external-functions.md

title	External Functions (F#)
description	Learn about the F# language support for calling functions in native code.
keywords	visual f#, f#, functional programming
author	cartermp
ms.author	phcart
ms.date	05/16/2016
ms.topic	language-reference
ms.prod	.net
ms.technology	devlang-fsharp
ms.devlang	fsharp
ms.assetid	c26b6124-ceaa-471c-9dc9-861b4dfa332a

External Functions

This topic describes F# language support for calling functions in native code.

Syntax

[<DllImport( arguments )>]
extern declaration

Remarks

In the previous syntax, arguments represents arguments that are supplied to the System.Runtime.InteropServices.DllImportAttribute attribute. The first argument is a string that represents the name of the DLL that contains this function, without the .dll extension. Additional arguments can be supplied for any of the public properties of the System.Runtime.InteropServices.DllImportAttribute class, such as the calling convention.

Assume you have a native C++ DLL that contains the following exported function.

#include <stdio.h>
extern "C" void __declspec(dllexport) HelloWorld()
{
    printf("Hello world, invoked by F#!\n");
}

You can call this function from F# by using the following code.

open System.Runtime.InteropServices

module InteropWithNative =
    [<DllImport(@"C:\bin\nativedll", CallingConvention = CallingConvention.Cdecl)>]
    extern void HelloWorld()

InteropWithNative.HelloWorld()

Interoperability with native code is referred to as platform invoke and is a feature of the CLR. For more information, see Interoperating with Unmanaged Code. The information in that section is applicable to F#.

See Also

Functions