モダン .NET 2026 完全ガイド - .NET 9 · C# 13 · F# 9 · ASP.NET Core · Blazor United · MAUI · Aspire · EF Core 9 徹底解説

# 現在のSDKを確認
dotnet --list-sdks

# .NET 10 のインストール(macOS Homebrew)
brew install --cask dotnet-sdk

# global.json で特定バージョンを固定
cat > global.json <<EOF
{
  "sdk": {
    "version": "10.0.100",
    "rollForward": "latestFeature"
  }
}
EOF

# 新規プロジェクト
dotnet new console -n HelloWorld
cd HelloWorld
dotnet run

// C# 12 まで — params object[] のみ
void Log(params object[] args) { }

// C# 13 — params ReadOnlySpan<object>
void Log(params ReadOnlySpan<object> args)
{
    foreach (var arg in args)
        Console.WriteLine(arg);
}

// 呼び出し時に配列の確保がない。Span はスタック割り当て。
Log("hello", 42, DateTime.Now);

using System.Threading;

public class Counter
{
    private readonly Lock _lock = new();
    private int _count;

    public void Increment()
    {
        lock (_lock)
        {
            _count++;
        }
    }
}

public partial class ViewModel
{
    public partial string Name { get; set; }
}

// 別ファイルでソースジェネレータが実装を提供
public partial class ViewModel
{
    private string _name = "";
    public partial string Name
    {
        get => _name;
        set { _name = value; OnPropertyChanged(); }
    }
}

Console.WriteLine("\e[31mRed text\e[0m");

let rec fib n =
    match n with
    | 0 | 1 -> n
    | _ -> fib (n - 1) + fib (n - 2)

[0..10]
|> List.map fib
|> List.iter (printfn "%d")

type Shape =
    | Circle of radius: float
    | Rectangle of width: float * height: float
    | Triangle of a: float * b: float * c: float

let area shape =
    match shape with
    | Circle r -> System.Math.PI * r * r
    | Rectangle (w, h) -> w * h
    | Triangle (a, b, c) ->
        let s = (a + b + c) / 2.0
        sqrt (s * (s - a) * (s - b) * (s - c))

printfn "%f" (area (Circle 5.0))

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

app.MapGet("/", () => "Hello, World!");
app.MapGet("/hello/{name}", (string name) => $"Hello, {name}!");
app.MapPost("/items", (Item item) => Results.Created($"/items/{item.Id}", item));

app.Run();

record Item(int Id, string Name);

dotnet publish -c Release -p:PublishAot=true
# bin/Release/net10.0/<rid>/publish/MyApi (シングルバイナリ)

[JsonSerializable(typeof(User))]
public partial class AppJsonContext : JsonSerializerContext { }

// AOT フレンドリーなシリアライズ
var json = JsonSerializer.Serialize(user, AppJsonContext.Default.User);

@page "/counter"
@rendermode InteractiveServer

<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>

@code {
    private int currentCount = 0;
    private void IncrementCount() => currentCount++;
}

dotnet new maui -n MyApp
cd MyApp

# iOS
dotnet build -t:Run -f net10.0-ios
# Android
dotnet build -t:Run -f net10.0-android
# macOS
dotnet build -t:Run -f net10.0-maccatalyst
# Windows
dotnet build -t:Run -f net10.0-windows10.0.19041.0

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MyApp.MainPage">
    <VerticalStackLayout Padding="20" Spacing="10">
        <Label Text="Hello, MAUI!" FontSize="32" />
        <Button Text="Click me" Clicked="OnClicked" />
        <Label x:Name="CounterLabel" Text="0" />
    </VerticalStackLayout>
</ContentPage>

public partial class MainPage : ContentPage
{
    int count = 0;
    public MainPage() { InitializeComponent(); }
    void OnClicked(object sender, EventArgs e)
    {
        count++;
        CounterLabel.Text = count.ToString();
    }
}

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="HelloAvalonia.MainWindow"
        Title="HelloAvalonia">
  <StackPanel Margin="20">
    <TextBlock Text="Hello from Avalonia!" FontSize="24" />
    <Button Content="Click me" Click="OnClick" />
  </StackPanel>
</Window>

