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How do code coverage tools like NCover know what parts of the code were executed and what parts were not?
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Test coverage is defined as a technique which determines whether our test cases are actually covering the application code and how much code is exercised when we run those test cases. If there are 10 requirements and 100 tests created and if 90 tests are executed then test coverage is 90%.
Your code coverage tool will monitor the execution of your test suite and tell you how much of the statements, branches, functions and lines were run as part of your tests.
To calculate the code coverage percentage, simply use the following formula: Code Coverage Percentage = (Number of lines of code executed by a testing algorithm/Total number of lines of code in a system component) * 100.
It requires that you run your tests once with code coverage analysis enables, and then simply counts the number of blocks (that is, scope blocks) covered and compares to the total number of blocks in the project(s) you are testing.
Here's a technical paper on How to Implement test coverage tools for arbitrary languages.
My company builds a family of test coverage tools for Java, C#, C++, PHP, COBOL, PLSQL, ... based on this principle.
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Quote straight from the NCover FAQ: NCover reports the percentage of branches in the code that have been taken throughout the course of your automated testing. It achieves this by instrumenting the source code at each branch, and writing the 'hit' points to a file. These 'hit' points are then compared to the total possible points that could have been 'hit'.
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