VB.NET - Enum Examples - Dot Net Perls

Enum. An VB.NET Enum type stores special values. These are named constants. With an Enum, we replace magic constants throughout a program.

Enums make code clearer: they organize code and make programs easier to maintain. We use the Enum keyword, and place enums in Select Case and If-statements.

Enum.Parse

Example. Here we use an Enum named Importance. The Enum represents a constant value. It indicates a value's priority. The names of the constants in an Enum can be any valid identifiers.

Here In Main, we use a Select statement on an enum variable. This enables a fast match for the correct Enum value.

Select Case

Result The case for Importance.Critical is selected. The output of the program is "True."

Module Module1

    ''' <summary>
    ''' Levels of importance.
    ''' </summary>
    Enum Importance
        None = 0
        Trivial = 1
        Regular = 2
        Important = 3
        Critical = 4
    End Enum

    Sub Main()
        Dim value As Importance = Importance.Critical
        ' Select the enum and print a value.
        Select Case value
            Case Importance.Trivial
                Console.WriteLine("Not true")
                Return
            Case Importance.Critical
                Console.WriteLine("True")
                Exit Select
        End Select
    End Sub

End Module
True

If, ElseIf. Often we use the If and ElseIf constructs to implement checks for certain Enum values. With conditionals, we check an Enum Dim variable against named constants in an Enum type.

Here We represent a markup tag, such as HTML, in a conceptual model as an Enum value. We test the tags with the If and ElseIf statements.

Module Module1

    ''' <summary>
    ''' Represents tag type.
    ''' </summary>
    Enum TagType
        None = 0
        BoldTag = 1
        ItalicsTag = 2
        HyperlinkTag = 3
    End Enum

    Sub Main()
        ' Create enum type variable.
        Dim value As TagType = TagType.HyperlinkTag
        ' Use enum in If-statement.
        ' ... Also use ElseIf statement.
        If value = TagType.BoldTag Then
            Console.WriteLine("Bold")
        ElseIf value = TagType.HyperlinkTag Then
            Console.WriteLine("Not true")
        End If
    End Sub

End Module
Not true

ToString. Suppose we want a string representation. We need to invoke ToString explicitly on an Enum variable when we pass it to the Console.Write or WriteLine subs.

Important If we do not call ToString, we will receive the numeric value of the Enum, not the string representation.

Here In this example, we show the ToString method on an Enum variable called AnimalType.

Detail The Enum represents a type of animal, which has options for cats and dogs. We finally call Console.WriteLine.

Console.WriteLine

Module Module1

    ''' <summary>
    ''' Type of animal.
    ''' </summary>
    Enum AnimalType
        None = 0
        Cat = 1
        Dog = 2
    End Enum

    ''' <summary>
    ''' Type of visibility.
    ''' </summary>
    Enum VisibilityType
        None = 0
        Hidden = 2
        Visible = 4
    End Enum

    Sub Main()
        Dim dog As AnimalType = AnimalType.Dog
        Dim hidden As VisibilityType = VisibilityType.Hidden
        ' Write to the Console with the ToString method.
        Console.WriteLine(dog.ToString)
        Console.WriteLine(hidden.ToString)
    End Sub

End ModuleDog
Hidden

Stack. Here we use a Stack to represent a markup tree. If we have an HTML document with tags, we could use a Stack of Enum values (TagType) to represent this tree.

Note You could use this functionality for HTML validation as well. Opening and closing tags could be matched.

Tip To use the Stack collection, we typically use the Push, Pop as well as Peek methods.

Stack

Module Module1

    Enum TagType
        None = 0
        BoldTag = 1
        ItalicsTag = 2
        HyperlinkTag = 3
    End Enum

    Sub Main()
        ' Create a new Stack generic instance.
        Dim stack As New Stack(Of TagType)
        ' Add a BoldTag to it.
        stack.Push(TagType.BoldTag)
        ' Add an ItalicsTag to it.
        stack.Push(TagType.ItalicsTag)
        ' Pop the top element.
        ' ... Then write it to the Console.
        Dim popped As TagType = stack.Pop()
        Console.WriteLine(popped)
        Console.WriteLine(popped.ToString)
    End Sub

End Module2
ItalicsTag

Default. An Enum has a default value. This corresponds to the value that equals 0 within the Enum type, which is by default the first listed one.

Note The default values of an enum proceed up from 0. So the first is zero, the second is one, and the third is two.

