Enumerations, also known as enums, are integral types where labels are assigned to a subset of the values. You can use them in place of literals to make code more readable and maintainable.


type enum-name =
| value1 = integer-literal1
| value2 = integer-literal2


An enumeration looks much like a discriminated union that has simple values, except that the values can be specified. The values are typically integers that start at 0 or 1, or integers that represent bit positions. If an enumeration is intended to represent bit positions, you should also use the Flags attribute.

The underlying type of the enumeration is determined from the literal that is used, so that, for example, you can use literals with a suffix, such as 1u, 2u, and so on, for an unsigned integer (uint32) type.

When you refer to the named values, you must use the name of the enumeration type itself as a qualifier, that is, enum-name.value1, not just value1. This behavior differs from that of discriminated unions. This is because enumerations always have the RequireQualifiedAccess attribute.

The following code shows the declaration and use of an enumeration.

// Declaration of an enumeration.
type Color =
    | Red = 0
    | Green = 1
    | Blue = 2
// Use of an enumeration.
let col1: Color = Color.Red

You can easily convert enumerations to the underlying type by using the appropriate operator, as shown in the following code.

// Conversion to an integral type.
let n = int col1

Enumerated types can have one of the following underlying types: sbyte, byte, int16, uint16, int32, uint32, int64, uint64, and char. Enumeration types are represented in the .NET Framework as types that are inherited from System.Enum, which in turn is inherited from System.ValueType. Thus, they are value types that are located on the stack or inline in the containing object, and any value of the underlying type is a valid value of the enumeration. This is significant when pattern matching on enumeration values, because you have to provide a pattern that catches the unnamed values.

The enum function in the F# library can be used to generate an enumeration value, even a value other than one of the predefined, named values. You use the enum function as follows.

let col2 = enum<Color> (3)

The default enum function works with type int32. Therefore, it cannot be used with enumeration types that have other underlying types. Instead, use the following.

type uColor =
    | Red = 0u
    | Green = 1u
    | Blue = 2u

let col3 = Microsoft.FSharp.Core.LanguagePrimitives.EnumOfValue<uint32, uColor>(2u)

Additionally, cases for enums are always emitted as public. This is so that they align with C# and the rest of the .NET platform.

See also