分配null以十進制MVC 3

Question

我試圖賦予null的變量IdSubCategory如果會話變量Session["SubCategory"]是空

爲什麼下面不工作？

decimal tmpvalue2; 
decimal? IdSubCategory = null;  
if (decimal.TryParse((string)Session["SubCategory"], out tmpvalue2)) 
    IdSubCategory = tmpvalue2; 

Answer 1

我通常將我的會話變量包裝在屬性中。

protected decimal? IdSubCategory 
    { 
     get 
     { 
      if (Session["SubCategory"] == null) 
       return null; 
      else 
       return decimal.Parse(Session["SubCategory"].ToString()); 
     } 
     set 
     { 
      Session["SubCategory"] = value; 
     } 
    } 

Answer 2

decimal.TryParse需要一個字符串轉換，但如果Session["SubCategory"]爲null，則你的代碼行正試圖施放一個空字符串將錯誤

這一個方法：if (decimal.TryParse((string)Session["SubCategory"], out tmpvalue2))

爲了解決它，請先檢查，如果Session["SubCategory"]不爲空，然後嘗試做十進制。嘗試Parse

Answer 3

什麼不起作用？你在Session["SubCategory"]存儲什麼？

這些測試存儲串表示在會話對象的ID時通過：

[Test] public void GivenWhenIntegerString_WhenTryParse_ThenValidInteger() 
{ 
    Dictionary<string, Object> fakeSession = new Dictionary<string, object>(); 
    fakeSession["SubCategory"] = "5"; 

    decimal tmp; 
    decimal? IdSubCategory = null; 
    if (decimal.TryParse((string)fakeSession["SubCategory"], out tmp)) 
     IdSubCategory = tmp; 

    Assert.That(IdSubCategory, Is.EqualTo(5d)); 
} 

[Test] public void GivenWhenNull_WhenTryParse_ThenNull() 
{ 
    Dictionary<string, Object> fakeSession = new Dictionary<string, object>(); 
    fakeSession["SubCategory"] = null; 

    decimal tmp; 
    decimal? IdSubCategory = null; 
    if (decimal.TryParse((string)fakeSession["SubCategory"], out tmp)) 
     IdSubCategory = tmp; 

    Assert.That(IdSubCategory, Is.EqualTo(null));    
} 

當在Session["SubCategory"]

[Test] 
public void GivenWhenInteger_WhenTryParse_ThenValidInteger() 
{ 
    Dictionary<string, Object> fakeSession = new Dictionary<string, object>(); 
    fakeSession["SubCategory"] = 5; 

    decimal tmp; 
    decimal? IdSubCategory = null; 
    if (decimal.TryParse((string)fakeSession["SubCategory"], out tmp)) 
     IdSubCategory = tmp; 

    Assert.That(IdSubCategory, Is.EqualTo(5d)); 
} 

存儲int或decimal在這種情況下該測試失敗，這將修復它：

decimal tmp; 
decimal? IdSubCategory = null; 
if (Session["SubCategory"] != null && 
    decimal.TryParse(Session["SubCategory"].ToString(), out tmp)) 
    IdSubCategory = tmp;