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Minimal APIs at a glance

Minimal APIs

WebApplication

Creating an application

var app = WebApplication.Create(args);

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

app.Run();

This listens to port http://localhost:5000 and https://localhost:5001 by default.

Changing the port

var app = WebApplication.Create(args);

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

app.Run("http://localhost:3000");

Multiple ports

var app = WebApplication.Create(args);

app.Urls.Add("http://localhost:3000");
app.Urls.Add("http://localhost:4000");

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

app.Run();

Reading the port from environment

var app = WebApplication.Create(args);

var port = Environment.GetEnvironmentVariable("PORT") ?? "3000";

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

app.Run($"http://localhost:{port}");

HTTPS with development certificate

var app = WebApplication.Create(args);

app.Urls.Add("https://localhost:3000");

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

app.Run();

HTTPS with custom certificate

var builder = WebApplication.CreateBuilder(args);

// Configure the cert and the key
builder.Configuration["Kestrel:Certificates:Default:Path"] = "site.crt";
builder.Configuration["Kestrel:Certificates:Default:KeyPath"] = "site.key";

var app = builder.Build();

app.Urls.Add("https://localhost:3000");

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

app.Run();

Reading the environment

var app = WebApplication.Create(args);

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/oops");
}

app.MapGet("/", () => "Hello World");
app.MapGet("/oops", () => "Oops! An error happened.");

app.Run();

Reading configuration

var app = WebApplication.Create(args);

Console.WriteLine($"The configuration value is {app.Configuration["key"]}");

app.Run();

Logging

var app = WebApplication.Create(args);

app.Logger.LogInformation("The application started");

app.Run();

WebApplicationBuilder

Changing the content root, application name and environment

var builder = WebApplication.CreateBuilder(new WebApplicationOptions
{
    ApplicationName = typeof(Program).Assembly.FullName,
    ContentRootPath = Directory.GetCurrentDirectory(),
    EnvironmentName = Environments.Staging
});

Console.WriteLine($"Application Name: {builder.Environment.ApplicationName}");
Console.WriteLine($"Environment Name: {builder.Environment.EnvironmentName}");
Console.WriteLine($"ContentRoot Path: {builder.Environment.ContentRootPath}");

var app = builder.Build();

Changing the content root, application name and environment

These may also be specified via the following environment variables:

  • ASPNETCORE_ENVIRONMENT
  • ASPNETCORE_CONTENTROOT
  • ASPNETCORE_APPLICATIONNAME

OR via command line arguments:

  • --applicationName
  • --environment
  • --contentRoot

Adding configuration providers

var builder = WebApplication.CreateBuilder(args);

builder.Configuration.AddIniFile("appsettings.ini");

var app = builder.Build();

Reading configuration

By default the WebApplicationBuilder reads configuration from:

  • appSettings.json
  • appSettings.{environment}.json
  • environment variables
  • The command line
var builder = WebApplication.CreateBuilder(args);

// Reads the ConnectionStrings section of configuration and looks for a sub key called Todos
var connectionString = builder.Configuration.GetConnectionString("Todos");

Console.WriteLine($"My connection string is {connectionString}");

var app = builder.Build();

Reading the environment

var builder = WebApplication.CreateBuilder(args);

if (builder.Environment.IsDevelopment())
{
    Console.WriteLine($"Running in development");
}

var app = builder.Build();

Adding logging providers

var builder = WebApplication.CreateBuilder(args);

// Configure JSON logging to the console
builder.Logging.AddJsonConsole();

var app = builder.Build();

Adding services

var builder = WebApplication.CreateBuilder(args);

// Add the memory cache services
builder.Services.AddMemoryCache();

// Add a custom scoped service
builder.Services.AddScoped<ITodoRepository, TodoRepository>();

var app = builder.Build();

Customizing the IHostBuilder

Existing extension methods on IHostBuilder can be accessed using the Host property.

var builder = WebApplication.CreateBuilder(args);

// Wait 30 seconds for graceful shutdown
builder.Host.ConfigureHostOptions(o => o.ShutdownTimeout = TimeSpan.FromSeconds(30));

var app = builder.Build();

Customizing the IWebHostBuilder

Existing extension methods on IWebHostBuilder can be accessed using the WebHost property.

var builder = WebApplication.CreateBuilder(args);

