对验证输入时显示的默认消息进行多语言支持

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Localization;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Globalization;

namespace LocalizationDefaultValidation
{
  public class Startup
  {
    // 省略

    public void ConfigureServices(IServiceCollection services)
    {
      services.AddControllersWithViews();

      services.AddMvc()
        // ローカライズに必要。Resx ファイルのフォルダパスを指定
        .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix, opts => { opts.ResourcesPath = "Resources"; })
        // DataAnnotations のローカライズに必要
        .AddDataAnnotationsLocalization(options =>
        {
          // DataAnnotation を使ったときのメッセージは SharedResource に集約する
          options.DataAnnotationLocalizerProvider = (type, factory) => factory.Create(typeof(SharedResource));
        });
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
      if (env.IsDevelopment())
      {
        app.UseDeveloperExceptionPage();
      }
      else
      {
        app.UseExceptionHandler("/Home/Error");
        // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
        app.UseHsts();
      }

      // 標準の機能で切り替えたい言語を定義します。
      var supportedCultures = new[]
      {
        new CultureInfo("ja"),
        new CultureInfo("en"),
        new CultureInfo("es"),
      };

      // 標準の言語切り替え機能を有効にします。対応しているのは「クエリ文字列」「Cookie」「Accept-Language HTTP ヘッダー」です。
      app.UseRequestLocalization(new RequestLocalizationOptions
      {
        DefaultRequestCulture = new RequestCulture("ja"),
        SupportedCultures = supportedCultures,
        SupportedUICultures = supportedCultures
      });

      app.UseHttpsRedirection();
      app.UseStaticFiles();

      // 省略
    }
  }
}

using LocalizationDefaultValidation.Models;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;

namespace LocalizationDefaultValidation.Controllers
{
  public class HomeController : Controller
  {
    // 省略

    public IActionResult Create()
    {
      SetDayOfWeeksViewData();
      return View();
    }

    [HttpPost]
    public IActionResult Create(UserViewModel model)
    {
      SetDayOfWeeksViewData();

      // エラーなら差し戻し
      if (ModelState.IsValid == false) return View(model);

      // 正常に登録できた場合は Index に戻る
      return RedirectToAction(nameof(Index));
    }

    /// <summary>曜日の一覧を ViewData に設定します。</summary>
    private void SetDayOfWeeksViewData()
      => ViewData["DayOfWeeks"] = ((DayOfWeek[])Enum.GetValues(typeof(DayOfWeek))).Select(x => new SelectListItem(x.ToString(), ((int)x).ToString()));
  }
}

@{
  ViewData["Title"] = "Home Page";
}

<div class="text-center">
  <h1 class="display-4">Welcome</h1>
  <p>Learn about <a href="https://docs.microsoft.com/aspnet/core">building Web apps with ASP.NET Core</a>.</p>
</div>

<p>ユーザー作成画面に遷移します。リンクごとに選択した言語で表示できます。</p>
<ul>
  <li><a asp-action="Create">Create</a></li>
  <li><a asp-action="Create" asp-route-culture="ja">Create (ja)</a></li>
  <li><a asp-action="Create" asp-route-culture="en">Create (en)</a></li>
  <li><a asp-action="Create" asp-route-culture="es">Create (es)</a></li>
</ul>

@model LocalizationDefaultValidation.Models.UserViewModel
@using System.Globalization;
@{
  ViewData["Title"] = "Create";
}

