>>I put my project under source code control using Visual Source Safe. I then tried to modify a class of mine via the Project window. After I clicked the class name I clicked on the Modify button. I was then asked to verify that I wanted to check out the VCX file that contained the class. When I confirmed the checkout I got an error that said "Source control cannot operate on open file xxx.vcx. Close the file and try again." I click on the OK button and I am in the class designer working on a read-only version of the class.
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>>I tried this again after restarting VFP and reopening the project. This time it worked! What do I need to do to close the class library file prior to making modifications?

>You may have a SET CLASSLIB to the questionable class somewhere. I usually SET CLASSLIBs when I startup VFP. Issuing a RELEASE ALL or CLEAR ALL; I do both will solve this.

I often experience this problem after I've run a form which uses the class. My guess is that VFP isn't releasing a reference to the class. I have a macro set up to rerun my startup program which sets global variables, issues SET CLASSLIBs, and, as Ramil suggests, issues RELEASE ALL or CLEAR ALL commands. I can then check out the class with no problem.