If Then, ElseIf. With logic (an If-statement) we direct control flow. The condition is evaluated. On a true result, control moves to the statements inside the block.
Expressions. In an If-statement, multiple expressions are short-circuited. We use And, Or, AndAlso and OrElse to evaluate conditions. With an End we close our block.
First example. An If-statement uses the If-keyword and the Then-keyword. The ElseIf-statement has the "If" part capitalized in the middle.
And The Else-statement has no "Then" part. We end a block with an "End If" statement.
Detail We do not use 2 equals signs together in an If-statement—we just use one.
Tip Visual Studio will automatically insert the "Then" and "End If" parts of an If-statement.
Module Module1
Function Test(ByVal s As String) As Integer
' Check input with If, ElseIf and Else.
If s = "cat" Then
Return 1
ElseIf s = "dog" Then
Return 2
Else
Return 0
End If
End Function
Sub Main()
' Call Test function.
Console.WriteLine(Test("dog"))
Console.WriteLine(Test("cat"))
Console.WriteLine(Test("elephant"))
End Sub
End Module2
1
0
If Not. An If-statement can also test a negative. Instead we must apply the Not-keyword. The If-statement then reads "If not this, then."
Detail We also can apply the Not-keyword to the ElseIf clause. We cannot use the "!=" operator.
Module Module1
Sub Main()
' An integer variable.
Dim i As Integer = 100
' Test integer against 100 and 200.
If Not i = 100 Then
Console.WriteLine("i not 100")
ElseIf Not i = 200 Then
Console.WriteLine("i not 2")
End If
End Sub
End Modulei not 2
And, Or. With the "And" and "Or" keywords, we test complex expressions. These expressions are chained. Both And and Or will lead to the evaluation of all parts (no early termination).
And Please consider the OrElse and AndAlso operators for early termination—this can lead to a performance improvement.
Module Module1
Sub Main()
Dim left As Integer = 10
Dim right As Integer = 100
' Use "and" in expression.
If left = 10 And right = 100 Then
Console.WriteLine(1)
End If
' Use "or" in expression.
If left = 5 Or right = 100 Then
Console.WriteLine(2)
End If
End Sub
End Module1
2
OrElse. This is important—the OrElse operator supports short-circuiting with an Or operator. This program demonstrates the OrElse operator.
Note If we use Or instead of OrElse, both IsBird() and IsCat() will be executed in this VB.NET program.
Important If we want to finish the expression as soon as a true result is found, we must use OrElse.
Module Module1
Function IsBird() As Boolean
Console.WriteLine("IsBird returns true")
Return True
End Function
Function IsCat() As Boolean
Console.WriteLine("IsCat not reached")
Return False
End Function
Sub Main()
If IsBird() OrElse IsCat() Then
Console.WriteLine("DONE")
End If
End Sub
End ModuleIsBird returns true
DONE
AndAlso. This operator is the version of And that supports short-circuiting (early termination). If a false value is encountered, things that come after AndAlso will not be tested.
Module Module1
Function IsRed() As Boolean
Console.WriteLine("IsRed: false")
Return False
End Function
Function IsBlue() As Boolean
Console.WriteLine("IsBlue: true")
Return True
End Function
Sub Main()
If IsRed() AndAlso IsBlue() Then
Console.WriteLine("DONE")
End If
End Sub
End ModuleIsRed: false
Boolean Function. An If-statement does not always directly test values. Sometimes it calls another Function and tests the value returned by that Function.
So If the Integer is within the specified range (10 to 100 inclusive) the inner block of the If-statement is reached.
Module Module1
''' <summary>
''' See if size is valid.
''' </summary>
Function IsValid(ByVal size As Integer) As Boolean
' Returns true if size is within this range.
Return size >= 10 And size <= 100
End Function
Sub Main()
' Size variable.
Dim size As Integer = 50
' Call IsValid function in an If-expression.
If IsValid(size) Then
Console.WriteLine("Valid size")
End If
End Sub
End ModuleValid size
Locals. If-statements can become complex and hard to understand. It is sometimes possible to simplify them by adding local variables.
Here We introduce a Boolean that stores whether the animal and size variables indicate a "big cat."
Then We can test the isBigCat variable in the two If-statements. This reduces code repetition and makes the program easier to read.
Also In some programs, we can use this approach to reduce expensive method calls by storing their results for later reuse.
Module Module1
Sub Main()
Dim animal As String = "cat"
Dim size As Integer = 10
Dim color As String = "grey"' Store expression result in local variable.
Dim isBigCat As Boolean = (animal = "cat" And size >= 8)
' Test local variable.
If isBigCat And color = "grey" Then
Console.WriteLine(True)
End If
If isBigCat And color = "white" Then
Console.WriteLine(False)
End If
End Sub
End ModuleTrue
Not equal operator. In VB.NET we can use the "not equals" operator instead of the "Not" keyword. This works in an if-statement or loops like the while-loop.
Tip This operator is available on Integers, Strings, Doubles and other types. It can help keep VB.NET code shorter.
Module Module1
Sub Main()
Dim temp As Integer = 10
' Use "not equal" operator.
If temp <> 0 Then
Console.WriteLine("Temp not zero")
End If
' Inner statement not reached here.
If temp <> 10 Then
Console.WriteLine("???")
End If
End Sub
End ModuleTemp not zero
Benchmark, reorder Ifs. Suppose we test values in an If-statement. If the most common encountered value is 3, we should put that test first. This reduces the number of checks required.
Version 1 This code tests the most commonly-encountered value last. So multiple checks must be done in the common case.
Version 2 This version checks the most common value first. It provides a "fast path" for the most frequent situation.
Result It is faster to check for the most common value first. Other optimizations, like Select Case, can be even better in performance.
Module Module1
Sub Main()
Dim m As Integer = 10000000
Dim s1 As Stopwatch = Stopwatch.StartNew
' Version 1: test most common value last.
For i As Integer = 0 To m - 1
Version1(100, 3)
Next
s1.Stop()
Dim s2 As Stopwatch = Stopwatch.StartNew
' Version 2: test most common value first.
For i As Integer = 0 To m - 1
Version2(100, 3)
Next
s2.Stop()
Dim u As Integer = 1000000
Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
End Sub
Function Version1(ByVal max As Integer, ByVal test As Integer) As Integer
' Test 3 last.
Dim result As Integer = 0
For i As Integer = 0 To max - 1
If test = 1 Then
result += 1
ElseIf test = 2 Then
result -= 1
ElseIf test = 3 Then
result += 2
End If
Next
Return result
End Function
Function Version2(ByVal max As Integer, ByVal test As Integer) As Integer
' Test 3 first.
Dim result As Integer = 0
For i As Integer = 0 To max - 1
If test = 3 Then
result += 2
ElseIf test = 1 Then
result += 1
ElseIf test = 2 Then
result -= 1
End If
Next
Return result
End Function
End Module114.63 ns Test 1, 2, 3 (3 last)
89.32 ns Test 3, 1, 2 (3 first)
Summary. If-statements are used throughout programs. They use a slightly different syntax form from many languages. We use the Not-keyword to test negatives for truth.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jun 20, 2024 (edit link).
