C#アノテーションの効果的な使い方と実践例

using System;

class Program
{
    [Obsolete("This method is obsolete. Use NewMethod instead.")]
    static void OldMethod()
    {
        Console.WriteLine("Old method");
    }

    static void NewMethod()
    {
        Console.WriteLine("New method");
    }

    static void Main()
    {
        OldMethod(); // この行は警告を発生させます
        NewMethod();
    }
}

using System;

[Serializable]
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

using System;
using System.ComponentModel.DataAnnotations;

public class Product
{
    [Key]
    public int ProductId { get; set; }

    [Required(ErrorMessage = "Product name is required")]
    [StringLength(100, MinimumLength = 3, ErrorMessage = "Product name must be between 3 and 100 characters")]
    public string Name { get; set; }

    [Range(0.01, 9999.99, ErrorMessage = "Price must be between 0.01 and 9999.99")]
    public decimal Price { get; set; }

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

using System;
using System.ComponentModel.DataAnnotations;

public class UserRegistration
{
    [Required(ErrorMessage = "Username is required")]
    [StringLength(50, ErrorMessage = "Username cannot be longer than 50 characters")]
    public string Username { get; set; }

    [Required(ErrorMessage = "Email is required")]
    [EmailAddress(ErrorMessage = "Invalid Email Address")]
    public string Email { get; set; }

    [Required(ErrorMessage = "Password is required")]
    [StringLength(100, MinimumLength = 6, ErrorMessage = "Password must be at least 6 characters long")]
    public string Password { get; set; }

    [Required(ErrorMessage = "Confirm Password is required")]
    [Compare("Password", ErrorMessage = "Password and Confirm Password do not match")]
    public string ConfirmPassword { get; set; }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

public class Program
{
    public static void Main()
    {
        var user = new UserRegistration
        {
            Username = "JohnDoe",
            Email = "john.doe@example.com",
            Password = "password123",
            ConfirmPassword = "password123"
        };

        var validationResults = new List<ValidationResult>();
        var context = new ValidationContext(user, serviceProvider: null, items: null);
        bool isValid = Validator.TryValidateObject(user, context, validationResults, validateAllProperties: true);

        if (isValid)
        {
            Console.WriteLine("User registration is valid.");
        }
        else
        {
            foreach (var validationResult in validationResults)
            {
                Console.WriteLine(validationResult.ErrorMessage);
            }
        }
    }
}

using System;
using System.ComponentModel.DataAnnotations;
using System.Text.RegularExpressions;

public class CustomPatternAttribute : ValidationAttribute
{
    private readonly string _pattern;

    public CustomPatternAttribute(string pattern)
    {
        _pattern = pattern;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value != null)
        {
            var stringValue = value as string;
            if (!Regex.IsMatch(stringValue, _pattern))
            {
                return new ValidationResult($"The field {validationContext.DisplayName} does not match the required pattern.");
            }
        }
        return ValidationResult.Success;
    }
}

using System;

public class UserInput
{
    [CustomPattern("^[a-zA-Z0-9]*$", ErrorMessage = "Only alphanumeric characters are allowed.")]
    public string AlphanumericField { get; set; }
}

using System;
using System.Reflection;

public class Example
{
    [Obsolete("This method is obsolete")]
    public void OldMethod() { }

    public static void Main()
    {
        MethodInfo method = typeof(Example).GetMethod("OldMethod");
        var attributes = method.GetCustomAttributes(typeof(ObsoleteAttribute), false);
        if (attributes.Length > 0)
        {
            Console.WriteLine("OldMethod is obsolete.");
        }
    }
}

using System;
using System.Collections.Generic;
using System.Reflection;

public static class AttributeCache
{
    private static readonly Dictionary<MethodInfo, object[]> _cache = new Dictionary<MethodInfo, object[]>();

    public static object[] GetAttributes(MethodInfo method)
    {
        if (!_cache.TryGetValue(method, out var attributes))
        {
            attributes = method.GetCustomAttributes(false);
            _cache[method] = attributes;
        }
        return attributes;
    }
}

using Microsoft.AspNetCore.Mvc;
using System.ComponentModel.DataAnnotations;

public class RegisterModel
{
    [Required(ErrorMessage = "Username is required")]
    [StringLength(50, MinimumLength = 3, ErrorMessage = "Username must be between 3 and 50 characters")]
    public string Username { get; set; }

    [Required(ErrorMessage = "Email is required")]
    [EmailAddress(ErrorMessage = "Invalid Email Address")]
    public string Email { get; set; }

    [Required(ErrorMessage = "Password is required")]
    [StringLength(100, MinimumLength = 6, ErrorMessage = "Password must be at least 6 characters long")]
    public string Password { get; set; }
}

public class AccountController : Controller
{
    [HttpPost]
    public IActionResult Register(RegisterModel model)
    {
        if (ModelState.IsValid)
        {
            // 登録処理
            return RedirectToAction("Success");
        }
        return View(model);
    }
}

using Microsoft.AspNetCore.Mvc;
using System.ComponentModel.DataAnnotations;

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    public class ProductInputModel
    {
        [Required(ErrorMessage = "Product name is required")]
        [StringLength(100, MinimumLength = 3, ErrorMessage = "Product name must be between 3 and 100 characters")]
        public string Name { get; set; }

        [Range(0.01, 9999.99, ErrorMessage = "Price must be between 0.01 and 9999.99")]
        public decimal Price { get; set; }
    }

    [HttpPost]
    public IActionResult Create(ProductInputModel model)
    {
        if (ModelState.IsValid)
        {
            // 商品の作成処理
            return Ok(new { Message = "Product created successfully" });
        }
        return BadRequest(ModelState);
    }
}

[Required(ErrorMessage = "Username is required")]
[StringLength(50, ErrorMessage = "Username cannot be longer than 50 characters")]
public string Username { get; set; }

[EmailAddress(ErrorMessage = "Invalid Email Address")]
public string Email { get; set; }

[Range(0.01, 9999.99, ErrorMessage = "Price must be between 0.01 and 9999.99")]
public decimal Price { get; set; }

public class CustomValidation : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        // 複雑なバリデーションロジック
    }
}