CAD tip # 2311:

Windows settings for the DWG file type define how are DWG opened when dbl-clicked. Open Windows Explorer and in Tools, Folder options, File types tab select DWG - AutoCAD Drawing. Click on Details, select AutoCAD open in the list and click the 'Modify...' button. Now check 'Use DDE' if you want to open DWGs in a single session or uncheck it for multiple sessions.

When Use DDE is selected, the DDE Message should be set to [open("%1")] and Theme to System.

You may also handle (open) the DWG files with "AutoCAD DWG launcher" application (AcLauncher.exe, not AutoCAD itself).

In Windows Vista, Win7, Win8 and Win10, the actions for the individual file types can be set only in the Registry or using additional tools - like e.g. the applications FileTypes or File Type Doctor.

But is is usually possible just to re-associate .DWG files with the application "AutoCAD DWG Launcher" - in Windows Explorer, right-click on a DWG file and choose "Open with" > "Choose another application > "AutoCAD DWG Launcher" and check "Always use the selected program to open this kind of file". If "AutoCAD DWG Launcher" is not listed, Browse to another application and select the aclauncher.exe manually.

If it still doesn't work, try to reset your AutoCAD profile, or try to temporarily set the Properties>Compatibility for your acad.exe and aclauncher.exe to "Run this program as an administrator".

Important: Please note that in any case, AutoCAD SDI system variable should be set to 0 if you want to open DWGs in a single session (otherwise multiple sessions are used even when DDE is set).