# ASP.Net7 Exception Attribute
###### tags: `C#`

## 定義自己的 exception

using System;

public class TestException : Exception
{
    public TestException()
    {
    }

    public TestException(string message)
        : base(message)
    {
    }

    public TestException(string message, Exception inner)
        : base(message, inner)
    {
    }
}

#### 相關連結
[如何建立使用者定義的例外狀況](https://learn.microsoft.com/zh-tw/dotnet/standard/exceptions/how-to-create-user-defined-exceptions)

## 定義自己的 exception attribute
1. 定義自己的 exception attribute，繼承 `IActionFilter`，接著實作 `OnException()`
可以判斷不同的 exception 做不同的事

    using Microsoft.AspNetCore.Mvc;
    using Microsoft.AspNetCore.Mvc.Filters;

    public class TestExceptionAttribute : IExceptionFilter
    {
        private readonly ILogger<TestExceptionAttribute> _logger;

        public TestExceptionAttribute(ILogger<TestExceptionAttribute> logger)
        {
            _logger = logger;
        }

        public void OnException(ExceptionContext context)
        {
            object result = null;
            //統一回傳的格式
            if (context.Exception is UserException)
            {
                var gameConnectException = (UserException)context.Exception;
                result = new
                {
                    status = 500,
                    msg = $"code:{gameConnectException.ErrorCode.ToString("D")}",
                    desc = gameConnectException.Message
                };
            }
            else
            {
                result = new
                {
                    status = 500,
                    desc = context.Exception.Message
                };
            }

            _logger.LogError(context.Exception.ToString());
            context.Result = new OkObjectResult(result);
        }
    }

2. 註冊 attribute 在 `Startup.cs ConfigureServices()`

    public void ConfigureServices(IServiceCollection services)
    {
           services.AddDbContext<MovieContext>(options =>
               options.UseSqlServer(Configuration.GetConnectionString("sqlConString")));
           services.AddScoped<TestExceptionAttribute>();
           services.AddControllers();
    }

3. 在 service 使用 attribute。可依 method 或直接掛在 controller 上，要記得 DI 註冊

    [HttpPost]
    [ServiceFilter(typeof(TestExceptionAttribute))]
    public IActionResult Post([FromBody] Movie movie)
    {
         _context.Movies.Add(movie);
         _context.SaveChanges();
         return CreatedAtRoute("MovieById", new { id = movie.Id }, movie);
    }

### 相關連結
[Implementing Action Filters in ASP.NET Core (.net7)](https://code-maze.com/action-filters-aspnetcore/)
[ASP.NET Core 2 系列 - Filters](https://ithelp.ithome.com.tw/articles/10195407)
[ASP.NET Web API Exception Filter](https://www.huanlintalk.com/2013/01/aspnet-web-api-exception-filter.html)