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Control statements give you additional means to control the processing within
the applications you develop. This section explores the syntax and function of
the if, select case, do-while, foreach, goto, exit, continue, and return
statements.
If-then-else
The if statement has three forms: single selection, if-then-else selection, and
multicase selection. Listing 5.23 contains an example of each form.
Listing 5.23: If-Else-ElseIf Example 1
'single selection
If i > 0 Then
Console.WriteLine("The
number {0} is positive", i)
End If
'if-then-else selection
If i > 0
Then
Console.WriteLine("The
number {0} is positive", i)
Else
Console.WriteLine("The
number {0} is not positive", i)
End If
'multicase selection
If i = 0
Then
Console.WriteLine("The
number is zero")
ElseIf i > 0
Then
Console.WriteLine("The
number {0} is positive", i)
Else
Console.WriteLine("The
number {0} is negative", i)
End
If
The variable i is the object of evaluation here. The expression in an if
statement must resolve to a boolean value type.
' Compiler Error
If 1
Then
Console.WriteLine("The if statement executed")
End If
Console.ReadLine()
When the VB.NET compiler compiles the preceding code, it generates the error
"Constant value 1 cannot be converted to bool."
Listing 5.24 shows how conditional or (OrElse) and conditional and (AndAlso) operators
are used in the same manner.
Listing 5.24: If-Then-Else Example 2
'Leap year
Dim year As
Integer = 1974
If (year Mod
4 = 0 AndAlso year
Mod 100 <> 0) OrElse year
Mod 400 = 0 Then
Console.WriteLine("The
year {0} is leap year ", year)
Else
Console.WriteLine("The
year {0} is not leap year ", year)
End
If
Switch
From the example in Listing 5.25, you can see that the select case is
similar to an if-else ifelse if-else form of an if statement.
Listing 5.25: Switch Example 1
Dim day
As String =
"Monday"
Console.WriteLine("enter
the day :")
day = Console.ReadLine()
Select Case
day
Case
"Mon"
Exit Select
Case
"Monday"
Console.WriteLine("day
is Monday: go to work")
Exit Select
Case Else
Console.WriteLine("default")
Exit Select
End
Select
Select
Case strVal1
Case
"reason1"
goto case "reason2"
' this is a jump to mimic
fall-through
Case
"reason2"
intOption = 2
Exit Select
Case
"reason 3"
intOption = 3
Exit Select
Case
"reason 4"
intOption = 4
Exit Select
Case
"reason 5"
intOption = 5
Exit Select
Case Else
intOption = 9
Exit Select
End
Select
Conclusion
Hope this article would have helped you in understanding control statements in
VB.NET.