Another problem with attributes, though in this case I think it has more to do with the way F# does interop rather than with the way it compiles attributes.

This fails:

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[DllImport("dwmapi.dll", PreserveSig=false)>]

extern bool DwmIsCompositionEnabled()

And this the exception I get (the same thing happens in both interactive and compiled code):

System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

at FSI_0089.DwmIsCompositionEnabled()

at <StartupCode$FSI_0089>.$FSI_0089.main@() in C:\Users\rei\Documents\Visual Studio 2008\Projects\UIUtil\GlassHelper.fs:line 29

stopped due to error

This also fails for the same reason (PreserveSigAttribute defaults false, according to the documentation):

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[DllImport("dwmapi.dll")>]

[PreserveSig>]

extern bool DwmIsCompositionEnabled()

Examining the resulting IL, it turns out that in both cases, preservesig is being ignored:

.method public static pinvokeimpl("dwmapi.dll" winapi) bool DwmIsCompositionEnabled() cil managed preservesig

{

}

My current workaround is to simply call the method with the native signature and hide it in a module because extern doesn't support private (and also because name clashes are likely):

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#nowarn "51"

module Interop =

    [DllImport("dwmapi.dll")>]

    extern int DwmIsCompositionEnabled(bool* enabled)

let IsCompositionEnabled() =

    let mutable enabled = false

    if Interop.DwmIsCompositionEnabled(&&enabled) <> 0 then failwith "IsCompositionEnabled failed"

    enabled

By the way, I have to turn off warning 51, which nags at me for using the && prefix. Is there any way to get around this?