設置可爲空屬性默認值沒有如所期望不工作

Question

我有一個屬性，該屬性類型是Integer一個默認值NothingNullable，如下所示：

Property TestId As Integer? = Nothing 

以下代碼評估屬性TestId爲Nothing（如想）

Dim test As RadTreeNode = rtvDefinitionCreate.FindNodeByValue(DefinitionHeaderEnum.Test) 
If test Is Nothing Then 
    definition.TestId = Nothing 
Else 
    definition.TestId = test.Nodes(0).Value 
End If 

但下面的代碼的計算結果爲0（默認值Integer，是Integer?與默認值即使當Nothing）

Dim test As RadTreeNode = rtvDefinitionCreate.FindNodeByValue(DefinitionHeaderEnum.Test) 
definition.TestId = If(IsNothing(test), Nothing, test.Nodes(0).Value) 

上述代碼有什麼問題？任何幫助？

（稍後在代碼中調用屬性時，該屬性具有0）

Answer 1

這是因爲你編譯你Option Strict Off編碼。

如果您要用Option Strict On編譯您的代碼，編譯器會給您一個錯誤，告訴您它不能從String轉換爲Integer?，避免在運行時出現這樣的驚喜。

---

這是一個在古怪VB.NET使用properties/ternary operator/option strict off時。

考慮下面的代碼：

Class Test 
    Property NullableProperty As Integer? = Nothing 
    Public NullableField As Integer? = Nothing 
End Class 

Sub Main() 
    ' Setting the Property directly will lest the ternary operator evaluate to zero 
    Dim b = New Test() With {.NullableProperty = If(True, Nothing, "123")} 
    b.NullableProperty = If(True, Nothing, "123") 

    ' Setting the Property with reflection or setting a local variable 
    ' or a public field lets the ternary operator evaluate to Nothing 
    Dim localNullable As Integer? = If(True, Nothing, "123") 
    Dim implicitLocal = If(True, Nothing, "123") 
    b.NullableField = If(True, Nothing, "123") 
    b.GetType().GetMethod("set_NullableProperty").Invoke(b, New Object() {If(True, Nothing, "123")}) 
    b.GetType().GetProperty("NullableProperty").SetValue(b, If(True, Nothing, "123"), Nothing) 
End Sub 

---

另一個區別考慮：

Dim localNullable As Integer? = If(True, Nothing, "123") 

將評估爲Nothing但

Dim localNullable As Integer? = If(SomeNonConstantCondition, Nothing, "123") 

將評估爲0

---

您可以創建一個擴展方法來爲您執行討厭的工作。

<Extension()> 
Function TakeAs(Of T, R)(obj As T, selector As Func(Of T, R)) As R 
    If obj Is Nothing Then 
     Return Nothing 
    End If 
    Return selector(obj) 
End Function 

，並調用它像

definition.TestId = test.TakeAs(Of Int32?)(Function(o) o.Nodes(0).Value)