Tutorial: Create a minimal web API with ASP.NET Core

By Rick Anderson

Minimal APIs are architected to create HTTP APIs with minimal dependencies. They are ideal for microservices and apps that want to include only the minimum files, features, and dependencies in ASP.NET Core.

This tutorial teaches the basics of building a minimal web API with ASP.NET Core. For a tutorial on creating a web API project based on controllers that contains more features, see Create a web API.

Overview

This tutorial creates the following API:

API Description Request body Response body
GET / Browser test, "Hello World" None Hello World!
GET /todoitems Get all to-do items None Array of to-do items
GET /todoitems/complete Get completed to-do items None Array of to-do items
GET /todoitems/{id} Get an item by ID None To-do item
POST /todoitems Add a new item To-do item To-do item
PUT /todoitems/{id} Update an existing item   To-do item None
DELETE /todoitems/{id}     Delete an item     None None

Prerequisites

VS22 installer workloads

Create a Web API project

  • Start Visual Studio 2022 and select Create a new project.

  • In the Create a new project dialog:

    • Enter API in the Search for templates search box.
    • Select the ASP.NET Core Web API template and select Next. Visual Studio Create a new project
  • Name the project TodoApi and select Next.

  • In the Additional information dialog:

    • Select .NET 6.0 (Long-term support)
    • Remove Use controllers (uncheck to use minimal APIs)
    • Select Create

Additional information

Examine the code

The Program.cs file contains the following code:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

var summaries = new[]
{
    "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
};

app.MapGet("/weatherforecast", () =>
{
    var forecast = Enumerable.Range(1, 5).Select(index =>
       new WeatherForecast
       (
           DateTime.Now.AddDays(index),
           Random.Shared.Next(-20, 55),
           summaries[Random.Shared.Next(summaries.Length)]
       ))
        .ToArray();
    return forecast;
})
.WithName("GetWeatherForecast");

app.Run();

internal record WeatherForecast(DateTime Date, int TemperatureC, string? Summary)
{
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
}

The project template creates a WeatherForecast API with support for Swagger. Swagger is used to generate useful documentation and help pages for web APIs.

The following highlighted code adds support for Swagger:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

Run the app

Press Ctrl+F5 to run without the debugger.

Visual Studio displays the following dialog:

This project is configured to use SSL. To avoid SSL warnings in the browser you can choose to trust the self-signed certificate that IIS Express has generated. Would you like to trust the IIS Express SSL certificate?

Select Yes if you trust the IIS Express SSL certificate.

The following dialog is displayed:

Security warning dialog

Select Yes if you agree to trust the development certificate.

For information on trusting the Firefox browser, see Firefox SEC_ERROR_INADEQUATE_KEY_USAGE certificate error.

Visual Studio launches the Kestrel web server.

The Swagger page /swagger/index.html is displayed. Select GET > Try it out > Execute. The page displays:

  • The Curl command to test the WeatherForecast API.
  • The URL to test the WeatherForecast API.
  • The response code, body, and headers.
  • A drop down list box with media types and the example value and schema.

Copy and paste the Request URL in the browser: https://localhost:<port>/WeatherForecast. JSON similar to the following is returned:

[
  {
    "date": "2021-10-19T14:12:50.3079024-10:00",
    "temperatureC": 13,
    "summary": "Bracing",
    "temperatureF": 55
  },
  {
    "date": "2021-10-20T14:12:50.3080559-10:00",
    "temperatureC": -8,
    "summary": "Bracing",
    "temperatureF": 18
  },
  {
    "date": "2021-10-21T14:12:50.3080601-10:00",
    "temperatureC": 12,
    "summary": "Hot",
    "temperatureF": 53
  },
  {
    "date": "2021-10-22T14:12:50.3080603-10:00",
    "temperatureC": 10,
    "summary": "Sweltering",
    "temperatureF": 49
  },
  {
    "date": "2021-10-23T14:12:50.3080604-10:00",
    "temperatureC": 36,
    "summary": "Warm",
    "temperatureF": 96
  }
]

Update the generated code

This tutorial focuses on creating a web API, so we'll delete the Swagger code and the WeatherForecast code. Replace the contents of the Program.cs file with the following:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

The following highlighted code creates a WebApplicationBuilder and a WebApplication with preconfigured defaults:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

The following code creates an HTTP GET endpoint / which returns Hello World!:

app.MapGet("/", () => "Hello World!");

app.Run(); runs the app.

