What's the best way to specify structure layout difference on architectures?

[huoyaoyuan]

When trying to modernize P/Invoke in System.Drawing.Common, I saw this:

runtime/src/libraries/System.Drawing.Common/src/Interop/Windows/Interop.Comdlg32.cs

Lines 17 to 63 in 11c9394

	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
	internal sealed class PRINTDLG
	{
	internal int lStructSize;
	internal IntPtr hwndOwner;
	internal IntPtr hDevMode;
	internal IntPtr hDevNames;
	internal IntPtr hDC;
	internal int Flags;
	internal short nFromPage;
	internal short nToPage;
	internal short nMinPage;
	internal short nMaxPage;
	internal short nCopies;
	internal IntPtr hInstance;
	internal IntPtr lCustData;
	internal IntPtr lpfnPrintHook;
	internal IntPtr lpfnSetupHook;
	internal string? lpPrintTemplateName;
	internal string? lpSetupTemplateName;
	internal IntPtr hPrintTemplate;
	internal IntPtr hSetupTemplate;
	}
	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto, Pack = 1)]
	internal sealed class PRINTDLGX86
	{
	internal int lStructSize;
	internal IntPtr hwndOwner;
	internal IntPtr hDevMode;
	internal IntPtr hDevNames;
	internal IntPtr hDC;
	internal int Flags;
	internal short nFromPage;
	internal short nToPage;
	internal short nMinPage;
	internal short nMaxPage;
	internal short nCopies;
	internal IntPtr hInstance;
	internal IntPtr lCustData;
	internal IntPtr lpfnPrintHook;
	internal IntPtr lpfnSetupHook;
	internal string? lpPrintTemplateName;
	internal string? lpSetupTemplateName;
	internal IntPtr hPrintTemplate;
	internal IntPtr hSetupTemplate;
	}

Win32 and Win64 uses different packing size for the struct (confirmed in commdlg.h). This is not easy to define in managed code. Defining two types like this looks very error-prone. Does it worth compiling separately for different archs?

[jkotas]

You can define two types, but hide the difference behind interface. #33899 has real world example of this technique.

[nguyenhoanghai1605]

When targeting different architectures like x86 and x64 in P/Invoke scenarios, struct layout can indeed diverge due to alignment and pointer size differences.

While defining two separate types and abstracting via interface (as mentioned in #33899) is a practical approach, here are deeper alternatives and considerations:

🧩 1. Use #if Preprocessor Directives

If you're building different binaries for different targets, you can conditionally define the struct:

#if X64
    [StructLayout(LayoutKind.Sequential)]
    internal struct PRINTDLG
    {
        public int lStructSize;
        public IntPtr hwndOwner;
        // ...
        public short nFromPage;
        public short nToPage;
        public short nMinPage;
    }
#else
    [StructLayout(LayoutKind.Sequential)]
    internal struct PRINTDLG
    {
        public int lStructSize;
        public int hwndOwner;
        // ...
        public short nFromPage;
        public short nToPage;
        public short nMinPage;
    }
#endif

[huoyaoyuan]

For future readers: using conditional compilation is strictly non-answer for this problem. The total intent is to express the layout in the same managed assembly.