Indexed Properties

Indexed properties are properties that provide array-like access to ordered data. They come in three forms:

  • Item
  • Ordinal
  • Cardinal

An F# member must be named one of these three names to provide array-like access. IndexerName is used to represent any of the three options below:


// Indexed property that has both get and set defined.
member self-identifier.IndexerName
    with get(index-variable) =
    and set index-variablesvalue-variables =

// Indexed property that has get only.
member self-identifier.IndexerName(index-variable) =

// Alternative syntax for indexed property with get only
member self-identifier.IndexerName
    with get(index-variables) =

// Indexed property that has set only.
member self-identifier.IndexerName
    with set index-variablesvalue-variables = 


The forms of the previous syntax show how to define indexed properties that have both a get and a set method, have a get method only, or have a set method only. You can also combine both the syntax shown for get only and the syntax shown for set only, and produce a property that has both get and set. This latter form allows you to put different accessibility modifiers and attributes on the get and set methods.

When the IndexerName is Item, the compiler treats the property as a default indexed property. A default indexed property is a property that you can access by using array-like syntax on the object instance. For example, if obj is an object of the type that defines this property, the syntax obj.[index] is used to access the property.

The syntax for accessing a nondefault indexed property is to provide the name of the property and the index in parentheses. For example, if the property is Ordinal, you write obj.Ordinal(index) to access it.

Regardless of which form you use, you should always use the curried form for the set method on an indexed property. For information about curried functions, see Functions.


The following code example illustrates the definition and use of default and non-default indexed properties that have get and set methods.

type NumberStrings() =
   let mutable ordinals = [| "one"; "two"; "three"; "four"; "five";
                             "six"; "seven"; "eight"; "nine"; "ten" |]
   let mutable cardinals = [| "first"; "second"; "third"; "fourth";
                              "fifth"; "sixth"; "seventh"; "eighth";
                              "ninth"; "tenth" |]
   member this.Item
      with get(index) = ordinals.[index]
      and set index value = ordinals.[index] <- value
   member this.Ordinal
      with get(index) = ordinals.[index]
      and set index value = ordinals.[index] <- value
   member this.Cardinal
      with get(index) = cardinals.[index]
      and set index value = cardinals.[index] <- value

let nstrs = new NumberStrings()
nstrs.[0] <- "ONE"
for i in 0 .. 9 do
  printf "%s " (nstrs.[i])
printfn ""

nstrs.Cardinal(5) <- "6th"

for i in 0 .. 9 do
  printf "%s " (nstrs.Ordinal(i))
  printf "%s " (nstrs.Cardinal(i))
printfn ""


ONE two three four five six seven eight nine ten
ONE first two second three third four fourth five fifth six 6th
seven seventh eight eighth nine ninth ten tenth

Indexed Properties with Multiple Index Variables

Indexed properties can have more than one index variable. In that case, the variables are separated by commas when the property is used. The set method in such a property must have two curried arguments, the first of which is a tuple containing the keys, and the second of which is the value being set.

The following code demonstrates the use of an indexed property with multiple index variables.

open System.Collections.Generic

type SparseMatrix() =
    let mutable table = new Dictionary<(int * int), float>()
    member this.Item
        with get(key1, key2) = table.[(key1, key2)]
        and set (key1, key2) value = table.[(key1, key2)] <- value

let matrix1 = new SparseMatrix()
for i in 1..1000 do
    matrix1.[i, i] <- float i * float i

See Also