This behavior may have one of several causes:
A. This may be a consequence of the security settings on Windows 7. When you run MATLAB as an administrator and then try to open a MATLAB file from Windows Explorer, you are not a privileged user as far as Windows Explorer is concerned. As a result, MATLAB that is open cannot open this MATLAB file since MATLAB is a program that is run by a privileged user (administrator).
To work around this issue, you can enable writing to the pathdef.m file. The pathdef.m file is located in the directory:
where MATLABROOT is the result of executing the following in the MATLAB command prompt:
To enable writing to pathdef.m:
1. Open the directory with pathdef.m in the Windows Explorer, right-click on the file and open "Security" tab.
2. Click the "Edit" button to modify permissions.
3. Select the user or group that should be allowed to write to pathdef.m, i.e., Users.
4. Check the "Write" check box and click "Apply"
If you have previously set MATLAB to be run as an administrator, change it to run it as an ordinary user. You will be now able to save path to the pathdef.m file and each time you click on the MATLAB file, if MATLAB is already open, MATLAB file will be opened in the Editor of the open MATLAB instance.
If this does not work, try associating the file with an older version of MATLAB, open the file and then change the association to the new version. This resolved the issue for a customer who was transitioning from R2017b to R2018a
B. This issue may also arise if you uninstall an earlier release of MATLAB after installing a newer release. The newer release uses a different method to register file associations.
These associations are deleted from the registry when an older release is uninstalled. In this situation, please uninstall the new release and reinstall it.
If the file associations are not correct because you have manually registered the MATLAB file types with Windows Explorer, you can re-associate the MATLAB file types by following these instructions from the MATLAB documentation:
This page explains how to re-register the .MAT extension, but you can follow the same instructions for other file types.