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Select Case. In branches,
in selection statements,
programs change course. Select Case rapidly matches values. In it, we specify a set of constants (Integers, Chars, Strings).
Matching. Select Case evaluates an expression and goes to the matching clause. Cases do not fall through, and no "exit" statement is required. But we can stack cases that share statements.
First example. To create a Select Case statement,
and press tab. Then, edit the variable name. We read a line from the Console, call Integer.Parse on it, and then use Select.
Else:Case Else is the default case. When no other values match, this case is reached.
Based on: .NET 4.5 VB.NET program that uses Select Case Module Module1 Sub Main() Dim value As Integer = Integer.Parse(Console.ReadLine()) Select Case value Case 1 Console.WriteLine("You typed one") Case 2 Console.WriteLine("You typed two") Case 5 Console.WriteLine("You typed five") Case Else Console.WriteLine("You typed something else") End Select End Sub End Module Output 2 You typed two
Nested. Sometimes a nested Select Case statement is useful. For example, we can test characters in a String, one after another, with nested Selects.
And:We can use this style of logic to optimize StartsWith or EndsWith calls. This is only needed on performance-critical code.StartsWith
Chars:The example uses chars within a String as the Select Case expression. This is a common construct.Char
VB.NET program that uses nested Select Case Module Module1 Sub Main() Dim value As String = "cat" ' Handle first letter. Select Case value(0) Case "c" ' Handle second letter. Select Case value(1) Case "a" Console.WriteLine("String starts with c, a") Case "o" ' Not reached: Console.WriteLine("String starts with c, o") End Select End Select End Sub End Module Output String starts with c, a
Strings. Select Case may be used on a String. With this statement we match a variable against a set of values such as String literals.
But:On Strings, Select Case offers no performance advantage as with Integers (or other values).Strings
To start:Let's evaluate a program that reads an input from the Console. Then it uses the Select Case statement on that value.
Values:It matches against four possible values: "dot", "net", and "perls", and also all other values (Else).
VB.NET program that uses Select Case on String Module Module1 Sub Main() While True Dim value As String = Console.ReadLine() Select Case value Case "dot" Console.WriteLine("Word 1") Case "net" Console.WriteLine("Word 2") Case "perls" Console.WriteLine("Word 3") Case Else Console.WriteLine("Something else") End Select End While End Sub End Module Output dot Word 1 perls Word 3 test Something else
Internals. On Strings, does Select Case ever compile into anything other than a series of comparisons? I changed the above program to have nine String literals.
Result:No Dictionary was used by the compiler. VB.NET lacks the String Dictionary optimization for C# String Switch constructs.
Therefore:If you have to match a lot of string literals, building a Dictionary might be faster.Dictionary
Variables. VB.NET allows variable cases. But we may lose optimizations with this syntax. Each case must be evaluated and cannot be stored in a lookup table.
Note:The "value" Integer is set to 10. And we match it against the variable y, which also equals 10, in a Select Case statement.Integer
Note 2:An advanced compiler could analyze this program before execution so that no branches are evaluated at runtime.
VB.NET program that uses variable Cases Module Module1 Sub Main() Dim value As Integer = 10 Dim x As Integer = 5 Dim y As Integer = 10 ' Select with cases that are variables. Select Case value Case x ' Not reached. Console.WriteLine("Value equals x") Case y Console.WriteLine("Value equals y") End Select End Sub End Module Output Value equals y
Stacked cases. Multiple cases can be combined by specifying them one after another. In this example, both 99 and 100 will reach the same Console.WriteLine statement.
VB.NET program that uses stacked cases Module Module1 Sub Main() Dim value As Integer = Integer.Parse("100") Select Case value Case 99 Case 100 ' Both 99 and 100 will end up here. Console.WriteLine("99 or 100") Case 101 Console.WriteLine("Not reached") End Select End Sub End Module Output 99 or 100
Performance. With Integers, Select Case often is faster than an If-Statement. Consider this benchmark. It tests an If-ElseIf construct and an equivalent Select Case. The value equals 2.If Then
Result:The Select Case statement is faster. The results are the same for the constructs.
Thus:In many programs, Select Case is an optimization. But if we have one case that occurs most often, If may be faster—see the next test.
VB.NET program that times If Then, Select Case Module Module1 Sub Main() Dim m As Integer = 300000000 Dim value As Integer = 2 ' Version 1: Use If-Statement. Dim total As Integer = 0 Dim s1 As Stopwatch = Stopwatch.StartNew For i As Integer = 0 To m - 1 If value = 0 Then total -= 1 ElseIf value = 1 Then total -= 100 ElseIf value = 2 Then total += 1 End If Next s1.Stop() ' Version 2: Use Select Case. total = 0 Dim s2 As Stopwatch = Stopwatch.StartNew For i As Integer = 0 To m - 1 Select Case value Case 0 total -= 1 Case 1 total -= 100 Case 2 total += 1 End Select Next s2.Stop() Console.WriteLine((s1.Elapsed.TotalMilliseconds / 1000).ToString("0.00 s")) Console.WriteLine((s2.Elapsed.TotalMilliseconds / 1000).ToString("0.00 s")) End Sub End Module Results 1.47 s: If Then, value = 2 0.86 s: Select Case, value = 2
An experiment. I changed the benchmark to set the "value" Integer to 0, not 2. Now the first If-expression evaluates to true every time. Fewer branches are needed, and the If is faster.
Results 2 0.89 s: If Then, value = 0 1.14 s: Select Case, value = 0
A discussion. On values, Select Case is implemented with the switch opcode in the intermediate language. This is the same implementation as the switch keyword from the C# language.
Note:With this opcode, Select Case is faster (on certain programs and data) than an If-statement.
However:Performance of Select Case is highly dependent on both the cases, and the data, in your program.
And:If we have 1,000 cases, but one is most common, testing for the common one with an If-statement is fastest.
Frequencies. If the frequency of cases is equally distributed, and the values are close together, a Select Case is better. The performance difference in most situations is small.
Tip:It is often the best approach to just use whatever syntax form is clearest.
A summary. The Select Case statement optimizes selection from several constant cases. This special syntax form can be used to test a variable against several constant values.