Remove the two "launchUrl": "swagger", lines from the Properties/launchSettings.json file. When the launchUrl isn't specified, the web browser requests the / endpoint.

Run the app. Hello World! is displayed. The updated Program.cs file contains a minimal but complete app.

Add NuGet packages

NuGet packages must be added to support the database and diagnostics used in this tutorial.

  • From the Tools menu, select NuGet Package Manager > Manage NuGet Packages for Solution.
  • Select the Browse tab, and verify that Include prerelease is checked.
  • Enter Microsoft.EntityFrameworkCore.InMemory in the search box, and then select Microsoft.EntityFrameworkCore.InMemory.
  • Select the Project checkbox in the right pane and then select Install.
  • Follow the preceding instructions to add the Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore package.

Add the API code

Replace the contents of the Program.cs file with the following code:

using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDbContext<TodoDb>(opt => opt.UseInMemoryDatabase("TodoList"));
builder.Services.AddDatabaseDeveloperPageExceptionFilter();
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.MapGet("/todoitems", async (TodoDb db) =>
    await db.Todos.ToListAsync());

app.MapGet("/todoitems/complete", async (TodoDb db) =>
    await db.Todos.Where(t => t.IsComplete).ToListAsync());

app.MapGet("/todoitems/{id}", async (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? Results.Ok(todo)
            : Results.NotFound());

app.MapPost("/todoitems", async (Todo todo, TodoDb db) =>
{
    db.Todos.Add(todo);
    await db.SaveChangesAsync();

    return Results.Created($"/todoitems/{todo.Id}", todo);
});

app.MapPut("/todoitems/{id}", async (int id, Todo inputTodo, TodoDb db) =>
{
    var todo = await db.Todos.FindAsync(id);

    if (todo is null) return Results.NotFound();

    todo.Name = inputTodo.Name;
    todo.IsComplete = inputTodo.IsComplete;

    await db.SaveChangesAsync();

    return Results.NoContent();
});

app.MapDelete("/todoitems/{id}", async (int id, TodoDb db) =>
{
    if (await db.Todos.FindAsync(id) is Todo todo)
    {
        db.Todos.Remove(todo);
        await db.SaveChangesAsync();
        return Results.Ok(todo);
    }

    return Results.NotFound();
});

app.Run();

class Todo
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

class TodoDb : DbContext
{
    public TodoDb(DbContextOptions<TodoDb> options)
        : base(options) { }

    public DbSet<Todo> Todos => Set<Todo>();
}

The model and database context classes

The sample app contains the following model:

class Todo
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

A model is a class that represents data that the app manages. The model for this app is the Todo class.

The sample app also contains the following database context class:

class TodoDb : DbContext
{
    public TodoDb(DbContextOptions<TodoDb> options)
        : base(options) { }

    public DbSet<Todo> Todos => Set<Todo>();
}

The database context is the main class that coordinates Entity Framework functionality for a data model. This class is created by deriving from the Microsoft.EntityFrameworkCore.DbContext class.

The following highlighted code adds the database context to the dependency injection (DI) container and enables displaying database-related exceptions:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDbContext<TodoDb>(opt => opt.UseInMemoryDatabase("TodoList"));
builder.Services.AddDatabaseDeveloperPageExceptionFilter();
var app = builder.Build();

The DI container provides access to the database context and other services.

The following code creates an HTTP POST endpoint /todoitems to add data to the in-memory database:

app.MapPost("/todoitems", async (Todo todo, TodoDb db) =>
{
    db.Todos.Add(todo);
    await db.SaveChangesAsync();

    return Results.Created($"/todoitems/{todo.Id}", todo);
});

Install Postman to test the app

This tutorial uses Postman to test the API.

