.NET 8.0 + ASP.NET Core(MVC) で実装するルートベースローカライズ処理（多国言語対応）

前回作成した .NET 8.0 + ASP.NET Core(MVC) で実装するクエリベースローカライズ処理（多国言語対応）を、ルートベースローカライズへ変更したサンプルを作成しました。
ソースコードは GitHub で公開しています。

クエリベースローカライズ：クエリ文字列での言語切替
ルートベースローカライズ：URLパス先頭での言語切替　※ASP.NET Core 公式推奨

ソースコード変更内容を解説

WebApplication1/Program.cs

RequestCultureProvidersと UseRequestLocalizationを追加し、cultureは URLのルートプレフィックスから取得すると明示。
URLのパターンに cultureを追加。


using Microsoft.AspNetCore.Localization.Routing;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.Extensions.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddLocalization(options => options.ResourcesPath = "Resources");

builder.Services.Configure<RequestLocalizationOptions>(options =>
{
  var supportedCultures = new[] { "ja", "en" };
  options.SetDefaultCulture(supportedCultures[0])
    .AddSupportedCultures(supportedCultures)
    .AddSupportedUICultures(supportedCultures);
});

builder.Services.AddMvc()
  .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
  .AddDataAnnotationsLocalization();

// Add services to the container.
builder.Services.AddControllersWithViews()
  .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
  .AddDataAnnotationsLocalization();

builder.Services.Configure<RequestLocalizationOptions>(options =>
{
  var supportedCultures = new[]
  {
    new System.Globalization.CultureInfo("en"),
    new System.Globalization.CultureInfo("ja")
  };

  options.DefaultRequestCulture = new Microsoft.AspNetCore.Localization.RequestCulture("ja");
  options.SupportedCultures = supportedCultures;
  options.SupportedUICultures = supportedCultures;
  options.RequestCultureProviders = new[] {
    new RouteDataRequestCultureProvider { Options = options }
  };
});

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
  app.UseExceptionHandler("/Home/Error");
}
app.UseStaticFiles();

app.UseRouting();

app.UseRequestLocalization(
    app.Services.GetRequiredService<IOptions<RequestLocalizationOptions>>().Value
);

app.UseAuthorization();

app.MapControllerRoute(
  name: "default",
  pattern: "{culture=ja}/{controller=Home}/{action=Index}/{id?}");

app.Run();

using Microsoft.AspNetCore.Localization.Routing;

using Microsoft.AspNetCore.Mvc.Razor;

using Microsoft.Extensions.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddLocalization(options => options.ResourcesPath = "Resources");

builder.Services.Configure<RequestLocalizationOptions>(options =>

{

var supportedCultures = new[] { "ja", "en" };

options.SetDefaultCulture(supportedCultures[0])

.AddSupportedCultures(supportedCultures)

.AddSupportedUICultures(supportedCultures);

});

builder.Services.AddMvc()

.AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)

.AddDataAnnotationsLocalization();

// Add services to the container.

builder.Services.AddControllersWithViews()

.AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)

.AddDataAnnotationsLocalization();

builder.Services.Configure<RequestLocalizationOptions>(options =>

{

var supportedCultures = new[]

{

new System.Globalization.CultureInfo("en"),

new System.Globalization.CultureInfo("ja")

};

options.DefaultRequestCulture = new Microsoft.AspNetCore.Localization.RequestCulture("ja");

options.SupportedCultures = supportedCultures;

options.SupportedUICultures = supportedCultures;

options.RequestCultureProviders = new[] {

new RouteDataRequestCultureProvider { Options = options }

};

});

var app = builder.Build();

// Configure the HTTP request pipeline.

if (!app.Environment.IsDevelopment())

{

app.UseExceptionHandler("/Home/Error");

}

app.UseStaticFiles();

app.UseRouting();

app.UseRequestLocalization(

app.Services.GetRequiredService<IOptions<RequestLocalizationOptions>>().Value

);

app.UseAuthorization();

app.MapControllerRoute(

name: "default",

pattern: "{culture=ja}/{controller=Home}/{action=Index}/{id?}");

app.Run();

WebApplication1/Views/Shared/_Layout.cshtml

言語選択用のコンボボックスを追加。
コードブロックで cultureを取得し、コンボボックスの選択状態（selected）の条件分岐に使用。
コードブロックで controller、action、Queryを取得し、コンボボックスの onchangeイベントにある、JavaScriptでページ遷移する際の URLに使用。


