C# — Enum & Switch

Photo by Fernando Hernandez on Unsplash

前言

當前端有 radio 選項時後端可以用 Enum 來避免 Magic Number

參考文章：[C#] 使用 Enum 避免 Magic Number

🔖 文章索引

1. 前端
2. 後端
3. 總結

前端

radio name="images" value=" 1 | 2 | 3 "

<form style="width: 500px" class="pt-3 pb-4">
    <div class="mb-3">
        <div class="mb-0 h5 d-flex align-items-center">
            <div>選擇圖片：</div>
            <div style="flex-basis: 65px; line-height: 30px;">
                <input class="form-check-input" type="radio" name="images" id="img_one" value="1" checked>
                <label class="form-check-label" for="img_one">
                    img_one
                </label>
            </div>
            <div style="flex-basis: 105px; line-height: 30px;">
                <input class="form-check-input" type="radio" name="images" id="img_two" value="2">
                <label class="form-check-label" for="img_two">
                    img_two
                </label>
            </div>
            <div style="line-height: 30px;">
                <input class="form-check-input" type="radio" name="images" id="img_three" value="3">
                <label class="form-check-label" for="img_three">
                    img_three
                </label>
            </div>
        </div>
    </div>
    <button type="submit" id="confirmBtn" class="btn btn-info w-100">確定</button>
</form>

const confirmBtn = document.getElementById('confirmBtn') as HTMLButtonElement;
const images = document.getElementsByName("images");

confirmBtn.addEventListener('click', function(e) {
  e.preventDefault();
  let images_checked = "0";

  images.forEach((element) => {
    if ( (element as HTMLInputElement).checked ) {
      images_checked = (element as HTMLInputElement).value;
    }
  })

  if (!images_checked) {
      alert("請選擇圖片");
      return;
  }

  const formData = new FormData();
  formData.append('image', images_checked);

  // axios.post("/Home/Index", data)
  callApiPostHomeIndex(formData);
})

後端

使用 switch ( Value ) 當 Value 要和 case enum 型別 比對時

要將 Value 的類別強制轉成 enum 型別

public enum Images
{
  One = 1,
  Two,
  Three
}

public class HomeController : Controller
{
  ...

  [HttpPost]
  public IActionResult Images(IFormCollection formCollection)
  {
    try
    {
      if (!Int32.TryParse(formCollection["image"], out int image))
      {
          return BadRequest(new ErrorResponse() { Message = "圖片選項不在範圍內" });
      }
      
      // Error
      // switch( image )
      // {
      //   Error: Cannot convert expression of type "ProjectName.Controllers.Images" to type 'int'
      //   case Images.One:
      //     break;
      // }

      // Success
      switch ( (Images)image )
      {
        case Images.One:
          ...
          break;
        case Images.Two:
          ...
          break;
          
          ...
      }
    }
    catch (Exception e)
    {
      ...
    }
  }
}

總結

當看到 Error: Cannot convert expression of type "ProjectName.Controllers.EnumName" to type 'int' 我沒有想到 Enum 也會是一個型別，所以一直沒想到用強轉型別的方式，經過這次以後看到錯誤訊息就會多了對於型別的敏銳度 👏