VB.NET If Then

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The If-statement affects control flow. Its condition is evaluated by the execution engine. If the result is true, the inner statements are next processed. In VB.NET, we use If-Then, along with ElseIf, Else and End.

Based on:

.NET 4.5

Example

In VB.NET, the 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 with an "End If" statement.

Equals:In VB.NET, we do not use two 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.

Program that uses If, ElseIf and Else: VB.NET

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. In VB.NET we must apply the Not-keyword. The If-statement then reads "If not this, then." We also can apply the Not-keyword to the ElseIf clause.

Note:We cannot use the "!=" operator in VB.NET. We must apply the Not-operator and then an equals expression.

Program that uses If Not: VB.NET

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

Boolean Function

An If-statement does not always directly test values. Sometimes, it can call another Function and test the value returned by that Function. In this example, the If-statement calls the IsValid Function.

Then:The IsValid Function is evaluated. It returns True if the Integer argument is within a certain range.

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

Program that uses Function, If-statement: VB.NET

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

Select Case

The If-statement is the most common selection statement in VB programs. But it is not our only choice. We can instead use the Select-Case statement. This compares a value against a set of constants.

Also:The VB.NET language provides support for Select-Case statements on String variables.

Performance

There are many ways to optimize an If-statement in a VB.NET program. These statements are evaluated in sequential order. This means the most common cases should be tested first. This reduces the total number of tests in program execution.

In some programs, using a Select-Case statement to replace an If-statement will improve performance. The Select-Case is implemented with a switch opcode. The If-statement is implemented with branch opcodes.

Caution:A Select-Case is not always faster than an If-statement. Performance is highly dependent on the input being processed.

Summary

If-statements are used throughout VB.NET programs. They use a slightly different syntax form from many languages. We use the Not-keyword to test negatives for truth. And we end the statement block with an "End If" line.