Also When part of a class, an Enum does not need initialization to this default value. Initializing it to zero is a performance negative.

Module Module1

    Enum Level
        Normal
        Low
        High
    End Enum

    Class Item
        ''' <summary>
        ''' Public enum.
        ''' </summary>
        Public _level As Level
    End Class

    Sub Main()
        ' The default value for an enum is the one equal to 0.
        Dim v As Item = New Item
        Console.WriteLine(v._level.ToString())
    End Sub

End Module
Normal

Types. Enums have a backing type—a data representation like Integer or Byte. By default, Enums are represented as Integers. But we can, with the "As" keyword change this.

Here We create a Byte Enum with the "as Byte" keywords. Each instance of "Code" will only require one byte.

Byte, Sbyte

Tip This ability is useful when designing a Class that is instantiated many times in memory.

Tip 2 With smaller types, we can reduce memory usage. Four Bytes are packed into the size occupied by one Integer.

Integer

Module Module1

    Enum Code As Byte
        Small = 0
        Medium = 1
        Large = 2
    End Enum

    Sub Main()
        ' The value is represented in a byte.
        Dim value As Code = Code.Medium
        Console.WriteLine(value)
    End Sub

End Module
1

Other values. In VB.NET we can specify an enum variable has a value that is not in the enum. No warning or error is issued. So enums help with, but do not enforce, correct code.

Module Module1

    Enum PageColor
        None = 0
        Red = 1
        Blue = 2
        Green = 3
    End Enum

    Sub Main()

        ' This value does not exist in the enum type.
        Dim value As PageColor = 4

        ' We can still test an enum against any value.
        If value = 4 Then
            Console.WriteLine("Value is 4")
        End If
    End Sub

End Module
Value is 4

Enum argument. We can pass an enum as an argument to a Sub or Function. We specify the enum name as the type—here we use the name "Position."

Sub

Module Module1

    Enum Position
        Above
        Below
    End Enum

    Sub Main()
        ' Pass enum to function.
        TestValue(Position.Below)
    End Sub

    Sub TestValue(ByVal value As Position)
        ' Test enum argument.
        If value = Position.Below Then
            Console.WriteLine("POSITION IS BELOW")
        End If
    End Sub

End Module
POSITION IS BELOW

Enum Flags. We can use the Flags attribute on an Enum to enable some bitwise operations. With a Flags enum, we can use the numbers 0, 1, 2 and further to indicate values.

Enum Flags

Info The VB.NET language supports Or, Xor, and And for bitwise operators. We can set the Flags value in this way.

Next We can test for the existence of a flag with the HasFlag function. One enum value can thus have many values at once.

Tip We specify that an enum is a Flags enum by using an Attribute (named Flags). We surround it with angle brackets.

Attribute

Module Module1

    <Flags> Enum FlagData
        None = 0
        Cached = 1
        Current = 2
        Obsolete = 4
    End Enum

    Sub Main()

        ' The or operator supports bitwise operations.
        ' ... This is the same as 1 | 2.
        Dim attr As FlagData = FlagData.Cached Or FlagData.Current

        ' Check for flag existence.
        If attr.HasFlag(FlagData.Cached) Then
            Console.WriteLine("File is cached")
        End If

        If attr.HasFlag(FlagData.Current) Then
            Console.WriteLine("File is current")
        End If

        If attr.HasFlag(FlagData.Obsolete) Then
            Console.WriteLine("Not reached")
        End If

    End Sub

End ModuleFile is cached
File is current

DayOfWeek, enum. Some enums are built into the .NET Framework—we can reuse them in our programs. The DayOfWeek enum stores all 7 days of the week.

So If we wish to represent a day in a class or collection, we can use the DayOfWeek enum (it works just like any other enum).

Module Module1

    Sub Main()

        ' Add weekend day enums to list.
        Dim list = New List(Of DayOfWeek)()
        list.Add(DayOfWeek.Saturday)
        list.Add(DayOfWeek.Sunday)

        ' Loop over enums in list.
        For Each value As DayOfWeek In list
            Console.WriteLine("WEEKEND ENUM VALUE: {0}", value)
        Next
    End Sub

End ModuleWEEKEND ENUM VALUE: Saturday
WEEKEND ENUM VALUE: Sunday

Summary. Enums are used to represent a set of named constants. Typically, these named, symbolic constants are a selection of options for a specific variable.