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# Defining a Rule

Visual Studio 2015

The fromext represents the extension of a dependent file, and toext represents the extension of a target file.

```.fromext.toext:
commands
```

## Remarks

Extensions are not case sensitive. Macros can be invoked to represent fromext and toext; the macros are expanded during preprocessing. The period (.) preceding fromext must appear at the beginning of the line. The colon (:) is preceded by zero or more spaces or tabs. It can be followed only by spaces or tabs, a semicolon (;) to specify a command, a number sign (#) to specify a comment, or a newline character. No other spaces are allowed. Commands are specified as in description blocks.