vb.net - IIF 行为不正确

Question

我想知道为什么以下行行为不正确。当我使用 IIF 时，虽然当条件为 True 时，该函数返回 getMessage

Return CType(IIf(Object.Equals(_newValue, _oldValue), 
             msg, GetMessage(msg)), PooMessage)

但是以下几行表现得很好:

If Object.Equals(_newValue, _oldValue) Then
        Return msg
Else
        Return CType(GetMessage(msg), PooMessage)
End If

Answer 1

您应该从 IIf() 更改为 If() ，因为后者使用短路，而前者则不使用。对于 IIf() 版本，即使 bool 值计算结果为 true，也会调用 GetMessage()，这可能会导致副作用。使用 If() 时，仅计算正确的返回值:

Return CType(If(Object.Equals(_newValue, _oldValue), msg, GetMessage(msg)), PooMessage)

编辑:添加示例代码，以更清楚地了解 If() 与 IIF()，并使用 dotnetfiddle 示例 fiddle :https://dotnetfiddle.net/vuMPgK

代码:

Imports System
Imports Microsoft.VisualBasic

Public Module Module1
    Public Sub Main()

        Dim didItWork as Boolean = False
        Dim myTestObject as Test = Nothing

        ' works, due to IF(). Only the 'true' value is calculated
        didItWork = If(myTestObject Is Nothing, False, myTestObject.MyValue)

            Console.WriteLine("Did If() work?: " & didItWork.ToString())


            ' does not work, due to IIF().  Both True and False conditions are calculated regardless of the original test condition.
            '  it fails because myTestObject is null, so trying to access one of its properties causes an exception.
            Try
                didItWork = IIF(myTestObject Is Nothing, False, myTestObject.MyValue)
                Console.WriteLine("Did IIF() work?: " & didItWork.ToString())
            Catch ex as Exception
                Console.WriteLIne("Error was thrown from IIF")
            End Try

    End Sub
End Module

Public Class Test
        Public Property MyValue as Boolean = True
End class