Need to resolve HMONITOR --> deviceName (or deviceName --> HMONITOR) in windows - delphi

Question

EDIT - See Update at end
This is for Delphi 7.0 Build 4.453
Summary
I need to be able to take the Handle property from a TMonitor object (an element in the Monitors array in the TScreen component) which is a HMONITOR, and turn it into the string you would use in calls to EnumDisplaySettings as the lpszDeviceName parameter.
(my end goal is to get a list of device settings from a given HMONITOR value, by passing the resolved lpszDeviceName into calls to EnumDisplaySettings).
Detailed Information
As mentioned above, the Screen.Monitors[x].Handle property is of type HMONITOR and is normally used to pass into the GetMonitorInfo function, which returns, geometry information, but no lpszDeviceName. (note: there is a TMonitorInfoEx structure that has a szDevice field, but it does not seem to get filled in on my system, even though i am setting the cbSize field to the appropriate size).
Alternatively, if i can use a szDeviceName to get the equivalent HMONITOR value, i could plug it into the following function, which would use it in a comparison (I have inserted a call to fictitious function called hMonitorFromDeviceName in the code below) to indicate how it would be used.
function GetMonitorDeviceName(hmon : HMONITOR) : string;
var
DispDev : TDisplayDevice;
deviceName : string;
nDeviceIndex : integer;
begin
Result := '';
FillChar(DispDev, sizeof(DispDev),0);
DispDev.cb := sizeof(DispDev);
nDeviceIndex := 0;
while (EnumDisplayDevices(nil, nDeviceIndex, DispDev, 0)) do
begin
if ( hMonitorFromDeviceName(DispDev.DeviceString) = hmon ) then
begin
Result := StrPas(DispDev.DeviceString);
exit;
end;
inc(nDeviceIndex);
end;
end;
Update
Thanks to David Heffernan, I have tested his solution, and here is a sample function to get the monitor name from a given handle:
function GetMonitorName(hmon : HMONITOR) : string;
type
TMonitorInfoEx = record
cbSize: DWORD;
rcMonitor: TRect;
rcWork: TRect;
dwFlags: DWORD;
szDevice: array[0..CCHDEVICENAME - 1] of AnsiChar;
end;
var
DispDev : TDisplayDevice;
deviceName : string;
monInfo : TMonitorInfoEx;
begin
Result := '';
monInfo.cbSize := sizeof(monInfo);
if GetMonitorInfo(hmon,#monInfo) then
begin
DispDev.cb := sizeof(DispDev);
EnumDisplayDevices(#monInfo.szDevice, 0, DispDev, 0);
Result := StrPas(DispDev.DeviceString);
end;
end;

Answer 1

I think that you must be calling GetMonitorInfo incorrectly. This code:
{$APPTYPE CONSOLE}
uses
SysUtils, MultiMon, Windows, Forms;
var
i: Integer;
MonitorInfo: TMonitorInfoEx;
begin
MonitorInfo.cbSize := SizeOf(MonitorInfo);
for i := 0 to Screen.MonitorCount-1 do
begin
if not GetMonitorInfo(Screen.Monitors[i].Handle, #MonitorInfo) then
RaiseLastOSError;
Writeln(MonitorInfo.szDevice);
end;
Readln;
end.
produces this output on my machine:
\\.\DISPLAY1
\\.\DISPLAY2
I suspect that your call to GetMonitorInfo is failing in some way and perhaps you are not checking the return value for errors.
Having searched QualityCentral I suspect you have fallen victim to a known bug in older versions of Delphi: QC#3239. This is reported fixed in version 10.0.2124.6661 which is Delphi 2006.
Your comments confirm this diagnosis. To fix the problem you'll need a new TMonitorInfoEx definition. Here's one that will work on your pre-Unicode Delphi:
type
TMonitorInfoEx = record
cbSize: DWORD;
rcMonitor: TRect;
rcWork: TRect;
dwFlags: DWORD;
szDevice: array[0..CCHDEVICENAME - 1] of AnsiChar;
end;
If you add that to the code above (before you declare the variables of course) then I believe it will resolve your problem.
As an interesting aside, even in XE3, these structs have not been translated correctly: QC#114460. Admittedly the error is rather benign as it only affects PMonitorInfoExA and TMonitorInfoExA, but the error caught me out whilst trying to solve the problem in this question!