Dotnet Webapi

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:

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:

/// <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:

/// <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:

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 embeds response type information in the method signature, giving the OpenAPI generator richer metadata automatically.

When a handler returns multiple result types (e.g., Ok or NotFound), annotate the lambda with an explicit Results<T1, T2> return type. This lets you use TypedResults while still giving the compiler a common type:

async Task<Results<Ok<ProductResponse>, NotFound>> (int id, ...) => ...

Do not use TypedResults.Ok(x) and TypedResults.NotFound() in a bare ternary without an explicit return type annotation. Ok<T> and NotFound are different types with no common base the compiler can infer, which causes CS1593: Delegate 'RequestDelegate' does not take N arguments because the compiler falls back to matching RequestDelegate(HttpContext).

Fallback — Results factory: If a handler has many conditional branches (7+ result types), you may use the Results factory (Results.Ok(), Results.NotFound()) which returns IResult, sacrificing compile-time OpenAPI inference for simpler signatures.

Status codes:

POST 201 responses: Always return a Location header pointing to the newly created resource.

• Controllers: use CreatedAtAction(nameof(GetById), new { id = ... }, response)
• Minimal APIs: use TypedResults.Created($"/api/products/{id}", response)

CancellationToken: Accept CancellationToken in every endpoint signature and forward it through to all async calls (service methods, EF Core queries, HttpClient calls). This allows the server to stop work when a client disconnects.

// Controller example
[HttpGet("{id}")]
public async Task<ActionResult<ProductResponse>> GetById(
    int id, CancellationToken cancellationToken)
{
    var product = await _productService.GetByIdAsync(id, cancellationToken);
    return product is null ? NotFound() : Ok(product);
}

// Minimal API example — TypedResults with explicit return type (recommended)
app.MapGet("/api/products/{id}", async Task<Results<Ok<ProductResponse>, NotFound>> (
    int id, IProductService service, CancellationToken cancellationToken) =>
{
    var product = await service.GetByIdAsync(id, cancellationToken);
    return product is null ? TypedResults.NotFound() : TypedResults.Ok(product);
});

Step 4: Wire up OpenAPI

Every ASP.NET Core Web API should have OpenAPI documentation. Check whether the project already has OpenAPI configured before adding it.

For .NET 9+ projects, use the built-in ASP.NET Core OpenAPI support (builder.Services.AddOpenApi() + app.MapOpenApi() in development). This is all that is needed — no additional packages required.

Do NOT add any Swashbuckle.* NuGet package (Swashbuckle.AspNetCore, Swashbuckle.AspNetCore.SwaggerUI, Swashbuckle.AspNetCore.SwaggerGen, etc.) to .NET 9+ projects. Swashbuckle has known compatibility issues with .NET 9+ and .NET 10 OpenAPI types. For projects targeting .NET 8 or earlier, Swashbuckle is acceptable. If the project already has Swashbuckle installed, keep it unless the user asks to remove it.

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

OpenAPI metadata on endpoints: Add descriptive metadata so the generated documentation is useful, not just a list of routes. For minimal APIs, chain the metadata methods:

app.MapGet("/api/products/{id}", handler)
    .WithName("GetProductById")
    .WithSummary("Get a product by ID")
    .WithDescription("Returns the full product details including category.")
    .Produces<ProductResponse>(StatusCodes.Status200OK)
    .Produces(StatusCodes.Status404NotFound);

Enum serialization (strings by default): Configure JSON serialization so enums appear as readable strings in both API responses and OpenAPI schemas. Always add this configuration unless the user explicitly requests integer enum serialization. Configure it for both minimal APIs and controllers, as they use different option types:

// Minimal APIs
builder.Services.ConfigureHttpJsonOptions(options =>
    options.SerializerOptions.Converters.Add(new JsonStringEnumConverter()));

// Controllers / MVC
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
    });