How to create a COM object in F#

Question

I need IMessage Interface. In C# I could write

CDO.Message oMsg = new CDO.Message(); // ok

but when I try to do this in F#

let oMsg = new CDO.Message()

I get an error message:

'new' cannot be used on interface types. Consider using an object expression '{ new ... with ... }' instead.

How to solve this problem?

Answer 1

You need to use the version that has the "Class" added to the interface name:

open System

[<EntryPoint>]
[<STAThread>]
let main argv = 
    let oMsg =  new CDO.MessageClass()  
    oMsg.Subject <- "Hello World" 
    Console.WriteLine(oMsg.Subject)
    Console.ReadLine() |> ignore
    0 

Note: I also explicitly had to add a reference to Microsoft ActiveX Data Objects 6.0 Library to the project to make the object instantiation work.

In case you want to use the specific interface and not the Co-Class (as proposed by Hans Passant), use this version that casts the created Co-Class to the interface, thereby returning a pointer to the actual interface.

open System

[<EntryPoint>]
[<STAThread>]
let main argv = 
    let oMsg =  upcast new CDO.MessageClass()  : CDO.IMessage
    oMsg.Subject <- "Hello World" 
    Console.WriteLine(oMsg.Subject)
    Console.ReadLine() |> ignore
    0