VB 中的空检查

我所要做的就是检查一个对象是否为 null，但是不管我做什么，如果它编译，它就会抛出一个 NullReferenceException来尝试检查！我是这么做的:

    If ((Not (comp.Container Is Nothing)) And (Not (comp.Container.Components Is Nothing))) Then
For i As Integer = 0 To comp.Container.Components.Count() - 1 Step 1
fixUIIn(comp.Container.Components.Item(i), style)
Next
End If


If ((Not IsDBNull(comp.Container)) And (Not IsDBNull(comp.Container.Components))) Then
For i As Integer = 0 To comp.Container.Components.Count() - 1 Step 1
fixUIIn(comp.Container.Components.Item(i), style)
Next
End If


If ((Not IsNothing(comp.Container)) And (Not IsNothing(comp.Container.Components))) Then
For i As Integer = 0 To comp.Container.Components.Count() - 1 Step 1
fixUIIn(comp.Container.Components.Item(i), style)
Next
End If


If ((Not (comp.Container Is DBNull.Value)) And (Not (comp.Container.Components Is DBNull.Value))) Then
For i As Integer = 0 To comp.Container.Components.Count() Step 1
fixUIIn(comp.Container.Components.Item(i), style)
Next
End If

我已经浏览了 VB 的书籍，搜索了几个论坛，和应该工作的一切都没有！很抱歉问了这么一个补救性的问题，但我只是想知道。

正如您所知，调试器说空对象是 comp.Container

282020

小开

Your code is way more cluttered than necessary.

Replace (Not (X Is Nothing)) with X IsNot Nothing and omit the outer parentheses:

If comp.Container IsNot Nothing AndAlso comp.Container.Components IsNot Nothing Then
For i As Integer = 0 To comp.Container.Components.Count() - 1
fixUIIn(comp.Container.Components(i), style)
Next
End If

Much more readable. … Also notice that I’ve removed the redundant Step 1 and the probably redundant .Item.

But (as pointed out in the comments), index-based loops are out of vogue anyway. Don’t use them unless you absolutely have to. Use For Each instead:

If comp.Container IsNot Nothing AndAlso comp.Container.Components IsNot Nothing Then
For Each component In comp.Container.Components
fixUIIn(component, style)
Next
End If