.NET空字串判斷徹底研究
.NET下的空字串判斷整體來說大概可分為下列幾種方法：
- 用 is Nothing 判斷。e.x. If str Is Nothing
- 用 = Nothing 判斷(類別中只有字串可以用 = Nothing 來判斷)。e.x. If str = Nothing
- 用 = "" 判斷。e.x. If str = ""
- 用 = String.Empty 判斷。e.x. If str = String.Empty
- 用 Is String.Empty 判斷。e.x. If str Is String.Empty
- 用 String.IsNullOrEmpty(str) 判斷。e.x. If String.IsNullOrEmpty(str)
- 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：
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：
從上圖可看出幾個現象
- 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
- 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：
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。
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。