// Change the HTTP server implemenation to be HTTP.sys based
builder.WebHost.UseHttpSys();

var app = builder.Build();

Changing the web root

By default, the web root is relative to the content root in the wwwroot folder. This is where the static files middleware expects to find static files. You can change this by using the UseWebRoot method on the WebHost property:

var builder = WebApplication.CreateBuilder(args);

// Look for static files in webroot
builder.WebHost.UseWebRoot("webroot");

var app = builder.Build();

Custom dependency injection container

This example uses Autofac

var builder = WebApplication.CreateBuilder(args);

builder.Host.UseServiceProviderFactory(new AutofacServiceProviderFactory());

// Register your own things directly with Autofac here. Don't
// call builder.Populate(), that happens in AutofacServiceProviderFactory
// for you.
builder.Host.ConfigureContainer<ContainerBuilder>(builder => builder.RegisterModule(new MyApplicationModule()));

var app = builder.Build();

Adding Middleware

Any existing ASP.NET Core middleware can be configured on the WebApplication:

var app = WebApplication.Create(args);

// Setup the file server to serve static files
app.UseFileServer();

app.Run();

Developer Exception Page

The WebApplication has a the developer exception enabled by default when the environment is development:

var app = WebApplication.Create(args);

app.MapGet("/", () => { throw new InvalidOperationException(); });

app.Run();

Navigating to / will render a friendly page that shows the exception.

TODO: Add image

ASP.NET Core Middleware

Middleware Description API
Authentication Provides authentication support. app.UseAuthentication()
Authorization Provides authorization support. app.UseAuthorization()
CORS Configures Cross-Origin Resource Sharing. app.UseCors()
Exception Handler Globally handles exceptions thrown by the middleware pipeline. app.UseExceptionHandler()
Forwarded Headers Forwards proxied headers onto the current request. app.UseForwardedHeaders()
HTTPS Redirection Redirect all HTTP requests to HTTPS. app.UseHttpsRedirection()
HTTP Strict Transport Security (HSTS) Security enhancement middleware that adds a special response header. app.UseHsts()
Request Logging Provides support for logging HTTP requests and responses. app.UseHttpLogging()
W3C Request Logging Provides support for logging HTTP requests and responses in the W3C format. app.UseW3CLogging()
Response Caching Provides support for caching responses. app.UseResponseCaching()
Response Compression Provides support for compressing responses. app.UseResponseCompression()
Session Provides support for managing user sessions. app.UseSession()
Static Files Provides support for serving static files and directory browsing. app.UseStaticFiles(), app.UseFileServer()
WebSockets Enables the WebSockets protocol. app.UseWebSockets()

Routing

You can route to handlers using the various Map* methods on WebApplication. There's a Map{HTTPMethod} method to allow handling different HTTP methods:

app.MapGet("/", () => "This is a GET");
app.MapPost("/", () => "This is a POST");
app.MapPut("/", () => "This is a PUT");
app.MapDelete("/", () => "This is a DELETE");

Other HTTP methods:

app.MapMethods(new [] { "OPTIONS", "HEAD" }, () => "This is an options or head request ");

Route Parameters

You can capture route parameters as part of the route pattern definition:

app.MapGet("/users/{userId}/books/{bookId}", (int userId, int bookId) => $"The user id is {userId} and book id is {bookId}");

The route handler can declare the parameters that it wants to capture, and when a request is made to this route, the parameters will be parsed and passed to the handler. This makes it easy to capture the values in a type safe way (above, you can be sure that the userId and bookId are both int).

If the route values are not valid ints then an exception will be thrown. The following request will result in the exception below:

GET /users/hello/books/3
Microsoft.AspNetCore.Http.BadHttpRequestException: Failed to bind parameter "int userId" from "hello".
    at Microsoft.AspNetCore.Http.RequestDelegateFactory.Log.ParameterBindingFailed(HttpContext httpContext, String parameterTypeName, String parameterName, String sourceValue, Boolean shouldThrow)
    at lambda_method1(Closure , Object , HttpContext )
    at Microsoft.AspNetCore.Http.RequestDelegateFactory.<>c__DisplayClass37_0.<Create>b__0(HttpContext httpContext)

Wildcards/Catch all routes

app.MapGet("/posts/{rest*}", (string rest) => $"Routing to {rest}");