<h1>Create</h1>

<h4>UserViewModel</h4>
<hr />
<div class="row">
  <div class="col-md-4">
    <form asp-action="Create" enctype="multipart/form-data" >
      <div asp-validation-summary="ModelOnly" class="text-danger"></div>
      <div class="form-group">
        <label asp-for="ID" class="control-label"></label>
        <input asp-for="ID" class="form-control" />
        <span asp-validation-for="ID" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Name" class="control-label"></label>
        <input asp-for="Name" class="form-control" />
        <span asp-validation-for="Name" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Password" class="control-label"></label>
        <input asp-for="Password" class="form-control" />
        <span asp-validation-for="Password" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="ConfirmPassword" class="control-label"></label>
        <input asp-for="ConfirmPassword" class="form-control" />
        <span asp-validation-for="ConfirmPassword" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Email" class="control-label"></label>
        <input asp-for="Email" class="form-control" />
        <span asp-validation-for="Email" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Age" class="control-label"></label>
        <input asp-for="Age" class="form-control" />
        <span asp-validation-for="Age" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Gender"></label>
        <div>
          <label><input type="radio" asp-for="Gender" value="@(GenderType.None)" />@(GenderType.None)</label>
          <label><input type="radio" asp-for="Gender" value="@(GenderType.Man)" />@(GenderType.Man)</label>
          <label><input type="radio" asp-for="Gender" value="@(GenderType.Woman)" />@(GenderType.Woman)</label>
        </div>
        <span asp-validation-for="Gender" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Birthday" class="control-label"></label>
        <input asp-for="Birthday" class="form-control" />
        <span asp-validation-for="Birthday" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Phone" class="control-label"></label>
        <input asp-for="Phone" class="form-control" />
        <span asp-validation-for="Phone" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="PostalCode" class="control-label"></label>
        <input asp-for="PostalCode" class="form-control" />
        <span asp-validation-for="PostalCode" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="CreditCard" class="control-label"></label>
        <input asp-for="CreditCard" class="form-control" />
        <span asp-validation-for="CreditCard" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Money" class="control-label"></label>
        <input asp-for="Money" class="form-control" />
        <span asp-validation-for="Money" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="StartDateTime" class="control-label"></label>
        <input asp-for="StartDateTime" class="form-control" />
        <span asp-validation-for="StartDateTime" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="WakeUpTime" class="control-label"></label>
        <input asp-for="WakeUpTime" class="form-control" />
        <span asp-validation-for="WakeUpTime" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Homepage" class="control-label"></label>
        <input asp-for="Homepage" class="form-control" />
        <span asp-validation-for="Homepage" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="MyImage" class="control-label"></label>
        <input asp-for="MyImage" class="form-control" />
        <span asp-validation-for="MyImage" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="MyColor" class="control-label"></label>
        <input asp-for="MyColor" class="form-control" type="color" />
        <span asp-validation-for="MyColor" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="WorkingDays" class="control-label"></label>
        <select asp-for="WorkingDays" class="form-control" asp-items="@((IEnumerable<SelectListItem>)ViewData["DayOfWeeks"])" multiple></select>
        <span asp-validation-for="WorkingDays" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="VacationDay" class="control-label"></label>
        <select asp-for="VacationDay" class="form-control" asp-items="@((IEnumerable<SelectListItem>)ViewData["DayOfWeeks"])" multiple></select>
        <span asp-validation-for="VacationDay" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Comment" class="control-label"></label>
        <textarea asp-for="Comment" class="form-control"></textarea>
        <span asp-validation-for="Comment" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="FileName" class="control-label"></label>
        <input asp-for="FileName" class="form-control" />
        <span asp-validation-for="FileName" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="UploadFile" class="control-label"></label>
        <input asp-for="UploadFile" type="file" multiple />
        <span asp-validation-for="UploadFile" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Month" class="control-label"></label>
        <input asp-for="Month" class="form-control" type="month" />
        <span asp-validation-for="Month" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Search" class="control-label"></label>
        <input asp-for="Search" class="form-control" type="search" />
        <span asp-validation-for="Search" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Range" class="control-label"></label>
        <input asp-for="Range" class="form-control" type="range" min="10" max="100" />
        <span asp-validation-for="Range" class="text-danger"></span>
      </div>
      <div class="form-group">
        <label asp-for="Week" class="control-label"></label>
        <input asp-for="Week" class="form-control" type="week" />
        <span asp-validation-for="Week" class="text-danger"></span>
      </div>
      <div class="form-group form-check">
        <label class="form-check-label">
          <input class="form-check-input" asp-for="IsAccepted" /> @Html.DisplayNameFor(model => model.IsAccepted)
        </label>
      </div>
      <div class="form-group">
        <input type="submit" value="Create" class="btn btn-primary" asp-route-culture="@CultureInfo.CurrentCulture.Name" />
      </div>
    </form>
  </div>
</div>

<div>
  <a asp-action="Index">Back to List</a>
</div>

@section Scripts {
  @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

using Microsoft.AspNetCore.Http;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

namespace LocalizationDefaultValidation.Models
{
  public class UserViewModel
  {
    [Required]
    [Display(Name = "ID (半角英数字)")]
    [StringLength(20)]
    [RegularExpression(@"^[0-9a-zA-Z]*$")]
    public string ID { get; set; }

    [StringLength(50)]
    public string Name { get; set; }

    [StringLength(50, MinimumLength = 8)]
    [DataType(DataType.Password)]
    [RegularExpression(@"^[0-9a-zA-Z]*$")]
    public string Password { get; set; }

    [StringLength(50, MinimumLength = 8)]
    [DataType(DataType.Password)]
    [Compare(nameof(Password))]
    public string ConfirmPassword { get; set; }

    [EmailAddress]
    [DataType(DataType.EmailAddress)] // スキャフォールディングするときはコメントアウトする
    public string Email { get; set; }

    //[Int]
    [Range(0, 150)]
    public int Age { get; set; }

    [Required]
    [EnumDataType(typeof(GenderType))]
    public GenderType Gender { get; set; }

    [DataType(DataType.Date)]
    public DateTime Birthday { get; set; }

    [Phone()]
    [DataType(DataType.PhoneNumber)] // スキャフォールディングするときはコメントアウトする
    public string Phone { get; set; }

    [DataType(DataType.PostalCode)]
    public string PostalCode { get; set; }

    [CreditCard()]
    [DataType(DataType.CreditCard)] // スキャフォールディングするときはコメントアウトする
    public string CreditCard { get; set; }

    [DataType(DataType.Currency)]
    public decimal Money { get; set; }

    [DataType(DataType.DateTime)]
    public DateTime StartDateTime { get; set; }

    [DataType(DataType.Time)]
    public TimeSpan WakeUpTime { get; set; }

    [Url]
    [DataType(DataType.Url)] // スキャフォールディングするときはコメントアウトする
    public string Homepage { get; set; }

    [Url]
    [DataType(DataType.ImageUrl)] // スキャフォールディングするときはコメントアウトする
    public string MyImage { get; set; }

    public string MyColor { get; set; }

    [MaxLength(5)]
    public DayOfWeek[] WorkingDays { get; set; }

    [MinLength(3)]
    public DayOfWeek[] VacationDay { get; set; }

    [StringLength(200)]
    [DataType(DataType.MultilineText)]
    public string Comment { get; set; }

    [Display(Name = "ファイル名 (.png)")]
    [FileExtensions(Extensions = "png")]
    public string FileName { get; set; }

    [DataType(DataType.Upload)]
    public List<IFormFile> UploadFile { get; set; }

    public DateTime Month { get; set; }

    public string Search { get; set; }

    [Range(10, 100)]
    public int Range { get; set; }

    public string Week { get; set; }

    [Required]
    public bool IsAccepted { get; set; }
  }

  public enum GenderType
  {
    None,
    Man,
    Woman,
    Other,
  }
}

namespace LocalizationDefaultValidation
{
  // クラス名は作成した .resx のファイル名と同じにする必要がある
  public class SharedResource { }
}