An object expression is an expression that creates a new instance of a dynamically created, anonymous object type that is based on an existing base type, interface, or set of interfaces.
The following is the basic syntax of an object expression.
// When typename is a class:
{ new typename [type-params]arguments with
member-definitions
[ additional-interface-definitions ]
}
// When typename is not a class:
{ new typename [generic-type-args] with
member-definitions
[ additional-interface-definitions ]
}
typename: Represents an existing class type or interface type.type-params: Describes the optional generic type parameters.arguments: Used only for class types, which require constructor parameters.member-definitions: Overrides of base class methods or implementations of abstract methods from either a base class or an interface.Let's consider the following several different types of object expressions.
The following object expression specifies a System.Object but overrides the ToString method.
let obj1 =
{
new System.Object() with member x.ToString() = "This is F# Tutorial."
}
Console.WriteLine(obj1)
The following object expression implements the IDisplay interface.
type IDisplay =
abstract member Print : unit -> unit
let Point3D(x: float, y: float, z: float) =
{ new IDisplay with
member this.Print() = printfn "X: %f, Y: %f, Z: %f" x y z }
let point1 = Point3D(1.1, 2.2, 3.3)
point1.Print()
The following object expression implements both interfaces.
type IFirst =
abstract F : unit -> unit
abstract G : unit -> unit
type ISecond =
inherit IFirst
abstract H : unit -> unit
abstract J : unit -> unit
let implementer() =
{ new ISecond with
member this.H() = ()
member this.J() = ()
interface IFirst with
member this.F() = ()
member this.G() = () }