Oh, my mistake, we should not pass Null for the first parameter, it is an
invalid handle for GetDeviceCaps. We should first create a screen DC with
CreateDC and then pass this screen DC to the GetDeviceCaps. Below code
snippet works well on my side:

[DllImport("gdi32.dll", EntryPoint = "CreateDC", CharSet = CharSet.Auto,
SetLastError = true)]
private static extern IntPtr CreateDC(string lpszDriver, string
lpszDeviceName, string lpszOutput, IntPtr devMode);

[DllImport("gdi32.dll", SetLastError=true)]
private static extern Int32 GetDeviceCaps(IntPtr hdc, Int32 capindex);
private const int LOGPIXELSX = 88;

IntPtr hdc = CreateDC("DISPLAY", null, null, IntPtr.Zero);
if (hdc == IntPtr.Zero)
{
   MessageBox.Show(Marshal.GetLastWin32Error().ToString());
   return;
}
int _dpi = GetDeviceCaps(hdc, LOGPIXELSX);
if (_dpi == 0)
{
   MessageBox.Show(Marshal.GetLastWin32Error().ToString());
   return;
}

Hope it helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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