dotnet-webapi

What this skill does

# 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:

```csharp
// 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:

```csharp
/// <summary>Represents a product returned by the API.</summary>
public sealed record ProductResponse(
    int Id,
    string Name,
    decimal Price,
    Category Category,
    bool IsAvailable,
    DateTimeOffset CreatedAt);
```

**Request DTOs** — use sealed records with `init` properties so data annotations
work naturally:

```csharp
/// <summary>Payload for creating a new product.</summary>
public sealed record CreateProductRequest
{
    [Required, MaxLength(200)]
    public required string Name { get; init; }

    [Range(0.01, 999999.99)]
    public required decimal Price { get; init; }

    public required Category Category { get; init; }
}
```

Follow the same pattern for `Update{Entity}Request` records, adding any
additional properties the update requires (e.g., `IsAvailable`).

**Minimal API validation — register explicitly:** Data-annotation validation
(`[Required]`, `[MaxLength]`, `[Range]`, etc.) is automatic in MVC controllers,
but minimal APIs require explicit opt-in. For **.NET 10+** projects using minimal
APIs, add the validation services in `Program.cs`:

```csharp
builder.Services.AddValidation();
```

This wires up an endpoint filter that validates parameters decorated with data
annotations before the handler executes, returning a `400 Bad Request` with a
validation problem details response on failure.

Reference: https://learn.microsoft.com/aspnet/core/fundamentals/minimal-apis?view=aspnetcore-10.0

**Do not** use mutable classes (`{ get; set; }`) for DTOs. Mutable DTOs allow
accidental modification after construction and lose the self-documenting
immutability that records provide.

### Step 3: Implement the endpoints

Whether using controllers or minimal APIs, follow these HTTP conventions
consistently.

**Organizing minimal API endpoints:** For projects using minimal APIs, organize
endpoints by resource using static classes with a static `Map<Resource>` method.
This pattern keeps endpoint definitions grouped by resource type, making the
code more maintainable and easier to navigate as the API grows.

**Pattern structure:**

1. Create one static class per resource (e.g., `ProductEndpoints`, `CategoryEndpoints`).
2. Define a static `Map<Resource>(this WebApplication app)` extension method.
3. Inside the method, call `MapGet`, `MapPost`, `MapPut`, `MapDelete`, etc. for
   that resource's endpoints.
4. In `Program.cs`, call each resource's `Map` method in order.

**Minimal API return types — prefer `TypedResults`:**

Always prefer `TypedResults` over the `Results` factory. `TypedResults