ShellExecuteExW.cs

// Idea 1: SHELLEXECUTEINFOW with nested structs, Windows is flat
public static class SHELLEXECUTEINFOW
{
    [StructLayout(..., Pack = 1)]
    public struct X86 { ... }

    [StructLayout(...)]
    public struct X64 { ... }

    [StructLayout(...)]
    public struct Arm64 { ... }
}

public static class Windows
{
    [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
    public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X86* pExecInfo);

    [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
    public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X64* pExecInfo);

    [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
    public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.ARM64* pExecInfo);
}

// Idea 2: SHELLEXECUTEINFOW with nested structs (same as above), and Windows also. This also provides a pattern for dispatching anything else in Windows (e.g. consts) that may need arch-specific differences
public static class Windows
{
    public static class X86
    {
        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X86* pExecInfo);
    }

    public static class X64
    {
        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X64* pExecInfo);
    }

    public static class Arm64
    {
        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.ARM64* pExecInfo);
    }
}

ViaInterfaces.cs

// Idea 3 ... use interfaces to simplify something
namespace TerraFX.Interop
{
    public unsafe interface SHELLEXECUTEINFOW
    {
        ref uint cbSize { get; }
        ref uint fMask { get; }
        ref IntPtr hwnd { get; }
        ref ushort* lpVerb { get; }
        ref ushort* lpFile { get; }
        ref ushort* lpParameters { get; }
        ref ushort* lpDirectory { get; }
        ref int nShow { get; }
        ref IntPtr hInstApp { get; }
        ref void* lpIDList { get; }
        ref ushort* lpClass { get; }
        ref IntPtr hkeyClass { get; }
        ref uint dwHotKey { get; }
        ref IntPtr hIcon { get; }
        ref IntPtr hMonitor { get; }
        ref IntPtr hProcess { get; }

        public struct X64 : SHELLEXECUTEINFOW
        {
            ...
        }
    }

    public static class Windows
    {
        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X86* pExecInfo);

        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.X64* pExecInfo);

        [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
        public unsafe static extern int ShellExecuteExW(SHELLEXECUTEINFOW.ARM64* pExecInfo);

        public unsafe static int ShellExecuteExW<TSHELLEXECUTEINFOW>(TSHELLEXECUTEINFOW* pExecInfo)
            where TSHELLEXECUTEINFOW : unmanaged, SHELLEXECUTEINFOW
        {
             return _ShellExecuteExW(pExecInfo);
                
             [DllImport("shell32", ExactSpelling = true, SetLastError = true)]
             unsafe static extern int _ShellExecuteExW(void* pExecInfo);
        }
    }
}