/dotnet-webapi

name: dotnet-webapi
description: >
  Guides creation and modification of ASP.NET Core Web API endpoints with
  correct HTTP semantics, OpenAPI metadata, and error handling.
  USE FOR: adding new API endpoints (controllers or minimal APIs), wiring up
  OpenAPI/Swagger, creating .http test files, setting up global error handling
  middleware.
  DO NOT USE FOR: general C# coding style, EF Core data access or query
  optimization (use optimizing-ef-core-queries), frontend/Blazor work, gRPC
  services, or SignalR hubs.
license: MIT

ASP.NET Core Web API

Produce well-structured ASP.NET Core Web API endpoints with proper HTTP semantics, OpenAPI documentation, and error handling.

When to Use

Use this skill when working on ASP.NET Core HTTP APIs, including:

• adding or modifying Web API endpoints implemented with controllers or minimal APIs;
• wiring up OpenAPI/Swagger metadata and endpoint documentation;
• defining request/response DTOs and consistent HTTP status code behavior;
• adding `.http` files or similar request-based API testing artifacts;
• configuring centralized API error handling middleware or exception mapping.

When Not to Use

Do not use this skill for:

• general C# coding style or non-API refactoring;
• EF Core data modeling or query optimization work; use `optimizing-ef-core-queries`;
• frontend, Razor, or Blazor UI changes;
• gRPC services;
• SignalR hubs or real-time messaging flows.

Inputs / prerequisites

Before applying this skill, gather the project context needed to match the existing API style and wiring:

• the ASP.NET Core entry point, typically `Program.cs`;
• any existing controllers, especially classes inheriting `ControllerBase` or

using `[ApiController]`;

• any existing minimal API registrations such as `app.MapGet`, `app.MapPost`,

`app.MapPut`, or `app.MapDelete`;

• related DTO, model, validation, and error-handling types already used by the project;
• available build, run, and test commands so changes can be verified.

If the user asks for a new endpoint, inspect the current project structure first so the implementation follows the established conventions rather than mixing styles.

Workflow

Step 1: Determine the API style

Scan the project for existing endpoint patterns before writing any code.

1. Search for classes inheriting `ControllerBase` or decorated with `[ApiController]`. 2. Search `Program.cs` or endpoint files for `app.MapGet`, `app.MapPost`, etc. 3. If the project already uses **controllers**, continue with controllers. 4. If the project already uses **minimal APIs**, continue with minimal APIs. 5. If neither exists (new project), **default to minimal APIs** unless the user explicitly requests controllers.

Do not mix styles in the same project.

Step 2: Define request and response types

Create dedicated types for API input and output. Never expose EF Core entities directly in request or response bodies.

**Use `sealed record` for all DTOs.** Records enforce immutability, provide value-based equality, and produce concise code. Seal them to prevent unintended inheritance and enable JIT devirtualization (CA1852).

**Naming convention:**

| Role | Convention | Example | |------|-----------|---------| | Input (create) | `Create{Entity}Request` | `CreateProductRequest` | | Input (update) | `Update{Entity}Request` | `UpdateProductRequest` | | Output (single) | `{Entity}Response` | `ProductResponse` | | Output (list) | `{Entity}ListResponse` | `ProductListResponse` |

**XML doc comments on all DTOs:** Add `<summary>` XML doc comments to every request and response type exposed in the API. These comments are automatically included in the generated OpenAPI specification, producing richer documentation without extra metadata calls.

Reference: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/openapi/openapi-comments

**Date and time values — use `DateTimeOffset`:** When a DTO includes a date or time property, always use `DateTimeOffset` instead of `DateTime`. `DateTimeOffset` preserves the UTC offset, avoids ambiguous timezone conversions, and serializes to ISO 8601 with offset information in JSON — which is what API consumers expect.

Reference: https://learn.microsoft.com/en-us/dotnet/api/system.datetimeoffset **JSON serialization options — preserve existing behavior by default:** For existing APIs, do **not** introduce stricter serialization/deserialization settings unless the project already uses them or the user explicitly asks for them. Settings such as case-sensitive property matching and strict number handling can break existing clients. For **new projects**, or when strict JSON handling is explicitly requested, configure options like the following to minimize the potential of processing malicious requests:

// Apply these settings only for new projects, when the existing project already
// uses them, or when the user explicitly requests stricter JSON behavior.
builder.Services.ConfigureHttpJsonOptions(options =>
{
    // disallow reading numbers from JSON strings
    options.SerializerOptions.NumberHandling = JsonNumberHandling.Strict;
    // match properties with exact casing during deserialization
    options.SerializerOptions.PropertyNameCaseInsensitive = false;
    // reject duplicate JSON property names during deserialization
    options.SerializerOptions.AllowDuplicateProperties = false;
    // omit null properties from serialized output
    options.SerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;
});

**Enum properties — serialize as strings by default:** Unless the user explicitly requests integer serialization, all enum properties should be serialized as strings. String-serialized enums are human-readable, less fragile when values are reordered, and produce better OpenAPI documentation. See Step 4 for the `JsonStringEnumConverter` configuration.

**Response DTOs** — use positional sealed records for concise, immutable output:

/// <summ