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in selection statements,
programs change course. Select Case rapidly matches values. In it, we specify a set of constants (Integers, Chars, Strings). Select Case evaluates an expression and goes to the matching clause.
To create a new Select Case statement, type Select and then press tab. Then, edit the variable name to be appropriate for your program. We read a line from the Console, call Integer.Parse on it, and then Select it from a set of cases.
Else:Case Else is the default case. When no other values match, this case is reached.
Based on: .NET 4.5 Program that uses Select Case: VB.NET 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
Sometimes a nested Select Case statement is useful.
we can test characters in a String,
one after another,
with nested Selects. This is an efficient, and fairly clear, way to test String data.
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
Program that uses nested Select Case: VB.NET 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
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).
Program that uses Select Case on String: VB.NET 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. No Dictionary collection was used by the compiler.
So:It seems that 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
Variable cases are allowed. But we may lose optimizations with this syntax—each case must be evaluated and cannot be stored in a lookup table. The compiled cannot use the switch opcode.
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.
Program that uses variable Cases: VB.NET 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
With Integers, Select Case often is faster than an If-Statement. Consider this benchmark.
It first tests,
in a tight loop,
an If-ElseIf construct. It then tests an equivalent Select Case statement. The value equals 2 here.
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.
Program that times If Then, Select Case: VB.NET 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 than the Select Case.
Results 2 0.89 s: If Then, value = 0 1.14 s: Select Case, value = 0
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. With this opcode, Select Case can be faster 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.
If the frequency of all cases is equally distributed, and the cases are close together in numeric value, then 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.
The Select Case statement in the VB.NET language provides an optimized way of selecting from several constant cases. This special syntax form can be used to test a variable against several constant values.