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?

http://stackoverflow.com/questions/37145234/how-to-convert-c-sharp-code-that-uses-shell-com-to-f/37150493#37150493 — Hans Passant, May 14 '16 at 12:25
Of interest: [COM Type Provider for F#](https://fsprojects.github.io/FSharp.ComProvider/) — Guy Coder, May 14 '16 at 13:07

NineBerry · Accepted Answer · 2016-05-14T14:36:24.450

11

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

edited May 14 '16 at 14:36

answered May 14 '16 at 12:47

NineBerry

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Works like a charm. Thanks a lot. – Oleg Leonov May 14 '16 at 13:17
Do glance at the linked duplicate to see why type inference is not a great idea. – Hans Passant May 14 '16 at 13:47
Added variant that uses the interface and not the co-class to access further members after creation – NineBerry May 14 '16 at 14:13

How to create a COM object in F#

1 Answers1