  • Install Postman
  • Start the web app.
  • Start Postman.
  • Disable SSL certificate verification
    • From File > Settings (General tab), disable SSL certificate verification.

      Warning

      Re-enable SSL certificate verification after testing the controller.

Test posting data

The following instructions post data to the app:

  • Create a new request.

  • Set the HTTP method to POST.

  • Set the URI to https://localhost:<port>/todoitems. For example: https://localhost:5001/todoitems

  • Select the Body tab.

  • Select raw.

  • Set the type to JSON.

  • In the request body enter JSON for a to-do item:

    {
      "name":"walk dog",
      "isComplete":true
    }
    
  • Select Send. Postman with Post request details

Examine the GET endpoints

The sample app implements several GET endpoints using calls to MapGet:

API Description Request body Response body
GET / Browser test, "Hello World" None Hello World!
GET /todoitems Get all to-do items None Array of to-do items
GET /todoitems/{id} Get an item by ID None To-do item
app.MapGet("/", () => "Hello World!");

app.MapGet("/todoitems", async (TodoDb db) =>
    await db.Todos.ToListAsync());

app.MapGet("/todoitems/complete", async (TodoDb db) =>
    await db.Todos.Where(t => t.IsComplete).ToListAsync());

app.MapGet("/todoitems/{id}", async (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? Results.Ok(todo)
            : Results.NotFound());

Test the GET endpoints

Test the app by calling the two endpoints from a browser or Postman. For example:

  • GET https://localhost:5001/todoitems
  • GET https://localhost:5001/todoitems/1

The call to GET /todoitems produces a response similar to the following:

[
  {
    "id": 1,
    "name": "Item1",
    "isComplete": false
  }
]

Test the GET endpoints with Postman

  • Create a new request.
  • Set the HTTP method to GET.
  • Set the request URI to https://localhost:<port>/todoitems. For example, https://localhost:5001/todoitems.
  • Select Send.

This app uses an in-memory database. If the app is restarted, the GET request doesn't return any data. If no data is returned, first POST data to the app.

Return values

ASP.NET Core automatically serializes the object to JSON and writes the JSON into the body of the response message. The response code for this return type is 200 OK, assuming there are no unhandled exceptions. Unhandled exceptions are translated into 5xx errors.

The return types can represent a wide range of HTTP status codes. For example, GET /todoitems/{id} can return two different status values:

  • If no item matches the requested ID, the method returns a 404 status NotFound error code.
  • Otherwise, the method returns 200 with a JSON response body. Returning item results in an HTTP 200 response.

Examine the PUT endpoint

The sample app implements a single PUT endpoint using MapPut:

app.MapPut("/todoitems/{id}", async (int id, Todo inputTodo, TodoDb db) =>
{
    var todo = await db.Todos.FindAsync(id);

    if (todo is null) return Results.NotFound();

    todo.Name = inputTodo.Name;
    todo.IsComplete = inputTodo.IsComplete;

    await db.SaveChangesAsync();

    return Results.NoContent();
});

This method is similar to the MapPost method, except it uses HTTP PUT. A successful response returns 204 (No Content). According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.

Test the PUT endpoint

This sample uses an in-memory database that must be initialized each time the app is started. There must be an item in the database before you make a PUT call. Call GET to ensure there's an item in the database before making a PUT call.

Update the to-do item that has Id = 1 and set its name to "feed fish":

{
  "Id": 1,
  "name": "feed fish",
  "isComplete": false
}

Examine the DELETE endpoint

The sample app implements a single DELETE endpoint using MapDelete:

app.MapDelete("/todoitems/{id}", async (int id, TodoDb db) =>
{
    if (await db.Todos.FindAsync(id) is Todo todo)
    {
        db.Todos.Remove(todo);
        await db.SaveChangesAsync();
        return Results.Ok(todo);
    }

    return Results.NotFound();
});

Use Postman to delete a to-do item:

  • Set the method to DELETE.
  • Set the URI of the object to delete (for example https://localhost:5001/todoitems/1).
  • Select Send.