// AppHost/Program.cs
var builder = DistributedApplication.CreateBuilder(args);

var redis = builder.AddRedis("cache");
var postgres = builder.AddPostgres("db").AddDatabase("appdb");

var api = builder.AddProject<Projects.MyApi>("api")
    .WithReference(redis)
    .WithReference(postgres);

builder.AddProject<Projects.MyWeb>("web")
    .WithReference(api);

builder.Build().Run();

dotnet run --project AppHost
# ブラウザで https://localhost:17000 — Aspire Dashboard

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public BlogMetadata Metadata { get; set; } = new();
}

public class BlogMetadata
{
    public string Author { get; set; } = "";
    public List<string> Tags { get; set; } = new();
}

public class BloggingContext : DbContext
{
    public DbSet<Blog> Blogs => Set<Blog>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>().OwnsOne(b => b.Metadata, b => b.ToJson());
    }
}

// JSON 内のフィールドを LINQ でクエリ
var blogs = await db.Blogs
    .Where(b => b.Metadata.Tags.Contains("dotnet"))
    .ToListAsync();

dotnet tool install --global dotnet-ef
dotnet ef migrations add InitialCreate
dotnet ef database update

using Dapper;
using Npgsql;

await using var conn = new NpgsqlConnection(connStr);
var users = await conn.QueryAsync<User>(
    "SELECT id, name FROM users WHERE active = @active",
    new { active = true });

foreach (var u in users)
    Console.WriteLine($"{u.Id}: {u.Name}");

var store = DocumentStore.For(opts =>
{
    opts.Connection(connStr);
    opts.Events.AddEventType(typeof(OrderPlaced));
});

await using var session = store.LightweightSession();
session.Events.Append(orderId, new OrderPlaced(orderId, items));
await session.SaveChangesAsync();

public record GetUserById(int Id) : IRequest<User>;

public class GetUserByIdHandler : IRequestHandler<GetUserById, User>
{
    private readonly AppDbContext _db;
    public GetUserByIdHandler(AppDbContext db) { _db = db; }

    public async Task<User> Handle(GetUserById req, CancellationToken ct)
    {
        return await _db.Users.FindAsync(new object[] { req.Id }, ct)
            ?? throw new KeyNotFoundException();
    }
}

// コントローラ
public class UsersController(IMediator mediator) : ControllerBase
{
    [HttpGet("{id}")]
    public Task<User> Get(int id) => mediator.Send(new GetUserById(id));
}

public interface IUserGrain : IGrainWithIntegerKey
{
    Task<string> GetNameAsync();
    Task SetNameAsync(string name);
}

public class UserGrain : Grain, IUserGrain
{
    private string _name = "";
    public Task<string> GetNameAsync() => Task.FromResult(_name);
    public Task SetNameAsync(string name) { _name = name; return Task.CompletedTask; }
}

// クライアント
var user = client.GetGrain<IUserGrain>(123);
await user.SetNameAsync("Alice");
var name = await user.GetNameAsync();

public class ChatHub : Hub
{
    public async Task SendMessage(string user, string message)
    {
        await Clients.All.SendAsync("ReceiveMessage", user, message);
    }
}

// Program.cs
builder.Services.AddSignalR();
app.MapHub<ChatHub>("/chathub");

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply);
}

message HelloRequest { string name = 1; }
message HelloReply { string message = 1; }

public class GreeterService : Greeter.GreeterBase
{
    public override Task<HelloReply> SayHello(HelloRequest req, ServerCallContext ctx)
    {
        return Task.FromResult(new HelloReply { Message = $"Hello {req.Name}" });
    }
}

public class OrderService(ILogger<OrderService> logger)
{
    public void Process(Order order)
    {
        logger.LogInformation("Processing order {OrderId} for {Amount}",
            order.Id, order.Amount);
    }
}

Log.Logger = new LoggerConfiguration()
    .MinimumLevel.Information()
    .WriteTo.Console()
    .WriteTo.File("logs/app-.txt", rollingInterval: RollingInterval.Day)
    .WriteTo.Seq("http://localhost:5341")
    .CreateLogger();

Log.Information("User {UserId} logged in from {IP}", userId, ip);

builder.Services.AddOpenTelemetry()
    .WithTracing(t => t
        .AddAspNetCoreInstrumentation()
        .AddHttpClientInstrumentation()
        .AddOtlpExporter())
    .WithMetrics(m => m
        .AddAspNetCoreInstrumentation()
        .AddRuntimeInstrumentation()
        .AddOtlpExporter());

public class CalculatorTests
{
    [Fact]
    public void Add_TwoNumbers_ReturnsSum()
    {
        var sut = new Calculator();
        Assert.Equal(5, sut.Add(2, 3));
    }

