[.NET]VB.NET 使用 Replace 小心得到 Nothing 哦!

2015-08-12

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.NET

使用 Replace 函式 (Visual Basic)，Expression 的長度為零或 Nothing，或 Start 大於 Expression 的長度，回傳值就是會是 Nothing 哦!

今天跟同事在查一個「System.NullReferenceException: 並未將物件參考設定為物件的執行個體」的錯誤!

後來發現字串經過 Replace Function 處理過後就回傳 Nothing 。

所以就使用以下的 VB.NET Console 的程式來測試，如下，

Imports System.Text.RegularExpressions
Module Module1

    Sub Main()
        Dim exp As String = String.Empty
        Dim result As String = String.Empty
        '一般字串測試
        exp = "123Find"
        result = VBReplaceTest(exp)
        Console.WriteLine(String.Format("VB Replace, 123Find->find->replace:[{0}]", result))
        result = StringReplaceTest(exp)
        Console.WriteLine(String.Format("String.Replace, 123Find->find->replace:[{0}]", result))
        result = RegexReplaceTest(exp)
        Console.WriteLine(String.Format("Regex.Replace, 123Find->find->replace:[{0}]", result))

        '空白字串測試
        exp = " "
        result = VBReplaceTest(exp)
        Console.WriteLine(String.Format("VB Replace, [ ]->find->replace:[{0}]", result))
        result = StringReplaceTest(exp)
        Console.WriteLine(String.Format("String.Replace, [ ]->find->replace:[{0}]", result))
        result = RegexReplaceTest(exp)
        Console.WriteLine(String.Format("Regex.Replace, [ ]->find->replace:[{0}]", result))

        '空字串測試
        exp = String.Empty
        result = VBReplaceTest(exp)
        Console.WriteLine(String.Format("VB Replace, string.empty->find->replace:[{0}]", _
                                        IIf(IsNothing(result), "Nothing", result)))
        result = StringReplaceTest(exp)
        Console.WriteLine(String.Format("String.Replace, string.empty->find->replace:[{0}]", _
                                        IIf(IsNothing(result), "Nothing", result)))
        result = RegexReplaceTest(exp)
        Console.WriteLine(String.Format("Regex.Replace, string.empty->find->replace:[{0}]", _
                                        IIf(IsNothing(result), "Nothing", result)))
		Console.ReadLine
    End Sub

    Const finder As String = "find"
    Const replacement As String = "replace"

    Function VBReplaceTest(ByVal exp As String) As String
        Dim result As String = String.Empty
        result = Replace(exp, finder, replacement, 1, -1, CompareMethod.Text)
        Return result
    End Function


    Function StringReplaceTest(ByVal exp As String) As String
        Dim result As String = String.Empty
        result = exp.Replace(finder, replacement)
        Return result
    End Function


    Function RegexReplaceTest(ByVal exp As String) As String
        Dim r As Regex = New Regex(finder, RegexOptions.IgnoreCase)
        Dim result As String = String.Empty
        result = r.Replace(exp, replacement)
        Return result
    End Function

End Module

測試結果如下，