Enums in Visual Basic .NET

Our enum type ServerState has an underlying int type.

The possible values for ServerState are defined as constants. The special feature Enum generates successive constant values automatically; in this case 0, 1, 2 and so on.

By implementing the ToString method, values of ServerState can be printed out or converted to strings.

If we have a value of type Integer, we cannot pass it to Transition - the compiler will complain about type mismatch. This provides some degree of compile-time type safety for enums.

Transition emulates a state transition for a server; it takes the existing state and returns a new state.

Module Enums

    Enum ServerState
        StateIdle = 0
        StateConnected
        StateError
        StateRetrying
    End Enum

    Dim stateName As New Dictionary(Of ServerState, String) From {
        {ServerState.StateIdle, "idle"},
        {ServerState.StateConnected, "connected"},
        {ServerState.StateError, "error"},
        {ServerState.StateRetrying, "retrying"}
    }

    Function ServerStateToString(ByVal ss As ServerState) As String
        Return stateName(ss)
    End Function

    Sub Main()
        Dim ns As ServerState = Transition(ServerState.StateIdle)
        Console.WriteLine(ServerStateToString(ns))

        Dim ns2 As ServerState = Transition(ns)
        Console.WriteLine(ServerStateToString(ns2))
    End Sub

    Function Transition(ByVal s As ServerState) As ServerState
        Select Case s
            Case ServerState.StateIdle
                Return ServerState.StateConnected
            Case ServerState.StateConnected, ServerState.StateRetrying
                ' Suppose we check some predicates here to determine the next state...
                Return ServerState.StateIdle
            Case ServerState.StateError
                Return ServerState.StateError
            Case Else
                Throw New ArgumentException("unknown state: " & s.ToString())
        End Select
    End Function

End Module

To run the program, put the code in Enums.vb and use vbc to compile it.

$ vbc Enums.vb
$ Enums.exe
connected
idle

This example illustrates the use of enums by creating a simple state transition mechanism for a server. By defining an enum type and using a Dictionary to map enum values to strings, we can easily print out meaningful representations of the states.

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Enums in Visual Basic .NET