MainWindowFinder.cs

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using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
 
namespace System.Diagnostics;
 
internal struct MainWindowFinder
{
    private IntPtr _bestHandle;
 
    private int _processId;
 
    public unsafe static IntPtr FindMainWindow(int processId)
    {
        Unsafe.SkipInit(out MainWindowFinder mainWindowFinder);
        mainWindowFinder._bestHandle = IntPtr.Zero;
        mainWindowFinder._processId = processId;
        global::Interop.User32.EnumWindows((delegate* unmanaged<IntPtr, IntPtr, global::Interop.BOOL>)(delegate*<IntPtr, IntPtr, global::Interop.BOOL>)(&EnumWindowsCallback), (IntPtr)(&mainWindowFinder));
        return mainWindowFinder._bestHandle;
    }
 
    private static bool IsMainWindow(IntPtr handle)
    {
        if (global::Interop.User32.GetWindow(handle, 4) == IntPtr.Zero)
        {
            return global::Interop.User32.IsWindowVisible(handle) != global::Interop.BOOL.FALSE;
        }
        return false;
    }
 
    [UnmanagedCallersOnly]
    private unsafe static global::Interop.BOOL EnumWindowsCallback(IntPtr handle, IntPtr extraParameter)
    {
        MainWindowFinder* ptr = (MainWindowFinder*)(void*)extraParameter;
        int num = 0;
        global::Interop.User32.GetWindowThreadProcessId(handle, &num);
        if (num == ptr->_processId && IsMainWindow(handle))
        {
            ptr->_bestHandle = handle;
            return global::Interop.BOOL.FALSE;
        }
        return global::Interop.BOOL.TRUE;
    }
}