VB.NET If Then, ElseIf, Else Examples

Encode conditionals with the If Then statement. Use the If, ElseIf, Else and End keywords.

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.

Equals: 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.

VB.NET program that uses If, ElseIf and Else Module Module1 Sub Main() ' Get input. Dim s As String = Console.ReadLine() ' Check input with If, ElseIf and Else. If s = "cat" Then Console.WriteLine("You like cats") ElseIf s = "dog" Then Console.WriteLine("You like dogs") Else Console.WriteLine("No choice made") End If End Sub End Module Output dog You like dogs

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."

Not: We also can apply the Not-keyword to the ElseIf clause. We cannot use the "!=" operator.

VB.NET program that uses If Not 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 Module Output i 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.

VB.NET program that uses And, Or 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 Module Output 1 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.

VB.NET program that uses 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 Module Output IsBird 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.

Tip: For "||" in VB.NET use the OrElse operator. And for "&&" in VB.NET use the AndAlso operator.

VB.NET program that uses AndAlso 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 Module Output IsRed: 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.Function

Here: The If-statement calls the IsValid Function. IsValid returns True if the Integer argument is within a certain range.

Boolean

So: If the Integer is within the specified range (10 to 100 inclusive) the inner block of the If-statement is reached.

VB.NET program that uses Function, If-statement 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 Module Output Valid 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.

VB.NET program that tests local variable 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 Module Output True

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.

VB.NET program that uses not equal operator 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 Module Output Temp 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.

VB.NET program that reorders If-statements 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 Module Output 114.63 ns Test 1, 2, 3 (3 last) 89.32 ns Test 3, 1, 2 (3 first)

Select Case. The If-statement is the most common selection statement. But we can instead use a Select-Case statement. This compares a value against a set of constants.Select Case

Select Case, strings. For strings we also consider the Select-Case statement. This can lead to clearer syntax. But I found that no advanced Dictionary-based optimizations were applied.

So: Select Case on strings does not improve performance much over the If-statement in the VB.NET language.

Lookup tables. A Dictionary collection uses hash lookups to locate values. For large data sets, this is much faster. Arrays (for numeric results) can also be used.Dictionary Array

A 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.