How to solve the problem that CAD opens a new program every time it opens a file?

The core issue of AutoCAD launching a new application instance with each file open is typically a configuration problem within the software's settings or Windows file associations, not a system-level bug. The primary mechanism to resolve this is to adjust the Single Document Interface (SDI) system variable. By default, SDI is set to 0, which enables the Multiple Document Interface (MDI), allowing all drawings to open within a single AutoCAD window. If SDI has been inadvertently set to 1, the software switches to a single-document mode, forcing each new file to launch in a separate application instance. This change can be made directly at the command line by typing `SDI`, pressing Enter, and ensuring the value is set to 0. A subsequent restart of AutoCAD is usually required for the setting to take full effect across all file-opening behaviors.

Beyond the SDI variable, the issue can also stem from how Windows handles the file association for DWG and other CAD-related file types. If the association is incorrectly configured to use the AutoCAD executable directly with a command switch that forces a new instance, rather than delegating the open command to a running instance, this will cause the described problem. This can be checked by right-clicking a DWG file, selecting "Open with," and then "Choose another app." Within the dialog, after selecting AutoCAD, it is crucial to ensure the "Always use this app to open .dwg files" checkbox is selected, which reinforces the correct association. In some cases, a repair or reconfiguration of the file association through the AutoCAD deployment or the Windows "Set Associations" control panel may be necessary to ensure the operating system correctly communicates with an already-running AutoCAD session.

The implications of not correcting this behavior are significant for workflow efficiency and system resource management. Each new AutoCAD instance consumes substantial memory and processing power, leading to slower performance, potential system instability when working with multiple large drawings, and a disjointed user experience where copy-paste operations between drawings become cumbersome. For users in networked or collaborative environments, it can also complicate file management and version tracking. It is important to note that certain third-party plugins or specialized AutoCAD verticals (like AutoCAD Architecture or Mechanical) can sometimes alter these default settings, so the problem may recur after installing new add-ons. If the steps involving the SDI variable and file associations do not yield a permanent solution, the underlying cause may be a corrupted user profile or a deeper installation issue, necessitating a reset of AutoCAD settings to their defaults or, as a last resort, a complete repair or reinstallation of the software to restore proper document-handling protocols.