Route constraints

Route constraints are influence the matching behavior of a route.

app.MapGet("/todos/{id:int}", (int id) => db.Todos.Find(id));
app.MapGet("/todos/{text}", (string text) => db.Todos.Where(t => t.Text.Contains(text));
app.MapGet("/posts/{slug:regex(^[a-z0-9_-]+$)}", (string slug) => $"Post {slug}");
Path Match
/todos/1 /todos/{id:int}
/todos/something /todos/{text}
/posts/mypost /posts/{slug:regex(^[a-z0-9_-]+$)}
/posts/% No match

Built in contraints

constraint Example Example Matches Notes
int {id:int} 123456789, -123456789 Matches any integer
bool {active:bool} true, FALSE Matches true or false. Case-insensitive
datetime {dob:datetime} 2016-12-31, 2016-12-31 7:32pm Matches a valid DateTime value in the invariant culture. See preceding warning.
decimal {price:decimal} 49.99, -1,000.01 Matches a valid decimal value in the invariant culture. See preceding warning.
double {weight:double} 1.234, -1,001.01e8 Matches a valid double value in the invariant culture. See preceding warning.
float {weight:float} 1.234, -1,001.01e8 Matches a valid float value in the invariant culture. See preceding warning.
guid {id:guid} CD2C1638-1638-72D5-1638-DEADBEEF1638 Matches a valid Guid value
long {ticks:long} 123456789, -123456789 Matches a valid long value
minlength(value) {username:minlength(4)} Rick String must be at least 4 characters
maxlength(value) {filename:maxlength(8)} MyFile String must be no more than 8 characters
length(length) {filename:length(12)} somefile.txt String must be exactly 12 characters long
length(min,max) {filename:length(8,16)} somefile.txt String must be at least 8 and no more than 16 characters long
min(value) {age:min(18)} 19 Integer value must be at least 18
max(value) {age:max(120)} 91 Integer value must be no more than 120
range(min,max) {age:range(18,120)} 91 Integer value must be at least 18 but no more than 120
alpha {name:alpha} Rick String must consist of one or more alphabetical characters, a-z and case-insensitive.
regex(expression) {ssn:regex(^\\d{{3}}-\\d{{2}}-\\d{{4}}$)} 123-45-6789 String must match the regular expression. See tips about defining a regular expression.
required {name:required} Rick Used to enforce that a non-parameter value is present during URL generation

Parameter Binding

Parameter binding is the process of turning request data into strongly typed parameters that are expressed by route handlers. A binding source determines where parameters are bound from. Binding sources can be explict or inferred based HTTP method and parameter type.

GET, HEAD, OPTIONS, DELETE

var builder = WebApplication.CreateBuilder(args);

// Added as service
builder.AddSingleton<Service>();

var app = builder.Build();

app.MapGet("/{id}", (int id, int page, Service service) => { });

class Service { }
Parameter Binding Source
id route value
page query string
service provided by dependency injection

Other verbs (POST, PUT, PATCH, etc)

var builder = WebApplication.CreateBuilder(args);

// Added as service
builder.AddSingleton<Service>();

app.MapPost("/", (Person person, Service service) => { });

class Service { }

record Person(string Name, int Age);
Parameter Binding Source
person body (as JSON)
service provided by dependency injection

Explicit Parameter Binding

Attributes can be used to explicitly declare where parameters should be bound from.

using Microsoft.AspNetCore.Mvc;

app.MapGet("/{id}", ([FromRoute]int id, 
                     [FromQuery("p")]int page, 
                     [FromServices]Service service, 
                     [FromHeader("Content-Type")]string contentType) => { });
Parameter Binding Source
id route with the name id
page query string with the name "p"
service provided by dependency injection
contentType header with the name "Content-Type"

Binding from form values is not supported at this time.

Optional parameters

Parameters declared in route handlers will be treated as required. This means if a request matches the route, the route handler will only execute if all required paramters are provided in the request. Failure to do so will result in an error.

app.MapGet("/products", (int pageNumber) => $"Requesting page {pageNumber}");

Since the pageNumber paramter is required, this route handler won't execute if the query string pageNumber isn't provided. To make it optional define the type as nullable.

app.MapGet("/products", (int? pageNumber) => $"Requesting page {pageNumber ?? 1}");

This also works with methods that have a default value:

string ListProducts(int pageNumber = 1) => $"Requesting page {pageNumber}";

app.MapGet("/products", (int? pageNumber) => ListProducts);

The above will default to 1 if the pageNumber isn't specified in the query string.

