[Performance][VB.NET].NET空字串判斷徹底研究

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.NET下的空字串判斷整體來說大概可分為下列幾種方法:

  1. 用 is Nothing 判斷。e.x. If str Is Nothing
  2. 用 = Nothing 判斷(類別中只有字串可以用 = Nothing 來判斷)。e.x. If str = Nothing
  3. 用 = “” 判斷。e.x. If str = “”
  4. 用 = String.Empty 判斷。e.x. If str = String.Empty
  5. 用 Is String.Empty 判斷。e.x. If str Is String.Empty
  6. 用 String.IsNullOrEmpty(str) 判斷。e.x. If String.IsNullOrEmpty(str)
  7. 用 String.Length = 0 判斷。e.x. If str.Length = 0

上述所列出的空字串判斷方法,其所跑出的效果不盡相同,為了看出其差異性,這邊撰寫了簡單的測試程式如下:


Sub Main()

Dim testStrings() As String = New String() {Nothing, “”, String.Empty}

For Each str As String In testStrings

TestString(str)

Console.WriteLine(“=======================”)

Next

End Sub


Private Sub TestString(ByVal str As String)

If str Is Nothing Then Console.WriteLine(“str Is Nothing”)

If str = Nothing Then Console.WriteLine(“str = Nothing”)

If str = “” Then Console.WriteLine(“str = ““”“”)

If str = String.Empty Then Console.WriteLine(“str = String.Empty”)

If str Is String.Empty Then Console.WriteLine(“str Is String.Empty”)

If String.IsNullOrEmpty(str) Then Console.WriteLine(“String.IsNullOrEmpty(str)”)


Try

If str.Length = 0 Then Console.WriteLine(“str.Length = 0”)

Catch ex As Exception


End Try

End Sub

執行結果如下圖所示:

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由上圖可知, “” 與 String.Empty 結果是一樣的。比較有趣的是,當字串值是””與String.Empty時, str is Nothing 是不會成立的,但是 str = Nothing 卻會成立。而當字串參考是 Nothing 時,str is string.Empty 不成立、 str = string.Empty 成立。這是因為當使用=運算子判斷時,Nothing 、 “” 與 String.Empty 是被視為一樣的,才會造成此有趣的現象。

接著我們來比較一下空字串判斷的執行速度,同樣的我們須撰寫如下測試程式:


Sub Main()

Dim testStrings() As String = New String() {Nothing, “”, String.Empty}

For Each str As String In testStrings

TestStringSpeed(str, 10000000)

Console.WriteLine(“=======================”)

Next

End Sub


Private Sub TestStringSpeed(ByVal str As String, ByVal count As Integer)

Dim n As Integer = count

Dim sw As New Stopwatch

Console.WriteLine(“str Is Nothing”)

sw.Start()

For i As Integer = 1 To n

If str Is Nothing Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str = Nothing”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = Nothing Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str = ““”“”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = “” Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str = String.Empty”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = String.Empty Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str Is String.Empty”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str Is String.Empty Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“String.IsNullOrEmpty(str)”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If String.IsNullOrEmpty(str) Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Try

Console.WriteLine(“str.Length = 0”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str.Length = 0 Then


End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)

Catch ex As Exception


End Try

End Sub

執行結果如下:

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從上圖可看出幾個現象

  1. 用String.IsNullOrEmpty同時判斷Nothing與空字串會比只判斷是否為 Nothing 或為空還慢
  2. 當字串不為 Nothing 時要判斷字串是否為空的話,用Is String.Empty去判斷最快(文件上通常是說String.Length = 0最快)

接著再看個實驗來看當同時判斷Nothing與空字串的情況,測試程式如下:


Sub Main()

Dim testStrings() As String = New String() {Nothing, “”, String.Empty}

For Each str As String In testStrings

‘TestString(str)

TestStringSpeed(str, 10000000)

Console.WriteLine(“=======================”)

Next

End Sub



Private Sub TestStringSpeed(ByVal str As String, ByVal count As Integer)

Dim n As Integer = count

Dim sw As New Stopwatch

Console.WriteLine(“str Is Nothing & Is String.Empty”)

sw.Start()

For i As Integer = 1 To n

If str Is Nothing Then

Else

If str Is String.Empty Then

End If

End If


Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str Is Nothing & Is String.Empty”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str Is Nothing Then

Else

If str.Length = 0 Then

End If

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)



If str = Nothing Then Console.WriteLine(“str = Nothing”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = Nothing Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str = ““”“”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = “” Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)


Console.WriteLine(“str = String.Empty”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If str = String.Empty Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)



Console.WriteLine(“String.IsNullOrEmpty(str)”)

sw.Reset()

sw.Start()

For i As Integer = 1 To n

If String.IsNullOrEmpty(str) Then

End If

Next

Console.WriteLine(“花費時間: “ & sw.ElapsedMilliseconds)



End Sub

執行結果如下:

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可以看出 String.IsNullOrEmpty 函式的效能與自行混用 str Is Nothing 與 Is String.Empty 判斷方法來達到相同目的的效能差不多。image

經過以上三個實驗,對於字串為空的判斷相信已有相當的了解,若能清楚分辨其功用與使用的時機,多少都能增進程式的效能。像是使用上若只需判斷Nothing或為空的話,應避免誤用會同時判斷的方法(如下例),就可以避免不必要的OverHead。

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