While I'm modifying the file, this file was changed already on Source Safe.

How can I do?

one way can do, to merge code before check in.

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If my code base is too old from the code on Source Safe. How can I make sure that the code I merage won't affect to other people's, which causes complileing error.

1. to update my code base
1. get the lasest code on Source Safe and make sure it can be builded through the end
2. merge my code which im developoing to the new code base and make sure it can be builded through the end
2. check out the code what I want to merge
3. merge 2 to my new code base and make sure it can be builded throough the end
4. check in