This logic applies to all sources.

app.MapPost("/products", (Product? product) => () => { });

The above will call the method will a null product if no request body was sent.

Special types

There are some special types that the framework supports binding without any explicit attributes:

  • HttpContext - The context which holds all the information about the current http request/response.
  • HttpRequest - The http request
  • HttpResponse - The http reponse
  • CancellationToken - The cancellation token associated with the current http request.
  • ClaimsPrincipal - The user associated with the request (HttpContext.User).

Custom Binding

There are 2 ways to customize parameter binding:

  1. For route, query, and header binding sources, you may bind custom types by adding a static TryParse method to your type.
  2. You can completely take over the binding process by implementing a BindAsync method on your type.

TryParse

The TryParse method must be of the form(s):

public static bool TryParse(string value, T out result);
public static bool TryParse(string value, IFormatProvider provider, T out result);

Example

app.MapGet("/map/{point}", (Point point) => $"Point: {point.X}, {point.Y}");

public class Point
{
    public double X { get; set; }
    public double Y { get; set; }

    public static bool TryParse(string? value, IFormatProvider? provider, out Point? point)
    {
        // Format is "(12.3,10.1)"
        var trimmedValue = value?.TrimStart('(').TrimEnd(')');
        var segments = trimmedValue?.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
        if (segments?.Length == 2
            && double.TryParse(segments[0], out var x)
            && double.TryParse(segments[1], out var y))
        {
            point = new Point { X = x, Y = y };
            return true;
        }

        point = null;
        return false;
    }
}

A request to /point?point=(12.3,10.1) returns:

Point: 12.3,10.1

BindAsync

The BindAsync method must be of the form:

public static ValueTask<T?> BindAsync(HttpContext context, ParameterInfo parameter);

Example

app.MapGet("/products", (PagingData pageData) => $"SortBy:{pageData.SortBy}, SortDirection:{pageData.SortDirection}, CurrentPage:{pageData.CurrentPage}");

public class PagingData
{
    public string? SortBy { get; init; }
    public SortDirection SortDirection { get; init; }
    public int CurrentPage { get; init; } = 1;

    public static ValueTask<PagingData?> BindAsync(HttpContext context, ParameterInfo parameter)
    {
        const string sortByKey = "sortBy";
        const string sortDirectionKey = "sortDir";
        const string currentPageKey = "page";

        Enum.TryParse<SortDirection>(context.Request.Query[sortDirectionKey], ignoreCase: true, out var sortDirection);
        int.TryParse(context.Request.Query[currentPageKey], out var page);
        page = page == 0 ? 1 : page;

        var result = new PagingData
        {
            SortBy = context.Request.Query[sortByKey],
            SortDirection = sortDirection,
            CurrentPage = page
        };

        return ValueTask.FromResult<PagingData?>(result);
    }
}

public enum SortDirection
{
    Default,
    Asc,
    Desc
}

Binding Precedence

The rules for determining a binding source from a parameter are as follows:

  1. Explicit attribute defined on parameter (From* attributes) in the following order:
    1. Route values (FromRoute)
    2. Query string (FromQuery)
    3. Header (FromHeader)
    4. Body (FromBody)
    5. Service (FromServices)
  2. Special types
    1. HttpContext
    2. HttpRequest
    3. HttpResponse
    4. ClaimsPrincipal
    5. CancellationToken
  3. Parameter type has a valid BindAsync method.
  4. Parameter type is a string or has a valid TryParse method.
    1. If the parameter name exists in the route template e.g. app.Map("/todo/{id}", (int id) => {});, then it will be bound from the route.
    2. It will be bound from the query string.
  5. If the parameter type is a service provided by dependency injection, it will use that as the source.
  6. The parameter is from the body.