Prevent over-posting

Currently the sample app exposes the entire Todo object. Production apps typically limit the data that's input and returned using a subset of the model. There are multiple reasons behind this and security is a major one. The subset of a model is usually referred to as a Data Transfer Object (DTO), input model, or view model. DTO is used in this article.

A DTO may be used to:

  • Prevent over-posting.
  • Hide properties that clients are not supposed to view.
  • Omit some properties in order to reduce payload size.
  • Flatten object graphs that contain nested objects. Flattened object graphs can be more convenient for clients.

To demonstrate the DTO approach, update the Todo class to include a secret field:

public class Todo
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
    public string? Secret { get; set; }
}

The secret field needs to be hidden from this app, but an administrative app could choose to expose it.

Verify you can post and get the secret field.

Create a DTO model:

public class TodoItemDTO
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }

    public TodoItemDTO() { }
    public TodoItemDTO(Todo todoItem) =>
    (Id, Name, IsComplete) = (todoItem.Id, todoItem.Name, todoItem.IsComplete);
}

Update the code to use TodoItemDTO:

using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDatabaseDeveloperPageExceptionFilter();
builder.Services.AddDbContext<TodoDb>(opt => opt.UseInMemoryDatabase("TodoList"));
var app = builder.Build();

app.MapGet("/todoitems", async (TodoDb db) =>
    await db.Todos.Select(x => new TodoItemDTO(x)).ToListAsync());

app.MapGet("/todoitems/{id}", async (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? Results.Ok(new TodoItemDTO(todo))
            : Results.NotFound());

app.MapPost("/todoitems", async (TodoItemDTO todoItemDTO, TodoDb db) =>
{
    var todoItem = new Todo
    {
        IsComplete = todoItemDTO.IsComplete,
        Name = todoItemDTO.Name
    };

    db.Todos.Add(todoItem);
    await db.SaveChangesAsync();

    return Results.Created($"/todoitems/{todoItem.Id}", new TodoItemDTO(todoItem));
});

app.MapPut("/todoitems/{id}", async (int id, TodoItemDTO todoItemDTO, TodoDb db) =>
{
    var todo = await db.Todos.FindAsync(id);

    if (todo is null) return Results.NotFound();

    todo.Name = todoItemDTO.Name;
    todo.IsComplete = todoItemDTO.IsComplete;

    await db.SaveChangesAsync();

    return Results.NoContent();
});

app.MapDelete("/todoitems/{id}", async (int id, TodoDb db) =>
{
    if (await db.Todos.FindAsync(id) is Todo todo)
    {
        db.Todos.Remove(todo);
        await db.SaveChangesAsync();
        return Results.Ok(new TodoItemDTO(todo));
    }

    return Results.NotFound();
});

app.Run();

public class Todo
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
    public string? Secret { get; set; }
}

public class TodoItemDTO
{
    public int Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }

    public TodoItemDTO() { }
    public TodoItemDTO(Todo todoItem) =>
    (Id, Name, IsComplete) = (todoItem.Id, todoItem.Name, todoItem.IsComplete);
}


class TodoDb : DbContext
{
    public TodoDb(DbContextOptions<TodoDb> options)
        : base(options) { }

    public DbSet<Todo> Todos => Set<Todo>();
}

Verify you can't post or get the secret field.

Differences between minimal APIs and APIs with controllers

Use JsonOptions

The following code uses JsonOptions:

using Microsoft.AspNetCore.Http.Json;

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

app.MapGet("/", () => new Todo { Name = "Walk dog", IsComplete = false });

app.Run();

class Todo
{
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

The following code uses JsonSerializerOptions:

using System.Text.Json;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

var options = new JsonSerializerOptions(JsonSerializerDefaults.Web);

app.MapGet("/", () => Results.Json(new Todo { 
                      Name = "Walk dog", IsComplete = false }, options));

app.Run();

class Todo
{
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

The preceding code uses web defaults, which converts property names to camel case.

Test minimal API

For an example of testing a minimal API app, see this GitHub sample.

Additional resources