    [Theory]
    [InlineData(1, 1, 2)]
    [InlineData(2, 3, 5)]
    [InlineData(-1, 1, 0)]
    public void Add_Various_ReturnsExpected(int a, int b, int expected)
    {
        var sut = new Calculator();
        Assert.Equal(expected, sut.Add(a, b));
    }
}

// NSubstitute
var repo = Substitute.For<IUserRepository>();
repo.FindById(1).Returns(new User(1, "Alice"));

var sut = new UserService(repo);
var user = sut.GetUser(1);

repo.Received(1).FindById(1);

[MemoryDiagnoser]
public class StringBenchmarks
{
    [Benchmark]
    public string ConcatStrings()
    {
        var s = "";
        for (var i = 0; i < 100; i++) s += i;
        return s;
    }

    [Benchmark]
    public string StringBuilder()
    {
        var sb = new StringBuilder();
        for (var i = 0; i < 100; i++) sb.Append(i);
        return sb.ToString();
    }
}

BenchmarkRunner.Run<StringBenchmarks>();

dotnet watch run
# Program.cs を編集 → 自動再起動
# Razor ページを編集 → Hot Reload(再起動なしで反映)

dotnet tool install -g Microsoft.dotnet-interactive
dotnet interactive jupyter install

// .NET ノートブックのセル
#r "nuget: System.Text.Json"

using System.Text.Json;

var json = """{"name": "Alice", "age": 30}""";
var doc = JsonDocument.Parse(json);
display(doc.RootElement);

dotnet new sln -n MySolution
dotnet new classlib -n MyLibrary
dotnet sln add MyLibrary
dotnet build
dotnet test
dotnet pack
dotnet nuget push -s nuget.org -k <api-key>

class Build : NukeBuild
{
    public static int Main() => Execute<Build>(x => x.Compile);

    Target Clean => _ => _
        .Executes(() => Directory.Delete("bin", true));

    Target Compile => _ => _
        .DependsOn(Clean)
        .Executes(() => DotNetTasks.DotNetBuild(s => s
            .SetConfiguration("Release")));

    Target Test => _ => _
        .DependsOn(Compile)
        .Executes(() => DotNetTasks.DotNetTest(s => s
            .SetConfiguration("Release")
            .EnableNoBuild()));
}

./build.sh test
# または
nuke test

# パッケージ追加
dotnet add package Serilog --version 4.0.0

# 更新
dotnet add package Serilog

# 削除
dotnet remove package Serilog

# 復元
dotnet restore

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup>
    <PackageVersion Include="Serilog" Version="4.0.0" />
    <PackageVersion Include="Dapper" Version="3.0.0" />
  </ItemGroup>
</Project>

using UnityEngine;

public class PlayerController : MonoBehaviour
{
    [SerializeField] float speed = 5f;
    Rigidbody rb;

    void Start() { rb = GetComponent<Rigidbody>(); }

    void Update()
    {
        var h = Input.GetAxis("Horizontal");
        var v = Input.GetAxis("Vertical");
        rb.linearVelocity = new Vector3(h, 0, v) * speed;
    }
}

// WinForms
public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
        var btn = new Button { Text = "Click me", Location = new Point(20, 20) };
        btn.Click += (s, e) => MessageBox.Show("Hello!");
        Controls.Add(btn);
    }
}

az webapp up --name myapp --runtime "DOTNETCORE:10.0"

public class Function
{
    public async Task<APIGatewayHttpApiV2ProxyResponse> Handle(
        APIGatewayHttpApiV2ProxyRequest req, ILambdaContext ctx)
    {
        return new APIGatewayHttpApiV2ProxyResponse
        {
            StatusCode = 200,
            Body = $"Hello from {req.RequestContext.Http.Path}"
        };
    }
}

FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS base
WORKDIR /app
EXPOSE 8080

FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
COPY . /src
WORKDIR /src
RUN dotnet publish -c Release -o /app/publish

FROM base AS final
COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApi.dll"]

dotnet add package Microsoft.CodeAnalysis.NetAnalyzers

public partial class EmailValidator
{
    [GeneratedRegex(@"^[^@]+@[^@]+\.[^@]+$")]
    private static partial Regex EmailRegex();

    public bool IsValid(string s) => EmailRegex().IsMatch(s);
}