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Asked: 2026-05-11T10:28:33+00:00

SOLVED I am using delphi 2009. My program listens for usb drives being connected

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SOLVED

I am using delphi 2009. My program listens for usb drives being connected and remove. Ive used a very similar code in 10 apps over the past year. It has always worked perfectly. When i migrated i had to give up using thddinfo to get the drive model. This has been replaced by using WMI. The WMI query requires the physical disk number and i happen to already have a function in the app for doing just that.

As i test I put this in a button and ran it and it successfully determines the psp is physical drive 4 and returns the model (all checked in the debugger and in another example using show message):

function IsPSP(Drive: String):Boolean; var Model: String; DriveNum: Byte; begin   Result := False;   Delete(Drive, 2, MaxInt);   DriveNum := GetPhysicalDiskNumber(Drive[1]);   Model := (MagWmiGetDiskModel(DriveNum));   if Pos('PSP',Model) > 0 then Result := True; end;  procedure TfrmMain.Button1Click(Sender: TObject); var DriveNum: Byte; begin   IsPSP('I'); end;

It works perfectly that is until i allow the WMDeviceChange that ive been using for a year to call up the getphysicaldisknumber and the wmi query statement. Ive tried them by themselves theyre both a problem. GetPhysicalDiskNumber freezes real bad when its doing a CloseHandle on the logical disk but does return the number eventually. The WMI query fails with no error just returns ” debugger points into the wbemscripting_tlb where the connection just never happened. Keep in mind the only thing thats changed in a year is what im calling to get the model i was using an api call and now im using something else.

Below is the rest of the code involved at this time sans the ispsp that is displayed above:

procedure TfrmMain.WMDeviceChange(var Msg: TMessage); var Drive: String; begin   case Msg.wParam of     DBT_DeviceArrival: if PDevBroadcastHdr(Msg.lParam)^.dbcd_devicetype = DBT_DevTyp_Volume then       begin         Drive := GetDrive(PDevBroadcastVolume(Msg.lParam)) + '\';         OnDeviceInsert(Drive);       end;     DBT_DeviceRemoveComplete: if PDevBroadcastHdr(Msg.lParam)^.dbcd_devicetype = DBT_DevTyp_Volume then       begin         Drive := GetDrive(PDevBroadcastVolume(Msg.lParam)) + '\';         OnDeviceRemove(Drive);       end;   end; end;  Procedure TfrmMain.OnDeviceInsert(Drive: String); var PreviousIndex: Integer; begin   if (getdrivetype(Pchar(Drive))=DRIVE_REMOVABLE) then   begin     PreviousIndex := cbxDriveList.Items.IndexOf(cbxDriveList.Text);     cbxDriveList.Items.Append(Drive);     if PreviousIndex = -1 then //If there was no drive to begin with then set index to 0     begin       PreviousIndex := 0;       cbxDriveList.ItemIndex := 0;     end;     if isPSP(Drive) then     begin       if MessageDlg('A PSP was detect @ ' + Drive + #10#13 + 'Would you like to select this drive?',mtWarning,[mbYes,mbNo], 0) = mrYes then       cbxDriveList.ItemIndex := cbxDriveList.Items.IndexOf(Drive)       else cbxDriveList.ItemIndex := PreviousIndex;     end     else if MessageDlg('USB Drive ' + Drive + ' Detected' + #10#13 + 'Is this your target drive?',mtWarning,[mbYes,mbNo], 0) = mrYes then         cbxDriveList.ItemIndex := cbxDriveList.Items.IndexOf(Drive)     else cbxDriveList.ItemIndex := PreviousIndex;   end; end;  Procedure TfrmMain.OnDeviceRemove(Drive: String); begin   if not (getdrivetype(Pchar(Drive)) = DRIVE_CDROM) then   begin     if cbxDriveList.Text = (Drive) then ShowMessage('The selected drive (' + Drive + ') has been removed');     cbxDriveList.Items.Delete(cbxDriveList.Items.IndexOf(Drive));     if cbxDriveList.Text = '' then cbxDriveList.ItemIndex := 0;     if Drive = PSPDrive then //Check Detect PSP and remove reference if its been removed     begin       PSPDrive := '';     end;   end; end;

Rob has said something below about im not calling the inherited message handler, ive read the document i see a couple of things i can return… but im not really sure i understand but i will look into it. Im not a very good pascal programmer but ive been learning alot. The transition to 2009 has had some rough patches as well.

The USB drive detection and all that works perfectly. If i remove the two things from is psp the user is greeted right away with wis this your whatever and adds I:\ to the list. Its just the two new things that have changed in the app that fail when called by wmdevicechange and as said before they work on their own.

EDIT – SOLVED

Alright well im using a timer as suggested and the problem seems to be solved. One note is that when called by the timer very shortly after the wmdevicechange getting the physical disk number still seems to be slow. I attribute this to the device still being attached to the system.

On that note im using a P2 450 on the regular. I hooked the PSP and app to a 1.8Ghz Dual Core Laptop and the program detected the psp and notified the user very fast. So the app wont freeze unless there on a very very slow computer and on this slow onw its only for a matter of seconds and doesnt affect the operation of the program though isnt very cool. But i feel that all modern computers will run the detection fast especially because they can attach the device alot faster.

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1 Answer

  1. It’s possible that the information you’re querying becomes available only after the WMDeviceChange message handler runs. If the very same code works when called from a button, try this:

    1. Refactor your WMDeviceChange handler code into one or more separate methods.
    2. In the WMDeviceChange handler, activate a precreated timer and have it fire one second later, or something like that.
    3. Call the former WMDeviceChange handler code from the timer handler code.
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