屬性獲取器中使用的遞歸表達式

Question

如何避免屬性獲取器中的遞歸調用？這是我簡單的代碼

public class UploadAttribute : Attribute 
{ 
    private Type _resourceType; 
    private string _select; 
    private string _change; 
    private string _remove; 

    public Type ResourceType 
    { 
     get { return _resourceType; } 
     set { _resourceType = value; } 
    } 

    public string Select 
    { 
     get { return GetResourceText(m => m.Select, "Select..."); } 
     set { _select = value; } 
    } 

    public string Change 
    { 
     get { return GetResourceText(m => m.Change, "Change..."); } 
     set { _change = value; } 
    } 

    public string Remove 
    { 
     get { return GetResourceText(m => m.Remove, "Remove"); } 
     set { _remove = value; } 
    } 

    private string GetResourceText(Expression<Func<UploadAttribute, string>> expression, string @default) 
    { 
     var value = expression.Compile().Invoke(this); // here .net is creating new UploadAttribute instance and use it for expression fnc 
     var result = value ?? @default; 

     if (_resourceType != null && !string.IsNullOrEmpty(value)) 
     { 
      ResourceManager rm = new ResourceManager(_resourceType); 
      try 
      { 
       result = rm.GetString(value); 
      } 
      catch 
      { 
       // if string wasn't found in resource file than use what user specify; don't by big brother. 
      } 
     } 

     return result; 
    } 
} 

但是，如果你看一下方法GetResourceText，有行，我需要編譯並調用表達以獲取給定的屬性值。不幸的是，這個操作創建了UploadAttribute的新實例。在那一刻，.net通過所有屬性並調用getter，如果我沒有弄錯，並且在getter中.net編譯並調用表達式來獲取值或給定屬性，再次一次又一次地發生到StackOverlowException。 你能告訴我如何避免這種行爲，但簡單的解決方案？

編輯：該類的作用是爲按鈕提供標題 - 用戶設置了哪些用戶可以使用資源管理器中的多語言標題。 在上面的例子中，按鈕選擇從資源翻譯，對於更改按鈕，使用默認文本「更改...」和刪除按鈕標題「銷燬此@ &＃！」。因此，如果用戶沒有指定屬性值，那麼應用程序使用默認文本，否則嘗試在資源中查找文本，並且如果找到匹配，則使用來自資源的文本，否則使用用戶設置的內容。

[Required] 
[Upload(ResourceType = typeof(Resource), Select = "UploadSelect", Remove = "Destroy this @&#!")] 
public HttpPostedFileBase Logo { get; set; } 

Answer 1

我很漂亮的轉儲。它根本不會創建新的實例;我問了一會兒，我就回答了我的問題。

return GetResourceText(m => m.Select, "Select...");是遞歸沒有結束，但return GetResourceText(m => m._select, "Select...");不是，因爲我不是再次調用方法GetResourceText。

對不起男生的傻問題。

Answer 2

看來你試圖實現的是有一些方法來初始化這些屬性，如果他們沒有明確設置。你這樣做的方式是行不通的。

m => m.Remove類型表達式將導致在無限遞歸中再次調用屬性getter，直到發生堆棧溢出。

您可以使用lazy結構，如下圖所示。它的工作原理如下：

1. 如果用戶沒有爲屬性指定值，那麼當調用屬性getter時，它將返回硬編碼的默認值。
2. 如果用戶爲某個屬性指定了一個值，那麼該值將首先用作關鍵字，以試圖從資源中檢索相應的字符串值。如果未找到該資源，則將其用作該屬性的值，前提是該資源不爲空，否則將回退到硬編碼的默認值。

請注意，這種屬性屬性值的雙重用途導致了一個相當脆弱的設計解決方案。如果沒有找到具有「UploadSelect」鍵的資源，那將成爲該按鈕上的標題。

public class UploadAttribute : Attribute 
{ 
    private static readonly string kSelectDefaultCaption = "Select..."; 
    private static readonly string kChangeDefaultCaption = "Change..."; 
    private static readonly string kRemoveDefaultCaption = "Remove..."; 

    private Type _resourceType; 
    private Lazy<string> _select = new Lazy<string>(() => kSelectDefaultCaption); 
    private Lazy<string> _change = new Lazy<string>(() => kChangeDefaultCaption); 
    private Lazy<string> _remove = new Lazy<string>(() => kRemoveDefaultCaption); 

    public Type ResourceType 
    { 
     get { return _resourceType; } 
     set { _resourceType = value; } 
    } 

    public string Select 
    { 
     get { return _select.Value; } 
     set { _select = new Lazy<string>(() => GetResourceText(value, kSelectDefaultCaption)); } 
    } 

    public string Change 
    { 
     get { return _change.Value; } 
     set { _change = new Lazy<string>(() => GetResourceText(value, kChangeDefaultCaption)); } 
    } 

    public string Remove 
    { 
     get { return _remove.Value; } 
     set { _remove = new Lazy<string>(() => GetResourceText(value, kRemoveDefaultCaption)); } 
    } 

    private string GetResourceText(string key, string @default) 
    { 
     // initialize to default. 
     var result = @default; 
     if (_resourceType != null && !string.IsNullOrEmpty(key)) 
     { 
      // initialize to the value of the key, 
      // that could be a user supplied string literal 
      result = key; 

      // attempt to retrieve it from the resources. 
      ResourceManager rm = new ResourceManager(_resourceType); 
      try 
      { 
       result = rm.GetString(key); 
      } 
      catch 
      { 
       // could not retrieve key, using the key value as the result. 
      } 
     } 
     return result; 
    } 
} 

Answer 3

的輕微修改Alex的答案，如果你想保持你的編譯時安全使用的屬性名稱：

public class UploadAttribute : Attribute 
{ 
    private Type _resourceType; 
    private Lazy<string> _select; 
    private Lazy<string> _change; 
    private Lazy<string> _remove; 

    UploadAttribute() 
    { 
     _select = new Lazy<string>(() => GetResourceText(m => m.Select, "Select...")); 
     _change = new Lazy<string>(() => GetResourceText(m => m.Change, "Change...")); 
     _remove = new Lazy<string>(() => GetResourceText(m => m.Remove, "Remove...")); 
    } 

    public Type ResourceType 
    { 
     get { return _resourceType; } 
     set { _resourceType = value; } 
    } 

    public string Select 
    { 
     get { return _select.Value; } 
     set { _select = new Lazy<string>(() => value); } 
    } 

    public string Change 
    { 
     get { return _change.Value; } 
     set { _change = new Lazy<string>(() => value); } 
    } 

    public string Remove 
    { 
     get { return _remove.Value; } 
     set { _remove = new Lazy<string>(() => value); } 
    } 
    private string GetResourceText(Expression<Func<UploadAttribute, string>> expression, string @default) 
    { 
     var result = @default; 
     var memberExpression = expression.Body as MemberExpression; 
     if (_resourceType != null && memberExpression != null) 
     { 
      ResourceManager rm = new ResourceManager(_resourceType); 
      try 
      { 
       result = rm.GetString(memberExpression.Member.Name); 
      } 
      catch 
      { 
       // if string wasn't found in resource file than use what user specify; don't by big brother. 
      } 
     } 
     return result; 
    } 
}