

# Server Generator rwer tohttps://openapi-generator.tech/docs/usage#generate
# My preference is using input file and batch operation https://openapi-generator.tech/docs/usage#batch
openapi-generator-cli batch <input file>

Testing the Client and Server in API Design First Approach

Server-Side Testing

1. Planning and Initial Stages: Utilize the initial OAS definition to strategize various testing aspects, including unit, integration, and others, as needed by the application teams.
2. Unit Testing with .NET Libraries: Employ .NET libraries like NJsonSchema.Validation and assertion libraries (e.g., FluentAssertions) within testing frameworks (xUnit, NUnit, MSTest) to validate API responses against OAS schemas, ensuring .NET API contract fulfillment.
3. Mocking and Isolation: During testing, leverage .NET mocking libraries (e.g., WireMock.Net) or in-memory test servers (Microsoft.AspNetCore.Mvc.Testing), configured with OAS specifications, to isolate the .NET API and simulate dependent service behavior.
4. Deployment and Validation: The OAS is crucial for validating and managing deployed .NET APIs; create automated .NET integration tests that run post-deployment to verify live API endpoint adherence to OAS specifications.
5. API Gateway Integration: For internet-facing APIs managed by Azure API Management, the OAS serves as the structural definition, enabling APIM to enforce policies and generate developer portal documentation.

Client-Side Testing

1. Generated Client Integration: Add a project reference or include the generated client code files into your client application's test project (e.g., a separate xUnit, NUnit, or MSTest project).
2. Client Instantiation: Create an instance of the generated API client class in your test class, potentially providing it with an HttpClient instance configured to point to a mock server or controlled test environment for testing purposes
3. Mocking for Unit Testing: Utilize a mocking library like Moq to create mock implementations of the HttpClient or IHttpClientFactory to test client logic in isolation, simulating diverse API responses without actual network calls.
4. Mock Response Configuration: Set up the mock HttpClient to return specific HttpResponseMessage instances representing expected API endpoint responses (both success and error scenarios) as defined in your OAS, including status codes, headers, and content adhering to the response schema.
5. Client Method Invocation: Call the methods on your generated API client that correspond to the API endpoints you want to test.
6. Successful Response Assertion: For API calls returning data, assert correct client deserialization of the JSON response into generated C# model classes, verifying properties and values against OAS expectations.
7. Error Handling Assertion: Test generated client handling of different HTTP error status codes documented in your OAS, asserting expected exception throws (if configured) or appropriate error model instance returns.
8. Request Parameter Verification (with Mocking): Verify correct HTTP request construction by the generated client, including method, URL, headers, and content (request body serialization based on OAS).
9. Integration Tests (Test Environment): For integration tests, point your generated client to a running server instance in a dedicated test environment to verify end-to-end interaction between the client and actual API implementation.

Integrating OpenAPI Specifications Into Azure DevOps CI/CD Pipeline for .Net 

Integrating the OAS into the DevOps CI/CD pipelines automates crucial steps improvements to make it more robust and ensures a smooth and consistent API lifecycle based on your API design-first approach in the .NET ecosystem.

Build Pipeline:

1. OAS Validation: Validate OAS file schema.
2. OAS Linting: Lint OAS file for style and best practices.
3. Code Generation (Optional - Conditional): Generate .NET server stubs (if needed).
4. Client SDK Generation: Generate .NET client SDKs and package as NuGet artifacts.
5. Build .NET Application: Compile your .NET microservice.
6. Run Unit and Integration Tests: Verify the implementation against the OAS and other logic.
7. Container Image Generation: Build Docker image for the microservice.
8. Generate Helm Chart Configurations: Create Helm chart files for Kubernetes deployment.
9. Artifact Publication: Publish compiled application, Docker image, Helm charts, generated code, and client SDKs as build artifacts.

Release Pipeline:

1. Deployment to Kubernetes: Deploy Docker image and Helm chart to the target Rancher Kubernetes cluster using environment-specific configurations.
2. Verify Deployment Health: Check Kubernetes readiness and liveness probes or perform basic API health checks.
3. Post-Deployment Contract Verification: Run automated .NET integration tests against the deployed API to ensure adherence to the OAS.
4. Azure API Management Update: Import/update API definition in Azure API Management using the OAS file from the build artifacts.
5. (Optional) Apply API Policies (APIM): Configure or update API policies in Azure API Management.

Future Trends

Expect continued evolution in .NET tooling to further streamline the API design-first workflow, including more integrated code generation capabilities and enhanced support for contract testing and API governance. Here is a comparative study of popular frameworks in .NET ecosystem in 2025 and is expected to grow.

Note: The block and flow diagrams are generated using mermaid scripts.