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Object expressions generate instances of classes and interfaces. There are both simple object expressions (which do not generate a new private class under the hood) and more sophisticated object expressions that specify overrides and implement interfaces. Simple object expressions use the syntax "new":

  let myForm = new System.Windows.Forms.Form()
  myForm.Visible <- true
  myForm.Activate()

Object expressions that implement an interface use the syntax "{ new InterfaceType with ... and ... }":

  let myComparer = 
    { new System.Collections.Generic.IComparer<string>
        with Compare(a,b) = compare a.Length b.Length }

Object expressions that extend a class use the syntax "{ new ClassType(arg) with ... and ... }".

   { new System.Windows.Forms.Form() 
       with OnPaint(args) = Printf.printf "OnPaint\n"
       and  OnResize(args) = Printf.printf "OnResize\n" }

Object expressions may capture variables.

  let myObjectGenerator(n) =
    {new System.Windows.Forms.Form() 
      with OnPaint(args) = Printf.printf "OnPaint: n = %d\n" n
      and  OnResize(args) = Printf.printf "OnResize: n = %d\n" n }

Object expressions may reference members of the class being extended using "base":

    {new System.Windows.Forms.Form() as base
      with OnPaint(args) = base.OnPaint(args);  Printf.printf "OnPaint\n"
      and  OnResize(args) = base.OnResize(args); Printf.printf "OnResize\n" }

Object expressions may give additional interface implementations. The type of such an object expression is the type of the class.

  { new System.Collections.Generic.IEnumerator<string>
      with ...
      and  ...
    interface System.Collections.IEnumerator
      with ...
      and  ...
    interface System.IDisposable
      with ... }