When defining a Windows API interface in C#, do I have to define all members? Can I only define the methods I'm going to use?

Question

For example, this is the full definition of the IFileOpenDialog interface, a Windows Shell Interface, taken from the Pinvoke site:

[ComImport, Guid ( "d57c7288-d4ad-4768-be02-9d969532d960" ), InterfaceType ( ComInterfaceType.InterfaceIsIUnknown )]
interface IFileOpenDialog : IFileDialog
{
// Defined on IModalWindow - repeated here due to requirements of COM interop layer
// --------------------------------------------------------------------------------
[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime ), PreserveSig]
int Show ( [In] IntPtr parent );

// Defined on IFileDialog - repeated here due to requirements of COM interop layer
[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFileTypes ( [In] uint cFileTypes, [In] COMDLG_FILTERSPEC[] rgFilterSpec );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFileTypeIndex ( [In] uint iFileType );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetFileTypeIndex ( out uint piFileType );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void Advise ( [In, MarshalAs ( UnmanagedType.Interface )] IFileDialogEvents pfde, out uint pdwCookie );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void Unadvise ( [In] uint dwCookie );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetOptions ( [In] FOS fos );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetOptions ( out FOS pfos );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetDefaultFolder ( [In, MarshalAs ( UnmanagedType.Interface )] IShellItem psi );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFolder ( [In, MarshalAs ( UnmanagedType.Interface )] IShellItem psi );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetFolder ( [MarshalAs ( UnmanagedType.Interface )] out IShellItem ppsi );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetCurrentSelection ( [MarshalAs ( UnmanagedType.Interface )] out IShellItem ppsi );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFileName ( [In, MarshalAs ( UnmanagedType.LPWStr )] string pszName );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetFileName ( [MarshalAs ( UnmanagedType.LPWStr )] out string pszName );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetTitle ( [In, MarshalAs ( UnmanagedType.LPWStr )] string pszTitle );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetOkButtonLabel ( [In, MarshalAs ( UnmanagedType.LPWStr )] string pszText );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFileNameLabel ( [In, MarshalAs ( UnmanagedType.LPWStr )] string pszLabel );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetResult ( [MarshalAs ( UnmanagedType.Interface )] out IShellItem ppsi );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void AddPlace ( [In, MarshalAs ( UnmanagedType.Interface )] IShellItem psi, NativeMethods.FDAP fdap );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetDefaultExtension ( [In, MarshalAs ( UnmanagedType.LPWStr )] string pszDefaultExtension );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void Close ( [MarshalAs ( UnmanagedType.Error )] int hr );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetClientGuid ( [In] ref Guid guid );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void ClearClientData ( );

// Not supported:  IShellItemFilter is not defined, converting to IntPtr
[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetFilter ( [MarshalAs ( UnmanagedType.Interface )] IntPtr pFilter );

// Defined by IFileOpenDialog
// ---------------------------------------------------------------------------------
[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetResults ( [MarshalAs ( UnmanagedType.Interface )] out IShellItemArray ppenum );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void GetSelectedItems ( [MarshalAs ( UnmanagedType.Interface )] out IShellItemArray ppsai );
}

If I'm only going to use two methods from this interface, can I define it like:

[ComImport, Guid ( "d57c7288-d4ad-4768-be02-9d969532d960" ), InterfaceType ( ComInterfaceType.InterfaceIsIUnknown )]
interface IFileOpenDialog : IFileDialog
{
// Defined on IModalWindow - repeated here due to requirements of COM interop layer
// --------------------------------------------------------------------------------
[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime ), PreserveSig]
int Show ( [In] IntPtr parent );

[MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
void SetOptions ( [In] FOS fos );
}

Is it going to work? Or do I have to define the full interface with all methods?

Answer 1

No, this won't work. The CLR builds a dispatch table for the COM interface based on the declaration. The order of the function pointers in that table is set by the order of the method definitions in your declaration. The Show() method will occupy the first slot in both cases, no trouble there. SetOptions() however is going to end up calling the 2nd one, which is actually SetFileTypes(). They have different arguments, that's going to go kaboom in a nasty way when the implementation gets garbage arguments and the stack becomes unbalanced.

You can omit any declarations from the tail end. Also note that the actual declaration of the method doesn't matter when you don't call it. Which allows you to lie and avoid having to declare their argument types. Do make sure it is obvious that the method won't actually work, name it something like void DontCallMe2().

Note that these interfaces are already wrapped in the Windows API Code Pack as well as .NET 4.0's version of Microsoft.Win32.OpenFileDialog and .NET 3.5's version of System.Windows.Forms.OpenFileDialog

Answer 2

If you want to define a type that implements an interface you're obliged to implement all elements of the interface. If you don't then the best case is that you'll get "erratic" behaviour from your code, the worst case is that your application will crash as soon as you try to do anything with your implementation of the interface.

Bear in mind that these two interfaces are not the same:

[ComImport, Guid ( "d57c7288-d4ad-4768-be02-9d969532d960" ), InterfaceType ( ComInterfaceType.InterfaceIsIUnknown )]
interface IFileOpenDialog : IFileDialog
{
    [MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime ), PreserveSig]
    int Show ( [In] IntPtr parent );
}

[ComImport, Guid ( "d57c7288-d4ad-4768-be02-9d969532d960" ), InterfaceType ( ComInterfaceType.InterfaceIsIUnknown )]
interface IFileOpenDialog : IFileDialog
{
    [MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime ), PreserveSig]
    int Show ( [In] IntPtr parent );

    [MethodImpl ( MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime )]
    void SetOptions ( [In] FOS fos );
}

Even though they have the same name, the same Guid and both implement Show, the fact that only one implements SetOptions makes them different.

By all means, throw a NotImplemented exception from any methods of the interface you don't implement, but to say that you *implement interface ISomeInterface, you actually have to do so.