@{
  var culture = ViewContext.RouteData.Values["culture"]?.ToString()?.ToLower() ?? "ja";
  var ctrl = ViewContext.RouteData.Values["controller"]?.ToString() ?? "Home";
  var act = ViewContext.RouteData.Values["action"]?.ToString() ?? "Index";
  var query = Context.Request.QueryString.ToUriComponent(); // ?id=1&page=2 など
}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>@ViewData["Title"] - WebApplication1</title>
  <link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.min.css" />
  <link rel="stylesheet" href="~/css/site.css" asp-append-version="true" />
  <link rel="stylesheet" href="~/WebApplication1.styles.css" asp-append-version="true" />
</head>
<body>
  <header>
    <nav class="navbar navbar-expand-sm navbar-toggleable-sm navbar-light bg-white border-bottom box-shadow mb-3">
      <div class="container-fluid">
        <a class="navbar-brand" asp-area="" asp-controller="Home" asp-action="Index">WebApplication1</a>
        <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target=".navbar-collapse" aria-controls="navbarSupportedContent"
                aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>
        <div class="navbar-collapse collapse d-sm-inline-flex justify-content-between">
          <ul class="navbar-nav flex-grow-1">
            <li class="nav-item">
              <a class="nav-link text-dark" asp-area="" asp-controller="Home" asp-action="Index">Home</a>
            </li>
            <li class="nav-item">
              <a class="nav-link text-dark" asp-area="" asp-controller="Home" asp-action="Privacy">Privacy</a>
            </li>
          </ul>
        </div>
      </div>
    </nav>
  </header>

  <form method="get" style="display:inline;">
    <select name="culture" onchange="
      const c=this.value;
      const query='@query' ;
      let url='/' + c + '/@ctrl/@act' ;
      if (query && query.length>
      0) url += query;
      window.location.href = url;
    ">
      @if (culture == "ja")
      {
        <text>
        <option value="ja" selected>日本語</option>
        <option value="en">English</option>
        </text>
      }
      else
      {
        <text>
        <option value="ja">日本語</option>
        <option value="en" selected>English</option>
        </text>
      }
    </select>
  </form>

  <div class="container">
    <main role="main" class="pb-3">
      @RenderBody()
    </main>
  </div>

  <footer class="border-top footer text-muted">
    <div class="container">
      &copy; 2025 - WebApplication1 - <a asp-area="" asp-controller="Home" asp-action="Privacy">Privacy</a>
    </div>
  </footer>
  <script src="~/lib/jquery/dist/jquery.min.js"></script>
  <script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js"></script>
  <script src="~/js/site.js" asp-append-version="true"></script>
  @await RenderSectionAsync("Scripts", required: false)
</body>
</html>

var culture = ViewContext.RouteData.Values["culture"]?.ToString()?.ToLower() ?? "ja";

var ctrl = ViewContext.RouteData.Values["controller"]?.ToString() ?? "Home";

var act = ViewContext.RouteData.Values["action"]?.ToString() ?? "Index";

var query = Context.Request.QueryString.ToUriComponent(); // ?id=1&page=2 など

}

<!DOCTYPE html>

<head>

<title>@ViewData["Title"] - WebApplication1</title>

</head>

<body>

<a class="navbar-brand" asp-area="" asp-controller="Home" asp-action="Index">WebApplication1</a>

<button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target=".navbar-collapse" aria-controls="navbarSupportedContent"

aria-expanded="false" aria-label="Toggle navigation">

</button>

</li>

<a class="nav-link text-dark" asp-area="" asp-controller="Home" asp-action="Privacy">Privacy</a>

</li>

</ul>

</div>

</nav>

</header>

<select name="culture" onchange="

const c=this.value;

const query='@query' ;

let url='/' + c + '/@ctrl/@act' ;

if (query && query.length>

0) url += query;

window.location.href = url;

@if (culture == "ja")

{

<text>

<option value="en">English</option>

</text>

}

else

{

<text>

<option value="en" selected>English</option>

</text>

}

</select>

</form>

@RenderBody()

</main>

</div>

</div>

</footer>

@await RenderSectionAsync("Scripts", required: false)

</body>

</html>