Enumerations

An enumeration is a

  • Programmer-defined primitive data type
  • Collection of named values
  • Value type
  • Better substitute for "Magic Numbers"

Magic Numbers - The Land Before enum

int status;
// ...
if (status == 5) // What does 5 mean?
  • We wanted to distinguish different states (such as the status of something)
  • The number of different states is typically more than 3
    • Otherwise we could have simply used a bool
  • Simple integer values are easy to distinguish between each other
  • The "meaning" of the integer values was known only to the programmer
    • "What was the intent of the value?" - Hard to know

Magic Numbers - The Land Before enum

First, we tried to use Constants to distinguish our "Magic Numbers"

const int NOT_STARTED = 0;
const int STARTED = 1;
// ...
const int FINISHED = 5;

But it got complicated as we needed more of them...

const int SHIPMENT_STARTED = 1;
// ...
const int RETURN_MINOR_DAMAGE = 1;

Magic Numbers - The Land Before enum

"Magic Numbers" are an imperfect solution

  • How do you share constants among classes/objects?
  • How do you stop the "explosion" of simple constants?
  • How do you "group" constants to distinguish them (aside from just using a prefix)?
  • How do you limit/control the range of acceptable values?

enum - A Better Approach

public enum ReturnState { NoDamage, MinorDamage, SignificantDamage }

public enum RepairStatus { NotStarted, Started, /*...*/ Finished}

Enumerations allow us to

  • Use meaningful names
  • Clearly distinguish values
    • ReturnState.NoDamage is different from ReturnState.MinorDamage
  • Group the values together
    • You can't "cross-contaminate" the enum values
      ReturnState.Started would be a compiler error

The Parts of an Enumeration

public enum ReturnState { NoDamage, MinorDamage, SignificantDamage }
  • ReturnState is the data type
  • NoDamage, MinorDamage and SignificantDamage are distinct values
ReturnState shipmentItem = ReturnState.NoDamage;
  • shipmentItem is a variable of type ReturnState with an initial value of NoDamage

Controlling the underlying values

By default, enum values

  • are int values under-the-hood
  • start at zero
  • increment by 1 automatically
public enum ReturnState
{
    NoDamage,          // = 0
    MinorDamage,       // = 1
    SignificantDamage  // = 2
}

Controlling the underlying values

We can set the underlying values and even change the underlying data type to any whole-number data type (int, short, long).

public enum DayOfWeek : short
{
    Unknown = -1,
    Sun,
    Mon,
    Tue,
    Wed,
    Thu,
    Fri,
    Sat
}

Enums As Flags

[Flags]
public enum WeaponStatus
{                 // Binary Representation
    Offline = 0,  // 00000000
    Online = 1,   // 00000001
    Armed = 2,    // 00000010
    Firing = 4,   // 00000100
    Damaged = 8   // 00001000
}

WeaponStatus torpedo = WeaponStatus.Armed | WeaponStatus.Online; // bitwise OR
// ...
if(torpedo.HasFlag(WeaponStatus.Online))
// ...

Enums in switch Statements

Enumerations are handy in simple switch statements.

switch(status)
{
    case ReturnState.NoDamage;
        // ...
        break;
    case ReturnState.MinorDamage;
        // ...
        break;
    case ReturnState.SignificantDamage;
        // ...
        break;
}

When to Use Enumerations

  • Distinguish between simple states (all you need is a custom primitive type)
  • Have three or more distinct states
  • Have three or more overlapping states (use with [Flags])
  • There is no intrinsic behaviour to the state

The End

void Main() { WeaponStatus torpedo = WeaponStatus.Firing | WeaponStatus.Online; if(torpedo == WeaponStatus.FireInTheHole) Console.WriteLine(torpedo); if(torpedo.HasFlag(WeaponStatus.Online)) Console.WriteLine("Phasers Ready"); torpedo = WeaponStatus.Armed | WeaponStatus.Damaged | WeaponStatus.Firing; if(torpedo == WeaponStatus.FireInTheHole) Console.WriteLine("EVACUATE"); } // Define other methods, classes and namespaces here [Flags] public enum WeaponStatus { // Binary Representation Offline = 0, // 00000000 Online = 1, // 00000001 Armed = 2, // 00000011 Firing = 4, // 00000101 Damaged = 8, // 00001000 FireInTheHole = 14 // 00001110 }