Responses

The following example uses the built in result types to customize the response:

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

JSON

app.MapGet("/hello", () => Results.Json(new { Message = "Hello World" }));

Custom Status Code

app.MapGet("/405", () => Results.StatusCode(405));

Text

app.MapGet("/text", () => Results.Text("This is some text"));

Stream

var proxyClient = new HttpClient();
app.MapGet("/pokemon", async () => 
{
    var stream = await proxyClient.GetStreamAsync("http://myurl/pokedex.json")
    // Proxy the response as JSON
    return Results.Stream(stream, "application/json");
});

Redirect

app.MapGet("/old-path", () => Results.Redirect("/new-path"));

File

app.MapGet("/download", () => Results.File("foo.text"));

Built in results

Description Response type Status Code API
Write a JSON response with advanced options application/json 200 Results.Json
Write a JSON response application/json 200 Results.Ok
Write a text response text/plain (default), configurable 200 Results.Text
Write the response as bytes application/octet-stream (default), configurable 200 Results.Bytes
Write a stream of bytes to the response application/octet-stream (default), configurable 200 Results.Stream
Stream a file to the response for download with the content-disposition header application/octet-stream (default), configurable 200 Results.File
Set the status code to 404, with an optional JSON response N/A 404 Results.NotFound
Set the status code to 204 N/A 204 Results.NoContent
Set the status code to 422, with an optional JSON response N/A 422 Results.UnprocessableEntity
Set the status code to 400, with an optional JSON response N/A 400 Results.BadRequest
Set the status code to 409, with an optional JSON response N/A 409 Results.Conflict
Write a problem details JSON object to the response N/A 400 (default), configurable Results.Problem
Write a problem details JSON object to the response with validation errors N/A 400 (default), configurable Results.ValidationProblem

Customizing results

Authorization

Routes can be protected using authorization policies. These can be declared via the authorize attribute or by using the RequireAuthorization method call.

app.MapGet("/auth", [Authorize] () => "This endpoint requires authorization");

OR

app.MapGet("/auth", () => "This endpoint requires authorization")
   .RequireAuthorization();

Authorization policies can be configured as well:

app.MapGet("/admin", [Authorize("AdminsOnly")] () => "This endpoint is for admins only");

OR

app.MapGet("/admin", () => "This endpoint is for admins only")
   .RequireAuthorization("AdminsOnly");

Open API/Swagger

It's possible to describe the OpenAPI specification for route handlers using Swashbuckle.

Below is an example of a typical ASP.NET Core application with OpenAPI suppport:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new() { Title = builder.Environment.ApplicationName, Version = "v1" });
});

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", $"{builder.Environment.ApplicationName} v1"));
}

Describing response types

app.MapGet("/api/products", (int id, ProductDb db) => db.Products.Find(id) is Product product ? Results.Ok(product) : Results.NotFound())
   .Produces<Product>(200)
   .Produces(404);

Exclude Open API description

app.MapGet("/skipme", () => { })
   .ExcludeFromDescription();

Add operation ids to Open API

app.MapGet("/api/products", (ProductDb db) => db.Products.ToListAsync())
   .WithName("GetProducts");

Add tags to the Open API description (used for grouping)

app.MapGet("/api/products", (ProductDb db) => db.Products.ToListAsync())
   .WithTags("ProductsGroup");

Describe request body

app.MapGet("/upload", async (HttpRequest req) =>
{
    if (!req.HasFormContentType)
    {
        return Results.BadRequest();
    }

    var form = await req.ReadFormAsync();
    var file = form.Files["file"];

    if (file is null)
    {
        return Results.BadRequest();
    }

    var uploads = Path.Combine(uploadsPath, file.FileName);
    using var fileStream = File.OpenWrite(uploads);
    using var uploadStream = file.OpenReadStream();
    await uploadStream.CopyToAsync(fileStream);

    return Results.NoContent();
})
.Accepts<IFormFile>("multipart/form-data");

Building libraries for ASP.NET Core

The existing .NET ecosystem has built extensibility around IServiceCollection, IHostBuilder and IWebHostBuilder. These properties are exposed on the WebApplicationBuilder as Services, Host and WebHost.

The WebApplication implements both Microsoft.AspNetCore.Builder.IApplicationBuilder and Microsoft.AspNetCore.Routing.IEndpointRouteBuilder.

We expect library authors to continue targeting IHostBuilder, IWebHostBuilder, IApplicationBuilder and IEndpointRouteBuilder when building ASP.NET Core specific components. This will ensure that your middleware, route handler, or other extensibility points continue to work across different hosting models.

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